Electrical
Parallel conductors and NEC 310.10(G) for electrical crews
Run two or more conductors per phase as one circuit, keep every conductor identical so they share the current, and put a full-size ground in each raceway.
Direct answer
Paralleling conductors means running two or more conductors per phase as one circuit, joined at both ends, to carry current a single conductor cannot. The NEC permits it only in sizes 1/0 AWG and larger, and every paralleled conductor of a phase must be identical in length, material, size, and insulation. The adopted code edition controls.
Key takeaways
- NEC 310.10(G) (2023, formerly 310.10(H)) permits paralleling conductors only in sizes 1/0 AWG and larger.
- Every paralleled conductor of a phase must be identical in length, material, size (circular-mil area), insulation, and termination.
- Install a full-size equipment grounding conductor in each raceway sized to the OCPD, never divided across raceways, per 250.122(F).
- Each separate raceway must carry one conductor of each phase plus neutral and ground so magnetic fields cancel and steel does not overheat (300.20(A)).
- Figure ampacity per conductor with ambient and conductors-per-raceway derating, then sum the sets; do not add raw table values.
Paralleling conductors, and why you do it
Paralleling conductors means running two or more conductors per phase, joined together at both ends, so they act as one large conductor. A 2000 A service does not run on a single wire, because no single conductor the trade stocks can carry that current. So you split the phase across several conductors, land them on the same lug at each end, and the load shares across the set. The companion ampacity and conductor-type guides cover sizing and insulation for a single conductor. This one is about what changes when you run several of them as one.
Two reasons drive the decision. The first is necessity. Above a certain current there is no single conductor big enough, so paralleling is the only way to carry the load in conductors at all. The second is handling. A single 600 kcmil conductor is heavy, stiff, and fights you in a bend. Two smaller conductors carrying the same current pull easier, bend tighter, and land on lugs a crew can actually torque.
The rules around paralleling exist because the trick only works if the conductors truly share the current. Make them unequal and one carries more than its share, runs hot, and the set you sized for 2000 A is not carrying 2000 A safely. Most of this guide is about keeping the sharing equal.
Why run conductors in parallel?
You parallel conductors when the load current is larger than any single conductor can carry, or large enough that a single conductor becomes impractical to handle. Big services and feeders are the obvious case. A 1200 A, 2000 A, or 4000 A service is far past the ampacity of the largest building wire on the shelf, so the current gets split across two, three, four, or more conductors per phase.
The math is plain. The largest common conductor tops out at a few hundred amps in the usual termination column, so a 2000 A feeder needs the current divided into sets that each stay within a conductor's ampacity. Run several sets of a large conductor and each one carries a manageable share.
The benefits stack up beyond raw capacity. Smaller conductors cost less per foot of capacity in many cases, bend with a smaller radius so they fit the gutter and the gear, and weigh little enough that two electricians can pull and land them. On a long feeder the parallel set also lowers the effective resistance, which helps the voltage drop. The decision is rarely only about ampacity. It is ampacity, handling, and cost together.
What is the minimum size to parallel wire?
The minimum size you may parallel is 1/0 AWG. The NEC permits conductors to be run in parallel only in sizes 1/0 and larger, in copper, aluminum, or copper-clad aluminum. The rule is cited at 310.10(G) in the 2023 NEC, renumbered from the former 310.10(H), and recent code cycles have moved the parallel-conductor provisions, so confirm the citation against the edition the jurisdiction has adopted.
Below 1/0, you do not parallel power conductors. A pair of #2 conductors might look like it carries the same current as one 4/0, but the code does not allow it for general power. The smaller the conductor, the harder it is to guarantee equal sharing, and the easier it is for a slight difference in length or termination to throw the balance off.
There are narrow exceptions. The code allows conductors smaller than 1/0 in parallel for limited purposes such as control power to instruments, relays, and contactors, for higher-frequency circuits at 360 Hz and above, and certain other listed applications. Those are special cases with their own conditions. For ordinary feeders and services the threshold is firm: 1/0 and up, nothing smaller.
The identical-conductor rule
This is the rule that makes paralleling work, and the one that gets violated most. Every conductor making up one phase of a parallel set must be identical to the others in that phase. Same length. Same conductor material, all copper or all aluminum, never mixed. Same size, the same circular-mil area. Same insulation type. And terminated the same way at both ends.
The requirement applies per phase, not across phases. Phase A conductors must all match each other, phase B conductors must all match each other, and so on. Phase A does not have to match phase C, though in practice you pull them all the same. The neutral, where it is paralleled, follows the same identical rule within itself.
Where the sets run in separate raceways, the raceways themselves have to share the same physical characteristics and carry the same number of conductors, so no one raceway presents a different impedance to its set. Get any one of these wrong and the conductors stop sharing the current evenly. The whole point of paralleling is equal sharing. The identical rule is how you get it. Treat it as the first check, not the last.
| Property | Must match across the phase | Why |
|---|---|---|
| Length | Same routed length on every conductor | Unequal length means unequal impedance and unequal current |
| Material | All copper or all aluminum | Different metals have different resistance per foot |
| Size | Same circular-mil area (AWG or kcmil) | A smaller conductor has higher impedance and runs hotter |
| Insulation | Same type and temperature rating | Sets the ampacity column and the run conditions |
| Termination | Landed the same way at both ends | Different terminations add different contact resistance |
Why do parallel conductors have to be identical?
Because current follows the path of least impedance, and if the conductors are not identical, one path has less impedance than the others and hogs more than its share of the current. The conductor that hogs the current runs hotter than the set was designed for, while the others loaf along under-loaded. You sized the set for the total split evenly. It is not split evenly.
Length is the input that bites most often in the field. A conductor run two feet shorter than its mates has slightly lower impedance, so it carries slightly more current. On a heavily loaded service that small imbalance is enough to push one conductor past its rating while the panel ammeter still reads a normal total. The overload hides inside the set.
The failure shows up as heat at the short or undersized conductor and its terminations. You smell it or see the discoloration on the insulation at the lug before a breaker ever trips, because the breaker sees the total current, which is fine, not the one conductor that is overloaded. That is the danger of unequal paralleling. The protection cannot see the problem. The only defense is making the conductors identical so the current has no reason to favor one over another.
The set, the phases, and the neutral
A parallel set is one complete circuit's worth of conductors: one conductor of each phase, plus the neutral where the circuit has one, plus the equipment grounding conductor. A 2000 A three-phase feeder run in six parallel might be built as six sets, each holding one A, one B, one C, a neutral, and a ground.
Think in sets, not in loose conductors, because the set is the unit that has to stay balanced and grouped. The total ampacity of the feeder is the sum of what each set's phase conductor carries. Divide the design current by the number of sets to get the per-conductor current, and that per-conductor current is what each phase conductor in every set has to handle.
Counting sets keeps the install honest. If the feeder is six sets, you pull six conductors per phase, six neutrals where the neutral is paralleled at full count, and a full-size ground in each raceway. Lose track and you end up with five conductors on one phase and six on another, which is the identical rule broken before the first conductor is even landed. Lay the sets out on paper first, then pull to the count.
How do you figure the ampacity of parallel conductors?
The total ampacity is the sum of the per-conductor ampacities, but each conductor's ampacity is figured with the same derating any conductor gets, and that derating is what catches people. Start with one conductor's ampacity from the table for its size, insulation column, and the termination, then apply the ambient correction and the more-than-three-conductors adjustment for how many current-carrying conductors share the raceway. Multiply that corrected per-conductor ampacity by the number of sets.
The companion ampacity and derating guide carries the correction and adjustment math in full. The trap in a parallel run is the conductor count. Put all the sets in one big raceway and you can easily have a dozen or more current-carrying conductors bundled together, which pushes the adjustment factor down hard, so each conductor carries less than its table value and you may need more or larger conductors than the simple division suggested.
Run each set in its own raceway and the count per raceway drops, often to three current-carrying conductors plus the neutral and ground, so the adjustment is lighter. This is one reason separate raceways are common on large parallels. Figure the per-conductor ampacity with the real raceway fill, then sum. Do not sum table values and skip the derating.
| Step | Value |
|---|---|
| Feeder design current | 2000 A, three-phase |
| Parallel sets per phase | 6 |
| Per-conductor current (2000 / 6) | about 334 A each |
| Conductor (example) | 600 kcmil Cu, 75C column |
| Derating applied | Ambient and conductors-per-raceway per the tables |
| Total ampacity | Sum of the 6 derated per-conductor ampacities |
Separate raceways or one raceway
You can run parallel conductors two ways: each set in its own raceway, or all the sets together in one large raceway. Both are allowed. The choice drives the derating, the heating, and the labor.
Separate raceways are the common arrangement on large feeders. Each raceway carries one full set, one conductor of each phase plus the neutral and ground, so the conductor count per raceway stays low and the adjustment factor stays light. The raceways have to be identical to each other so every set sees the same impedance: same size, same length, same routing, same material.
All sets in one raceway happens on smaller parallels or where the layout forces it. It works, but the conductor count climbs fast, the adjustment factor drops, and you give up ampacity per conductor to crowding. There is also a heating rule in play. When sets share a raceway, the conductors of each phase still have to be arranged so the magnetic fields balance. Most large jobs land on separate identical raceways because it keeps the derating sane and the phase balance simple. Pick the arrangement before you size the conductor, because the arrangement changes the ampacity.
One conductor of each phase per raceway
When you run sets in separate raceways, each raceway has to carry one conductor of each phase, plus the neutral and the equipment grounding conductor for that set. You do not put all the phase A conductors in one raceway and all the phase B conductors in another. That is the mistake that cooks a steel conduit.
The reason is magnetic. The current in a phase conductor sets up a magnetic field, and the return currents in the other phases and the neutral set up opposing fields. Group one of each phase together and the fields nearly cancel, so almost no net field reaches the surrounding raceway. Put one phase alone in a raceway and there is nothing to cancel its field, so the field links the raceway wall and drives currents in it.
In a nonmagnetic raceway like PVC or aluminum that is mostly a non-issue. In a ferrous raceway, steel conduit, the unbalanced field induces heating in the steel and adds impedance to that conductor, which then throws off the sharing too. So the rule is one of each phase in every raceway, neutral and ground included, every set the same. Keep the set together and the physics takes care of itself.
Induced heating in a steel raceway
This is the specific hazard behind the phase-balance rule, and it is worth its own line because it surprises people. When alternating-current conductors run through a ferrous metal enclosure or raceway, they have to be arranged so they do not heat the surrounding steel by induction. The provision is commonly cited at 300.20(A).
Steel is magnetic, so an unbalanced magnetic field passing through it drives circulating eddy currents in the metal, and those currents make heat in the steel itself. Run a single phase conductor through a steel plate or a steel conduit with no balancing return nearby and you can heat the steel enough to feel it, sometimes enough to damage the conductor insulation against it. The same thing happens when you split a parallel phase off by itself into its own steel raceway.
The fixes are the ones the trade already uses. Keep all the conductors of a set, all phases plus neutral plus ground, together in the same raceway so the fields cancel. Where a single conductor must pass through a steel wall, cut a slot between the knockouts or use a nonmagnetic plate so the field has no closed steel loop to circulate in. The safe default for parallels is simple. Never let one phase travel alone in steel.
How do you size the ground for parallel conductors?
Where parallel conductors run in separate raceways, you put a full-size equipment grounding conductor in each raceway, and each one is sized to the circuit's overcurrent device, not divided among the raceways. This is the parallel-EGC rule, commonly cited at 250.122(F), and it is one of the most common errors on a parallel install.
The logic trips people because the phase conductors are divided and the ground is not. The phase current splits across the sets, so each phase conductor carries a share. But a ground fault can return on the EGC in any one raceway, so each EGC has to be sized to clear the full fault current the breaker will let through. You size each ground from the table for the feeder's overcurrent device rating, then put that full-size ground in every raceway. A 1200 A feeder in four raceways gets four grounds, each sized for 1200 A, not four grounds each sized for a quarter of it.
There is a refinement worth knowing. Recent code language clarifies that the EGC in any one raceway need not be larger than the largest ungrounded conductor in that same raceway. Verify the exact wording against the adopted edition. The rule that does not change: full-size ground in each raceway, sized to the OCPD, never divided.
Paralleling the neutral
The neutral, the grounded conductor, gets paralleled by the same rules as the phases when the circuit needs one. Each set carries its own neutral conductor, and all the neutral conductors of the circuit have to be identical to each other: same length, material, size, and insulation, terminated the same way. The neutral is sized for the circuit's neutral load, not automatically full size, but whatever size it is, the paralleled neutrals must match.
The neutral also counts in the balancing. On a wye system the neutral carries the unbalanced current and the harmonic content, so it belongs grouped with the phase conductors of its set in the same raceway, both for sharing and for the magnetic cancellation that keeps a steel raceway cool. Splitting the neutral off by itself breaks the same physics as splitting a phase.
One field note. On systems with heavy nonlinear load, the neutral can run as hot as the phases or hotter, so do not shortchange the paralleled neutral size on a hunch. Size it for the real neutral current, then keep every paralleled neutral identical. The identical rule is not just a phase-conductor rule. It applies to the neutral set too.
Terminations, lugs, and the gutter
Parallel conductors all have to land somewhere that accepts multiple conductors per phase, and the termination is where unequal sharing is created or avoided. Gear lugs come rated for a number of conductors per phase and a conductor range. Mechanical set-screw lugs and compression lugs both show up. Whatever the type, every conductor of a phase has to land the same way, on the same kind of lug, torqued to the same value, so no one conductor sees more contact resistance than its mates.
Torque is not optional here. An under-torqued lug on one conductor of a set adds contact resistance to that path, which shifts current to the others and makes heat right at the loose lug. Use a calibrated torque tool and the value stamped on the lug or in the manufacturer's instructions, and mark each one as you go.
Where the gear does not have enough lugs, the parallels land in a wireway, gutter, or junction box on a set of parallel lugs or a bus, then a single conductor or bus runs into the gear. That gutter is a common place for parallels to gather, tap, and transition. Keep the conductors of each phase grouped and equal length right through the gutter, because the identical rule does not stop at the box wall.
Matching length in the field
Equal length is the identical-rule input you actually have to fight for on a real building, because conduit does not run in straight equal lines. The conductor that takes the shorter raceway, the inside of the bend, or the more direct pull comes up shorter, and shorter means lower impedance and more current. You match length by routing the sets the same way and pulling them to the same length, not by hoping the runs come out even.
The practical method is to make every raceway for the parallel sets the same length and the same configuration, same number of bends, same routing, so the conductors in them come out matched. Where one raceway has to be a little longer, you can cut all the conductors of that circuit to the longest run's length and coil the slack equally, so every conductor is the same length even if the raceways are not exactly. The slack lives in the gutter or the gear, looped, not crammed.
What you do not do is cut each conductor tight to its own raceway and call a two-foot difference close enough. On a lightly loaded feeder it may never matter. On one running near its rating it is the difference between balanced sharing and one conductor cooking. Match the length.
Where parallels show up: services and feeders
Parallel conductors live on the big stuff: services and large feeders. The service entrance to a 2000 A or 4000 A switchboard, the feeder from a main switchboard to a distribution panel or a large piece of equipment, the tie between sections of gear. Anywhere the current is past what a single conductor carries, you find parallels landing on the gear lugs.
Switchgear and switchboards are built for it. The main lugs come rated for several conductors per phase, and the gear's wiring space and gutter are sized so the parallels can fan in, bend, and land without violating bending space or fill. Read the gear's lug rating and conductor range before you size the parallel, because the gear, not just the table, sets how many conductors per phase you can land and what sizes it accepts.
The feeder rules still apply on top of the parallel rules. The conductors are sized for the load and the ampacity, the overcurrent device protects them, and the voltage drop gets checked over the run. Paralleling is the method for carrying the current. It does not change the feeder math, it just splits the conductor that carries the result. Size the feeder, then decide how many sets carry it.
Parallel sets and voltage drop
Paralleling lowers the effective resistance of the run, because two equal conductors in parallel have half the resistance of one, so the voltage drop over a long feeder comes down as you add sets. On a long service or a feeder to distant gear, that drop reduction is sometimes a reason to parallel beyond what bare ampacity demands, the same way you would upsize a single conductor to hold the drop.
The companion voltage-drop guide carries the formula and the targets. For a parallel run, figure the drop on one conductor at the per-conductor current, the design current divided by the number of sets, over the routed one-way length. Because the current per conductor is the total divided across the sets, the drop comes out the same as treating the whole set as one larger conductor, which is the point.
Watch the same trap as ampacity. Use the per-conductor current, not the full feeder current, against a single conductor's resistance. And remember the identical rule feeds back here. If the conductors are not equal length, they do not share equally, and the neat parallel-resistance math stops describing what the run actually does. Equal conductors, equal sharing, predictable drop.
Conduit fill and sizing the raceway
Each raceway in a parallel run gets sized for the conductors it actually holds, by the fill rules, the same as any raceway. With one set per raceway, that is one conductor of each phase, the neutral, and the ground, and the conduit is sized so those conductors do not exceed the allowable fill percentage. Run all the sets in one raceway and the fill climbs fast, which is part of why big parallels spread across several conduits.
The fill math comes from the conductor areas and the conduit dimensions in the code's Chapter 9 tables, with the percent-fill limits for the number of conductors, and the prebuilt combinations in the annex tables save time. The companion conductor-type guide covers how insulation choice changes the conductor's cross-section and therefore the fill. Larger-insulation conductors eat conduit faster.
Two field points. Keep the raceways for the sets identical in size, because a smaller conduit on one set can change the heat and the handling even if the conductor is the same. And size the conduit for the pull, not just the static fill, because a parallel set of large stiff conductors needs room to be pulled and trained into the gutter without skinning the insulation. The fill table is the floor, not always the right size for the pull.
Busway as an alternative to many sets
On the largest feeders, busway is the alternative to running many parallel sets of conductors, and on a high-amp run it is often the better one. Busway is a prefabricated run of busbars in an enclosure, sold in standard lengths and fittings, rated up to several thousand amps. Instead of pulling six or eight conductors per phase through conduit, you bolt up a run of busway.
The trade-offs are real. Busway costs more per foot up front and wants a planned, mostly straight route with proper supports. But it installs faster on a long high-amp run, takes far less labor than pulling and landing dozens of large conductors, handles heat predictably, and makes tap-offs along its length easy where parallel conductors would force a junction. For a 3000 A or 4000 A riser feeding a stack of panels, busway often wins on installed cost and serviceability.
Parallel conductors win where the route is short, twisty, or one-off, where the gear is already lugged for conductors, and on smaller parallels where busway is overkill. The decision is a real estimating question on big jobs. Count the conductors, the conduit, and the labor for the parallel run against the busway material and supports, and price both. It is not automatic either way.
Large-feeder parallels on data center and industrial jobs
Data center and large industrial work is where parallels and busway both show up heaviest, because the services and feeders are enormous and the loads are dense. A data hall's power distribution can run thousands of amps to the row, and the designers choose between many parallel cable sets and busway for the big feeders and the distribution to the racks.
The pattern on these jobs is usually busway for the high-amp distribution risers and the overhead runs to the rows, with parallel cable sets where flexibility, routing, or existing gear favors conductors. Cable parallels give you routing freedom and lower first cost on a given run. Busway gives you fast installation, clean tap-offs at each rack row, and predictable heat at very high current. The choice is made feeder by feeder, not once for the building.
Whatever the method, the parallel rules do not relax because the job is big. The conductors still have to be identical, the grounds still go full-size in each raceway, and the sets still stay balanced and grouped. If anything the discipline matters more here, because a service running near its rating around the clock punishes an unequal set faster than an intermittent one ever would. Continuous load finds the weak conductor.
What does the inspector check on parallel conductors?
The inspector checks that the conductors are actually identical and that the grounds and the balance are right, because those are the things that fail quietly. Expect them to look at conductor size and material across every conductor of a phase, at the count of conductors per phase, and at the equipment grounding conductor in each raceway.
The checklist in their head runs short and specific. Are all the conductors of each phase the same size, material, and insulation. Is every conductor 1/0 or larger. Is there a full-size ground in each raceway, sized to the overcurrent device, not divided. Does each raceway carry one conductor of each phase so the set is balanced and no phase travels alone in steel. Are the terminations the same type and torqued, with the torque marked or witnessed.
Length is the hard one to verify after the fact, because the conductors are landed and the slack is coiled, so document it as you pull. The inspector who finds five conductors on one phase and six on another, or a ground sized for the per-conductor current instead of the feeder, will reject the work, and they should. Make the install pass its own checklist before you call for the inspection.
What to document
Write down what proves the set is balanced and the grounds are right, because once the conductors are landed and the slack is coiled, nobody can see the length match or the per-raceway count without taking it apart. The record is what answers the inspector and the next electrician.
Capture the conductor material, size, and insulation, the number of sets and conductors per phase, the length each set was cut to, the raceway arrangement and that each carries one of each phase, the equipment grounding conductor size in each raceway and the overcurrent device it was sized to, the neutral size where paralleled, and the termination type and torque value. If you coiled slack to match length, note it, so the next person knows the loop in the gutter is intentional.
| Requirement | Rule | Note |
|---|---|---|
| Minimum size | 1/0 AWG or larger | Commonly 310.10(G); confirm the adopted edition |
| Identical conductors | Same length, material, size, insulation, termination | Per phase; the first thing that fails |
| Conductors per raceway | One of each phase, plus neutral and ground | Keeps the set balanced and the steel cool |
| Parallel EGC | Full size in each raceway, sized to the OCPD | Commonly 250.122(F); never divided |
| Ampacity | Per-conductor value with derating, then summed | Count the conductors in the raceway |
| Terminations | Same type, torqued to spec, on all conductors | A loose lug shifts current and makes heat |
Common mistakes
- Paralleling conductors smaller than 1/0 AWG for a power feeder, outside the narrow code exceptions.
- Mixing non-identical conductors in a phase, different length, material, size, or insulation, so they share current unevenly and one overheats.
- Sizing the equipment grounding conductor for the per-conductor current and dividing it among raceways, instead of full size in each raceway to the overcurrent device.
- Splitting one phase alone into its own ferrous raceway, inducing heat in the steel and unbalancing the set.
- Cutting each conductor tight to its own raceway and accepting a length mismatch that throws off the sharing.
- Landing parallels on mismatched terminations or skipping the torque, adding contact resistance to one path.
- Summing the table ampacities and skipping the conductors-per-raceway derating, so the set is undersized for its crowding.
- Losing the set count and pulling a different number of conductors on different phases.
Field checklist
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Standards and references
The NEC, NFPA 70, is the framework. Paralleling itself is permitted and conditioned in the parallel-conductor provisions, cited at 310.10(G) in the 2023 NEC, formerly 310.10(H), which set the 1/0 minimum and the identical-conductor requirements: same length, material, circular-mil area, insulation, and termination. Recent code cycles have renumbered parts of Article 310, so confirm the exact citation against the adopted edition.
The equipment grounding conductor in parallel comes from 250.122, with the parallel-specific rule commonly at 250.122(F): a full-size ground in each raceway, sized to the overcurrent device, not divided, with recent language clarifying it need not exceed the largest ungrounded conductor in that raceway. Induced heating in ferrous raceways is addressed at 300.20(A), the basis for keeping a set's conductors grouped. Conductor ampacity and the derating that figures each conductor's share come from the ampacity tables and the correction and adjustment factors in 310.15. Conduit fill comes from the Chapter 9 tables and the annex combinations. Busway is Article 368.
Section numbers move between editions and jurisdictions amend them, so verify every citation against the code the AHJ has actually adopted before you put it on a submittal. The manufacturer's instructions and the gear's lug listing govern the terminations and the conductors-per-phase the equipment accepts, and where they are stricter, they control.
Units and terms
Parallel work carries its own vocabulary, and the same idea reads differently across a one-line, a gear submittal, and the spec.
Conductor size is AWG for smaller conductors and kcmil, thousands of circular mils, for the larger ones that usually get paralleled, while metric drawings use mm squared. A set is one conductor of each phase plus neutral and ground; conductors per phase is how many sets you are running. The EGC is the equipment grounding conductor, the OCPD is the overcurrent protective device, and the AHJ is the authority having jurisdiction that adopts and enforces the code.
- Parallel set
- One conductor of each phase, plus neutral and equipment ground, run together as one circuit's worth
- Conductors per phase
- How many conductors share one phase, equal to the number of parallel sets
- 1/0 AWG
- The minimum conductor size the NEC permits to be paralleled for power, with narrow exceptions
- Circular-mil area
- The conductor's cross-sectional area; paralleled conductors of a phase must match it
- EGC
- Equipment grounding conductor, run full size in each parallel raceway, sized to the OCPD
- Ferrous raceway
- Steel raceway or enclosure, where an unbalanced field induces heating, addressed at 300.20(A)
- Busway
- Prefabricated busbar run, the common alternative to many parallel sets on high-amp feeders
FAQ
What does it mean to parallel conductors?
Paralleling conductors means running two or more conductors per phase, joined at both ends, so they act as one larger conductor and share the load current. You do it when a single conductor cannot carry the current of a big feeder or service, or to ease handling, bending, and cost on a heavy run.
What is the minimum size to parallel wire?
The minimum size you may parallel for power is 1/0 AWG, in copper, aluminum, or copper-clad aluminum, commonly under NEC 310.10(G). Conductors smaller than 1/0 are not paralleled for general feeders. Narrow exceptions exist for control power, certain higher-frequency circuits, and similar uses, so verify the adopted code edition.
Do parallel conductors have to be the same length?
Yes. Every conductor of a phase in a parallel set must be the same length, along with the same material, size, and insulation. Unequal length means unequal impedance, so the shorter conductor carries more than its share and runs hot while the others loaf. Route the raceways alike and cut the conductors to matched length.
How do you size the ground for parallel conductors?
Where parallel conductors run in separate raceways, install a full-size equipment grounding conductor in each raceway, sized to the circuit's overcurrent device, commonly per NEC 250.122(F). You do not divide one ground across the raceways. A fault can return on any raceway's ground, so each must clear the full fault current.
Can you mix copper and aluminum in a parallel set?
No. All conductors of one phase must be the same material, so you cannot mix copper and aluminum in a parallel phase. The two metals have different resistance per foot, so a mixed set shares current unevenly and one conductor overheats. Run all copper or all aluminum, identical in size and length, across the phase.
How do you figure the ampacity of parallel conductors?
Figure each conductor's ampacity from the table for its size and insulation, apply the ambient and conductors-per-raceway derating, then add up the per-conductor ampacities across the sets. The derating is the catch: crowding many conductors in one raceway cuts each one's ampacity, so the total is less than the table values summed.
Why do parallel conductors have to be identical?
Because current follows the path of least impedance. If one conductor is shorter, smaller, or a different metal, it has lower impedance and carries more than its share, running hot while the others loaf. The overload hides inside the set, since the breaker only sees the balanced total. Identical conductors keep the sharing equal.
Should each raceway carry one conductor of each phase?
Yes. In a separate-raceway parallel, each raceway carries one conductor of each phase plus the neutral and ground, so the magnetic fields nearly cancel. Put one phase alone in a steel raceway and its field induces heating in the steel and unbalances the set. Keep the whole set grouped in every raceway.
When do you use busway instead of parallel conductors?
Busway is the common alternative on the largest feeders, often a 3000 A or 4000 A riser, where pulling six or eight conductors per phase gets costly. Busway installs faster, handles heat predictably, and makes tap-offs easy. Parallel cable wins on short, twisty, or one-off routes and where the gear is already lugged for conductors.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.