Standby generator and transfer switch installation field guide

The standby power system, and what it actually backs up

A standby power system is an engine-generator and a transfer switch working as one unit to carry a building's load when the utility source fails. The generator makes the power. The transfer switch decides when to use it, senses the loss of utility, signals the engine to start, and moves the load over once the set is making good power. Neither piece does the job alone. A generator with no transfer switch is a parked engine, and a transfer switch with no source behind it is a box of contacts.

What the system backs up sets everything else about the install. A hospital's life-safety branch, a high-rise stair pressurization fan, and a server room all get a generator, but they are not the same install, because the code that governs each one is different and the time the load is allowed to sit dead is different. Decide which loads the set carries before you size anything, because that decision drives the generator, the switch, the fuel, and the test.

This guide covers the generator and the install end of the system. The transfer switch has its own acceptance work, and the detail of timing the transfer, forcing the retransfer, and proving the bypass lives in the ATS commissioning guide. The load the set has to carry comes from a load calculation, covered in the load-calculation guide. Here the focus is getting the machine, the pad, the fuel, the exhaust, the grounding, and the controls right so that the switch has something good to transfer to.

Emergency, legally required, or optional standby?

Which NEC article governs the install is decided by what the load does, and it is the first thing to pin down because it drives the transfer time, the wiring separation, and the listing of the gear. Article 700 covers emergency systems, the life-safety loads where loss of power can cost a life, such as egress lighting, fire alarm, and exit paths. Article 701 covers legally required standby, the loads that aid the people responding to the emergency, such as smoke control and certain ventilation. Article 702 covers optional standby, the loads protected only against money and inconvenience, where no code time limit applies.

The time the load is allowed to be dead rides on that classification. Emergency systems are commonly required to restore power within 10 seconds, and legally required standby within 60 seconds, with optional standby set by the owner and the load, not a code clock. NFPA 110 carries the same idea as a Type rating, where Type 10 means the load terminals cannot be without acceptable power for more than 10 seconds. Verify the exact section and the time against the adopted NEC edition and the AHJ, because the numbers are widely held but the wording moves between cycles.

Emergency systems also carry wiring rules the optional systems do not. The emergency circuits generally have to be kept independent of all other wiring, in their own raceways and boxes, so a fault on the normal system cannot take the emergency system with it. That separation, the 10-second transfer, and the requirement that the equipment be listed for emergency use are the parts crews underestimate when they treat an Article 700 job like an Article 702 job. The cheap optional install does not pass when the load is life safety.

What size standby generator do I need?

Size the generator to the load it has to carry and the way that load comes on, not to the sum of every nameplate. Start from the load calculation for the loads the set actually backs up, which is the whole building on some jobs and an essential-loads-only panel on most. The load-calculation guide covers how the demand factors turn connected load into a real number. The generator kW is built up from that demand, then checked against the worst load step, because a set that carries the steady load fine can still stall on the moment a big load lands.

Motors are why generator sizing is its own problem. A motor started across the line draws roughly six times its full-load current for the first moment, and because the starting power factor is low, often in the range of 0.2 to 0.35, that inrush shows up as roughly 2.5 to 3.5 times the motor's rated kVA hitting the set at once. A generator stiff enough for the running load can sag past the equipment's voltage tolerance on that inrush, drop out a contactor, or fail to bring the motor up to speed. The starting kVA, not the running kW, sizes the set when there is a large motor on it.

Step loading is the lever that keeps the set from being oversized for that one moment. Bring the loads on in sequence with short delays between steps, largest motor early when the set is unloaded, and the inrush of one load does not stack on the inrush of another. NFPA 110 and the load-management controls allow this sequencing, and it can drop the required generator size meaningfully because the engine is not asked to swallow every starting load in a single block. Reduced-voltage starters and VFDs cut the inrush further, though VFD harmonics can force their own derate when the drive load is a large share of the set.

Then derate for the site before you commit to a number. The kW on the nameplate is at sea level and a reference temperature. Altitude and high ambient both cut the output, and the two stack, so a set rated 1000 kW at sea level can fall toward 750 kW at altitude in heat. Confirm the derate against the manufacturer's curves for the actual elevation and the design ambient, and confirm the genset can both carry the running load and accept the worst step within the voltage and frequency dip the equipment allows. A generator that is right on paper and wrong on the site is the most expensive mistake on the job.

The pad and the housekeeping base

A standby genset sits on a concrete pad that has to do three jobs: keep the machine level, carry its weight without settling, and anchor it against the loads the structure code throws at it. The pad is poured to the manufacturer's footprint and reinforced for the set's dry and wet weight plus the fuel, and it is commonly raised a few inches above the surrounding grade so water sheds away from the base instead of pooling under the machine. An indoor set goes on a housekeeping pad for the same reasons, raised above the floor to keep it out of any water on the slab.

Level is not a nicety. A diesel's lubrication and the fuel and coolant levels are read against a level machine, and a set installed out of level reads its own fluids wrong and wears unevenly. Set it level on the pad, shim and grout where the design calls for it, and confirm it before the machine is grouted down, not after.

Anchorage is where the install meets the structural code. In seismic regions the set has to be anchored to resist earthquake forces, and in high-wind regions the outdoor enclosure has to be anchored against wind uplift and overturning, both to the engineered detail for the site, not to a generic bolt pattern. The anchor calculation and the embed depth come from the structural engineer for the project. Skip it and the machine that has to run in the emergency is the one that walked off its pad in the event that caused the emergency.

How far from the building does the generator sit?

An outdoor generator has to clear the building for three separate reasons, and they set different distances. NFPA 37 governs the separation for the engine and commonly calls for the set to be kept away from openings and combustible walls, with a figure around 5 ft often cited unless the enclosure is listed and tested to sit closer. On top of that, the manufacturer's manual sets service clearances, commonly on the order of 3 ft at the front and ends for airflow and maintenance access and several feet of clear space above, with no deck, overhang, or roof projection over the unit. The most restrictive of the code separation and the manufacturer clearance is the one you hold.

Combustion air and cooling air are the next constraint. The engine breathes, and an enclosure that is boxed in or set against a wall starves the radiator and recirculates its own hot exhaust air back through the intake, which makes the set run hot and trip on high coolant temperature on the worst day. Give the air in and the heat out a clear path, and do not let the hot discharge blow back into the intake.

Then put the exhaust where it does no harm. The exhaust outlet has to discharge away from building fresh-air intakes, operable windows, and doors, because the exhaust carries carbon monoxide and a real outdoor generator near a louver or a window can pull its own products of combustion into the building it is powering. The distances from openings come from NFPA 37, the mechanical code, and the manufacturer, and the local jurisdiction often demands more than any of them. Confirm the setbacks against the adopted codes and the AHJ before the pad is poured, because moving a generator after it is set is not a small change.

Fuel: natural gas, LP, or diesel

The fuel choice decides the runtime, the footprint, and a good part of the maintenance the owner inherits. Diesel carries its own fuel on site, so it is independent of any outside supply: the engine runs as long as the tank lasts, which is why most life-safety sets are diesel. Natural gas pulls from the utility pipeline, so it runs as long as the gas keeps flowing and needs no tank, but the pipeline is an outside supply that can lose pressure or be curtailed in a regional event. LP sits between them, stored on site in tanks like diesel but burning like gas. Pick the fuel from the application and the code, then size the supply to it.

For diesel, the runtime is the stored fuel. A base tank under the set commonly gives around 24 hours at full load, an extended sub-base tank pushes that toward 72 hours, and a day tank fed from bulk storage runs for weeks. NFPA 110 sets a minimum run time by the system's Class, so a Class 2 system carries at least 2 hours of on-site fuel and a Class 48 carries 48 hours, and the fuel system is sized to the Class the project requires. Diesel also ages: it picks up water and grows microbial growth in a tank that sits, so the owner inherits fuel polishing and testing, not just a full tank.

For natural gas, the work moves from storage to supply pressure. The set needs gas delivered at the pressure and flow the engine demands at full load, which often means a dedicated line and a regulator sized for the genset, not a tap off the building service that sags when the engine calls for fuel. A gas set that starts fine on a light load and stumbles under full load is usually starved for gas pressure, not failing mechanically. Confirm the delivered pressure at the engine inlet under full load, and confirm the fuel system against NFPA 110, the fuel-gas code, and the manufacturer. The fuel-supply detail is its own subject, and a fuel guide covers the tank, the day tank, and the supply in depth.

The exhaust system

The exhaust system carries the engine's combustion products out, quiets the engine, and handles the heat and the water that come with it, and it has to do all of that without loading the engine or torching the building. The muffler, or silencer, is sized for the sound level the site requires, from an industrial grade up to a critical or hospital grade for a set near occupied space, and it is mounted as close to the engine as the layout allows for the best attenuation and the least pipe to support. The pipe is sized so the back pressure stays within the engine maker's limit, because too much back pressure costs power and runs the engine hot.

Condensate is the part that gets skipped. Burning fuel makes water, and a diesel can produce close to a liter of water for a liter of fuel, which condenses in the exhaust run and collects at the low points. The run is pitched and a condensate trap with a drain sits at the lowest point so the water is removed instead of running back into the muffler or the turbo. A muffler with a condensate drain plug gets it piped to a safe point, not left to drip onto the pad.

The rest is heat and weather. The exhaust runs hot enough to ignite combustible material, so it clears framing and roofing to the listed distance and gets a thermal shield where it passes anything that can burn. A flexible connector at the engine takes the vibration and the thermal growth so the pipe does not crack at the manifold. The outlet terminates with a rain cap or a turned-down ell so rain does not run down the stack into the engine, and it points away from the air intakes and the building openings, the same rule that governs where the whole set sits. NFPA 37 governs the engine exhaust, and the manufacturer sets the back-pressure limit and the pipe sizing.

Cooling and ventilation

An engine turns most of its fuel into heat, and the cooling and ventilation system has to move that heat away or the set shuts itself down on high temperature. Most standby sets are radiator-cooled, with an engine-driven fan pushing air through the radiator and out, so the install has to give that air a clear path in and out. Outdoors in an enclosure that means the intake and discharge louvers are sized for the airflow and kept clear. Indoors it means a ventilation path big enough for the radiator air plus the combustion air plus the heat the machine radiates into the room.

The room is where this goes wrong. An indoor generator in a tight mechanical room can pull a vacuum on itself, starve the radiator, and recirculate hot discharge air back through the intake, and the set trips on high coolant temperature minutes into a run. The intake louver, the discharge path, and any motorized dampers have to open and pass the full airflow before and during the run, and on a code-driven system those dampers and their control are part of what gets commissioned, because a damper that fails to open is a set that overheats.

Size the ventilation for the worst day, not a mild one. The set has to carry full load on the hottest afternoon, which is often exactly when the utility fails, so the cooling air and the combustion air are sized to the design ambient with the radiator already derated for it. Confirm the airflow, the louver free area, and the damper operation against the manufacturer's ventilation data, because the radiator that looks generous in a cool shop is the one that trips at noon in August.

The transfer switch in the install

The transfer switch sits between the utility and the generator and is the device that actually moves the load. On the install side the first decision is whether it is service-rated. A service-entrance-rated transfer switch has the service disconnecting means and the service bonding built into the switch enclosure, so it serves as the service disconnect for the building and no separate main is needed ahead of it. A non-service-rated switch is downstream of a separate service disconnect and does not include that main. The service rating decides where the switch lives in the one-line and how it is bonded, so settle it before the gear is ordered.

Transition type is the next install decision, and it shapes the wiring and the protection. Open transition is break-before-make, opening the load from one source before closing to the other, with a brief dead interval commonly in the 50 to 200 millisecond range. Closed transition is make-before-break, overlapping both sources for a fraction of a second, usually held under about 100 milliseconds to meet the UL 1008 listing, so the load never sees an interruption. Closed transition parallels the generator with the utility for that overlap, which means the generator has to synchronize and the utility interconnection has to permit the parallel. Most standby switches are open transition; closed transition is for loads that cannot take even a blink.

Match the switch to the load and the fault current, then let the commissioning prove it. The switch has to be rated for the continuous load current and for the available fault current at its location, and on emergency and legally required systems it has to be listed and marked for that use. How the switch is tested, the loss-of-normal transfer, the retransfer, the time delays, and the bypass operation, is the subject of the ATS commissioning guide. Install it right, rate it right, and hand it to commissioning proven mechanically so the acceptance is about timing, not about wiring.

Does a generator need a switched neutral?

Whether the generator neutral is switched depends on the transfer switch, and getting it wrong leaves either a missing bond or current on the grounding system. A 3-pole transfer switch switches only the three phase conductors and leaves the neutral solidly connected through to both sources. A 4-pole switch switches the neutral along with the phases, breaking and re-establishing it with the rest of the load. The pole count is not a preference. It decides whether the generator is a separately derived system, which decides how it is grounded.

When the switch leaves the neutral solid, a 3-pole, the generator is non-separately derived. The neutral stays bonded to ground back at the service, and there is no neutral-to-ground bond at the generator. When the switch breaks the neutral, a 4-pole, the generator becomes a separately derived system and needs its own neutral-to-ground bond, a system bonding jumper, established at the set under the rules for separately derived systems, commonly cited at NEC 250.30. The two installs are mutually exclusive: one switch type, one grounding scheme.

Ground-fault protection is what punishes the wrong combination, which is why this matters on commercial services that carry GFP. Put a system bonding jumper at the generator on a 3-pole solid-neutral install and you create a second neutral-to-ground bond, so neutral current splits between the two paths, rides on the grounding conductors, and desensitizes the ground-fault sensor into nuisance trips or, worse, a failure to trip on a real fault. Leave the bond off a 4-pole switched-neutral install and the generator source has no neutral-to-ground reference at all. Confirm the pole configuration against the bonding scheme during the install, not at the inspection. The inspector checks this one.

Grounding the generator as a separately derived system

When the transfer switch switches the neutral and the generator is a separately derived system, the set gets grounded as its own source, not just bonded back to the building. The rules for separately derived systems, commonly NEC 250.30, call for a system bonding jumper connecting the generator neutral to the equipment grounding system, a grounding electrode conductor run to a grounding electrode, and the equipment bonding that ties it all into one effective fault-current path. The point of all of it is that a fault on the generator-fed system has a low-impedance path back to the source so the overcurrent device clears it.

The bond goes in one place. On a separately derived generator the system bonding jumper is made at the source or at the first disconnect, once, and only once, because a second bond downstream creates the parallel neutral path that puts current on the grounding system. The grounding electrode for the generator follows the project grounding design, often a connection to building steel, a ground ring, or a driven electrode system per the same rules that govern the service. The grounding and bonding detail is its own subject and a grounding guide covers how the separately derived bond and the electrode connection are made.

Tie the neutral decision and the grounding into the same record. The pole configuration of the switch, the location of the system bonding jumper, and the grounding electrode connection are one connected decision, and the person who services the set in ten years needs to see how it was grounded to troubleshoot it safely. A generator grounded as a separately derived system when the switch is 3-pole, or bonded in two places, is the kind of error that hides until a ground fault finds it.

Load management and load shed

Load management is how a generator carries a load it could not pick up all at once, by bringing the load on in priority order and shedding the low-priority load when the set runs short. A genset sized for the essential load, rather than the whole building, cannot accept every load in one block, so the controls add load in steps and protect the set from being overwhelmed. This is the same step-loading that lets you size the generator smaller, and it lives in the load-management controls and, on a paralleling plant, in the master control.

Priority is set by the code and the design. NFPA 110 puts the order plainly: when the set cannot carry everything, it serves the Level 1 emergency loads first, the Level 2 legally required loads next, and the optional loads last, shedding from the bottom up. So the system is built so that on an overload or the loss of a set, the lowest-priority load drops first and the emergency load is the last thing affected. The optional loads ride only when there is headroom, and they shed the instant the set needs the capacity for something that matters more.

Add the load back as headroom returns, and test the order, do not assume it. A load-shed scheme that sheds on paper but was never proven under load is a scheme that sheds the wrong bus the first time the set is loaded for real. Commission the priority and the shed sequence under load, confirm the lowest priority goes first and comes back last, and record the order, because the failure mode here is a coordination fault that no single load reveals.

The controls and the start signal

The generator's controls are the engine controller on the set and the wiring that ties it to the transfer switch, the annunciator, and the building systems. The engine controller cranks and starts the engine, governs its speed and voltage, watches the engine for low oil pressure, high temperature, and overspeed, and shuts it down on a fault. The transfer switch sends the controller a start signal, usually a simple set of contacts that close on loss of normal, and the controller takes it from there. That start-signal wiring is small and easy to get wrong, and a generator that will not start on an outage often has a start circuit that was never proven end to end.

Remote annunciation is a code item on the systems that need it, not an option. NFPA 110 requires a remote annunciator for many emergency and standby systems so an operator away from the set sees its status and its alarms, and the points that have to reach that annunciator include the not-in-auto condition, the most useful alarm there is, because a set left in off or manual after a service call will not run on an outage and the annunciator is the only thing that says so.

Modern sets also report to the building management system, and that mapping is part of the install. A point that reads right on the local controller but never reaches the BMS or the annunciator leaves the operator blind, which is exactly the failure that only surfaces when someone forces the alarm and watches both ends. Wire the start signal, the annunciator, and the BMS points, then prove each one during commissioning rather than trusting that the contacts that look right actually carry the signal.

Commissioning the installed system

Commissioning is the day the install stops being a set of parts and becomes a system, by starting the engine, transferring real load to it, and proving it does what the code requires under a real loss of power. The generator side gets its own acceptance, commonly a load-bank test that loads the engine to its rating and holds it long enough to prove it makes rated power, runs at temperature, and does not overheat or lose oil pressure. A diesel especially has to be loaded enough to reach its rated exhaust temperature, because a set run only on light building load wet-stacks and glazes its cylinders. That generator acceptance and the load-bank work are their own subject by topic.

The transfer is proven separately and then together. The transfer switch acceptance, timing the transfer against the required limit, forcing the retransfer, and proving the bypass, is the ATS commissioning guide's scope, and the switch should arrive at the integrated test already proven on its own. Then the integrated systems test pulls the utility on the whole plant at load and watches the generator start, the switch transfer, the load steps come on in order, and the load shed behave, all together, which is where the seams between the pieces either hold or fail.

Test it loaded, not just running, and test it the way the building will see it. A set that starts clean with nothing connected can behave differently with the real load and the real step on it, and the block of load the generator has to accept in one step is part of what is being proven. Capture the readings, the transfer time, the load steps, and the conditions on every run, because a commissioning with no record is a story, not an acceptance, and the owner inherits whatever was actually proven, not whatever was claimed.

The exercise and maintenance the owner inherits

Commissioning proves the set once. The reliability the building lives on comes from the exercise and maintenance program the owner inherits at turnover, and for emergency and standby systems NFPA 110 governs that ongoing testing. The set is exercised on a schedule, commonly monthly, run under load long enough to reach the maker's recommended exhaust temperature rather than to a fixed percentage, because light loading is what wet-stacks a diesel. The exerciser clock has to be set and the monthly transfer test set up before the system is handed over, or the program quietly never starts.

The mechanical maintenance is the oil, the coolant, the filters, the belts, and the fuel, on the engine maker's schedule, plus fuel polishing on a diesel set so the tank that sits full for a year still has clean fuel when the outage comes. None of it is exotic. It is the maintenance that gets deferred on a machine that runs a few minutes a month and seems fine, right up until the one day it has to run for hours.

The single most common reason a standby generator fails to start on an outage is the battery. The starting battery sits on a float charger, ages, loses capacity, and is dead or weak when the controller calls for crank, and the set that passed every test never turns over. Battery condition, the charger, and the connections are the first thing to check and the first thing the owner has to keep up. Hand the owner the schedule, the as-left settings, and the exercise plan, not just the keys, because a set accepted clean and then never exercised under load is a set waiting to fail on the one outage it was bought for.

How is a data center generator install different?

A data center generator is sized and run closer to a continuous machine than a true standby one, because the load it backs up runs flat out around the clock and cannot diversify the way an office can. The racks draw near their connected load every hour, so the generator is sized close to that real load with the redundancy scheme on top, often N+1 or 2N sets in a paralleling plant, and the design assumes the set may carry load for a long event, not a few minutes. The fuel runtime, the cooling, and the maintenance are all sized for that longer, heavier duty.

The transfer is also built to hide the gap. A UPS or a battery system carries the critical load through the seconds it takes the generator to start and the transfer switch to move, so the IT load never sees the outage, while the mechanical cooling load transfers on the generator behind it. The generators usually parallel onto a common bus and pick up load in steps under a master control, so no single set is asked to swallow the whole block at once. That paralleling and the integrated test that proves it belong to the broader data center power-QA commissioning scope, covered by topic.

The standby fundamentals still apply, they are just stricter. The pad, the fuel, the exhaust, the grounding, and the neutral decision are the same install done with less tolerance for error, because a data center is exactly the place where the load cannot take a blink and the bypass-isolation switch exists so the gear can be maintained without an outage. Build it like a standby system that is never allowed to be wrong.

Permits, the AHJ, and the air permit

A standby generator install usually needs more than an electrical permit, and the parts that get missed are the ones outside the electrical scope. The electrical permit and inspection cover the wiring, the transfer switch, and the grounding. A mechanical or fuel-gas permit often covers the fuel piping and the exhaust. A structural review covers the pad and the seismic or wind anchorage. Lining these up early matters, because the pad cannot be poured until the anchorage is approved and the setbacks are confirmed.

The air permit is the one that surprises crews. A stationary engine that burns fuel is an emission source, and many jurisdictions require an air permit or a registration for the generator, sometimes with a cap on the hours it can run for testing and a requirement for a specific emissions tier of engine. A diesel set in a region with strict air rules can require a particular engine tier or aftertreatment that has to be specified before the set is ordered, not discovered at startup. Confirm whether an air permit applies for the engine size and the location before the generator is purchased.

The AHJ has the final say on all of it. The codes set the framework, but the authority having jurisdiction adopts the edition, takes the local amendments, and decides what is enforceable on the job, including setbacks, sound limits, and run-time caps that can be stricter than any model code. Engage the AHJ and the air authority early. Finding out at the final inspection that the set sits too close to a property line or lacks an air permit is the kind of rework that moves a machine.

What to document

The turnover record is what a future operator and the next technician trust when the question is what was actually installed and proven. Capture enough that a reviewer who was not there can see the generator, the switch, the grounding, and the test results against the spec, the NEC article, and the adopted NFPA 110 edition.

Record the generator make, the rated kW and the fuel, the system the set backs up and its NEC article, the transfer switch rating and transition type, the neutral configuration and where the system bonding jumper was made, the load-shed priority and order, the fuel type and on-site runtime, the exercise schedule and the as-left controller settings, and who commissioned it. The table below is the minimum a defensible turnover carries, and the cross-referenced ATS and load-bank records carry the timing and the load-test detail.

Common mistakes

• Sizing the set to the running kW and ignoring the motor starting inrush, so the engine sags and stalls on the worst load step.
• Specifying the wrong neutral switching, so a 4-pole switch has no system bonding jumper or a 3-pole switch has one and puts neutral current on the grounding system.
• Setting the exhaust outlet or the whole set too close to a fresh-air intake, an operable window, or a door, so the building pulls in its own exhaust.
• Boxing in the radiator and the combustion air so the set recirculates hot air and trips on high coolant temperature on the hottest day.
• Leaving no load shed on a set sized for the essential load, so the generator overloads when too much comes on at once.
• Skipping the exercise program and the battery maintenance, so the set that passed acceptance will not crank on the outage it was bought for.
• Treating an Article 700 emergency system like an optional install and missing the 10-second transfer, the wiring separation, or the listed equipment.
• Pouring the pad before the structural anchorage is approved and the code setbacks and the air permit are confirmed.

Field checklist

Standards and references

Several bodies govern different parts of a standby install, and naming the right one for the point is the difference between a credible record and a guess. The NEC, NFPA 70, sets which rules apply by the system type: Article 700 for emergency systems, Article 701 for legally required standby, and Article 702 for optional standby, with the transfer-time expectations, commonly around 10 seconds for emergency and 60 seconds for legally required, riding on those articles. The grounding of a separately derived generator follows the NEC grounding rules, with the system bonding jumper and the grounding electrode connection commonly cited at 250.30. Confirm the exact section and the time against the adopted edition and the AHJ, because the wording shifts between cycles.

NFPA 110, the standard for emergency and standby power systems, classifies the system by Type, Class, and Level, sets the minimum on-site fuel run time by Class, and governs the installation acceptance test, the remote annunciation, the load priority, and the ongoing exercise the owner inherits. The transfer switch itself is listed to UL 1008, which sets the safety and performance and the withstand and closing ratings on the switch label. NFPA 37 covers the stationary combustion engine, including the separation from structures and openings and the exhaust.

The engine emissions fall under the air-quality authority and often an air permit, and for field acceptance testing of the electrical gear, ANSI/NETA gives the inspection and test requirements before energization. Above all of these sit the manufacturer's instructions and the project specification, which set the actual numbers: the sizing, the clearances, the back-pressure limit, the ventilation, and the controller settings. When a standard and the spec disagree, the stricter controlling document wins, and the AHJ has the final say on what is enforceable.

Units and terms

The standby-power vocabulary reads differently across a spec, a submittal, and an engine data sheet, and the same idea carries a few names. Generator output is in kilowatts (kW) for real power and kilovolt-amperes (kVA) for apparent power, and the set has both a standby rating, for emergency duty a few hours at a time, and lower prime or continuous ratings for sustained running. Transfer time is in seconds, the count from loss of normal to acceptable power at the load.

The terms below are the ones that travel across the whole install. Read the manufacturer's data and the controller manual for how a given set names its ratings and its delays, because the labels differ by maker even when the function is the same, and confirm the NEC article and the NFPA 110 Class and Level against the adopted editions before citing a number.

EPSS

Emergency power supply system, the generator, the transfer equipment, and the controls taken together under NFPA 110

ATS

Automatic transfer switch, the device that moves the load between the utility and the generator

NEC 700 / 701 / 702

Emergency, legally required standby, and optional standby articles that set the transfer-time class and the wiring rules

Separately derived system

A source, such as a generator behind a 4-pole switched-neutral switch, that needs its own neutral-to-ground bond per NEC 250.30

3-pole / 4-pole

Solid neutral switched through, or neutral switched with the phases, which sets the generator grounding

Standby rating

The generator output rated for emergency-duty use during a utility outage, above the prime and continuous ratings

Step load

A block of load brought onto the set at once, the moment that motor inrush makes a generator sag

Load shed

Dropping the lowest-priority load when the set runs short, so the emergency load is the last affected

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