Electrical
Conductor ampacity and NEC derating for electrical crews
Start at the 90C ampacity, correct for ambient heat, adjust for bundling, then cap the result at the lowest-rated termination before you trust the number.
Direct answer
Conductor ampacity is the current a conductor carries continuously without exceeding its insulation temperature rating. The NEC base value in Table 310.16 assumes one set of conditions, so you correct it for ambient heat and adjust it for bundling, then cap the result at the lowest-rated termination. The adopted code edition and the AHJ control.
Key takeaways
- NEC Table 310.16 base ampacities assume 30C (86F) ambient and not more than three current-carrying conductors, in 60/75/90C columns.
- Derated ampacity = base 90C value x ambient correction factor (Table 310.15(B)(1)) x conductor-count adjustment (310.15(C)(1)).
- NEC 110.14(C) caps final ampacity at the lowest termination column, usually 75C; the 90C column is for the heat math only.
- Conductor-count adjustment: 4-6 conductors at 80%, 7-9 at 70%, 10-20 at 50%; the EGC never counts, harmonic neutrals do (310.15(E)).
- Worked example: 175A load, 6 conductors, 40C ambient forced 3/0 copper (163.8A) up to 4/0 (189.3A) to carry the load.
Conductor ampacity, and why you derate it
Conductor ampacity is the current a wire can carry continuously without driving its insulation past the temperature it is rated for. Push more current than that and the copper or aluminum runs hotter, the insulation cooks, and over time it goes brittle, cracks, and faults. The ampacity is not a property of the metal alone. It is the current at which the heat the conductor makes balances the heat it can shed to its surroundings.
That balance is the whole game, and it is why a single table value is never the final answer. The base ampacity in the NEC table is good for one set of conditions: a stated ambient, a stated number of conductors, a stated installation. Change the conditions and the conductor sheds heat differently. Two things wreck the balance on real jobs. Heat from the space around the conductor, the ambient, leaves less room to dump the conductor's own heat. And crowding, more than three current-carrying conductors bundled in a raceway or cable, means each one is trying to shed heat into neighbors that are also hot.
So you do two separate corrections. You correct for ambient temperature, and you adjust for the conductor count. Skip either and the conductor runs hotter than the table promised, the insulation ages early, and the failure shows up years later as a fault nobody can explain. The derating is not paperwork. It is the difference between a conductor that lives out its service life and one that fails in the wall.
The base ampacity table, NEC 310.16
The starting numbers come from NEC Table 310.16, the allowable ampacity table for insulated conductors rated up to 2000 V installed in a raceway, cable, or directly buried, based on an ambient of 30 degrees C (86 degrees F) and not more than three current-carrying conductors. Find the conductor size down the side, the metal, and the temperature column across the top, and the cell is the base ampacity before any conditions of use.
Three columns run across the table, one for each insulation temperature rating: 60, 75, and 90 degrees C. The column you read depends on the insulation type. A 90 degree C insulation like THHN or XHHW-2 sits in the 90 column. The same size in a 60 degree C insulation reads lower. The higher-temperature insulation carries more current in the same size because it tolerates more heat before it degrades.
Copper and aluminum get separate columns, because aluminum has higher resistance and lower ampacity for the same size. A 3/0 copper conductor reads 200 A in the 75 degree C column. The aluminum 3/0 reads less, which is why aluminum feeders usually run a size or two larger than the copper they replace. Pull the value for the metal on the reel, not the metal you are used to.
Note this is the table for the common case. Other installations, free air, direct burial, conductors over 2000 V, have their own tables and rules, and the conditions baked into 310.16 do not carry over to them.
What is the 75C termination rule?
The 75 degree C termination rule comes from NEC 110.14(C): the temperature rating used for a conductor's ampacity cannot exceed the lowest temperature rating of any termination, device, or conductor in the circuit. The breaker lugs, the equipment terminals, and the splices all have a temperature rating, and the lowest one caps what you are allowed to use, no matter how good the wire is.
Here is the rule that trips people. You can buy 90 degree C wire all day, but most equipment terminations are listed for 75 degrees C, and a good number of smaller terminations are 60 degrees C. So even with 90 degree C THHN, the final ampacity is read from the column that matches the termination, usually the 75 column. The 90 column is for the derating math only. The termination caps the answer.
As a practical split, 110.14(C) commonly works out to the 60 degree C column for circuits rated 100 A or less unless the terminations are listed for 75, and the 75 degree C column for circuits over 100 A, but the listing on the actual equipment is what controls. Read the rating stamped on the breaker, the lug, or in the equipment instructions, and confirm it against the adopted code edition. Using the 90 column at the termination is one of the most common ampacity errors there is, and it puts more current on the lug than it was built to handle.
The reason is heat at the connection. A termination rated 75 degrees C is not built to sit at the 90 degree C wire's temperature. Run the conductor at its 90 degree C ampacity into a 75 degree C lug and the lug runs hotter than its listing, which is exactly where loosening and failures concentrate.
Ambient temperature correction
The base table assumes 30 degrees C (86 degrees F) around the conductor. When the ambient is hotter, the conductor sheds less of its own heat, so its ampacity drops, and you multiply the base value by a correction factor from NEC Table 310.15(B)(1). The hotter the space, the smaller the factor. In a cool space below 30 degrees C the factor is above 1.0 and the ampacity goes up.
The factors run by column, because a 90 degree C insulation tolerates a hot ambient better than a 60 degree C one. Working in the 90 degree C column, a 36 to 40 degree C ambient (about 97 to 104 degrees F) commonly carries a 0.91 factor, and a 46 to 50 degree C ambient drops to about 0.82. The exact bands and values are in the table for the adopted edition, so read them there rather than from memory.
Ambient is the temperature the conductor actually lives in, not the comfortable 70 degrees the office sees. An attic, a boiler room, a south-facing wall cavity, or a raceway in the sun all run far hotter than the design ambient. Pick the real ambient for the worst point along the run, because the conductor is only as derated as its hottest stretch.
| Ambient (90C column) | Correction factor (approx.) |
|---|---|
| 21 to 25C (70 to 77F) | 1.04 |
| 26 to 30C (79 to 86F) | 1.00 |
| 31 to 35C (88 to 95F) | 0.96 |
| 36 to 40C (97 to 104F) | 0.91 |
| 41 to 45C (106 to 113F) | 0.87 |
| 46 to 50C (115 to 122F) | 0.82 |
Adjustment for more than three current-carrying conductors
Table 310.16 assumes not more than three current-carrying conductors in the raceway or cable. Put more in and they heat each other, so NEC 310.15(C)(1) makes you multiply the ampacity by an adjustment factor based on the count. This is separate from the ambient correction, and you apply both.
The factors step down as the count climbs: 4 to 6 conductors at 80 percent, 7 to 9 at 70 percent, 10 to 20 at 50 percent, 21 to 30 at 45 percent, 31 to 40 at 40 percent, and 41 and above at 35 percent. The jump from three to four conductors is the one people forget, because adding a single circuit to a pipe that already had three can drop every conductor in it to 80 percent.
Count current-carrying conductors, not total conductors. The equipment grounding conductor does not count, because it carries current only during a fault, not in normal operation. The grounded neutral usually does not count either, with the harmonic exception covered below. So a 208Y/120 V three-phase, four-wire feeder with a linear load counts as three current-carrying conductors and stays out of the adjustment. The same feeder on a heavy nonlinear load counts the neutral too, which can be the fourth conductor that triggers the 80 percent factor.
There are conditions that exempt a bundle from the adjustment, like short nipples and certain spacing, so check the exceptions in the section for the adopted edition before you assume a packed pipe is fine.
| Current-carrying conductors | Adjustment factor |
|---|---|
| 1 to 3 | 100 percent (no adjustment) |
| 4 to 6 | 80 percent |
| 7 to 9 | 70 percent |
| 10 to 20 | 50 percent |
| 21 to 30 | 45 percent |
| 31 to 40 | 40 percent |
| 41 and above | 35 percent |
How do you calculate derated ampacity?
Start at the base ampacity for the conductor, then multiply by the ambient correction factor and the conductor-count adjustment factor. The result is the derated ampacity for the conditions of use. Then compare it to the ampacity in the termination column and take the smaller of the two. That smaller number is what the conductor is actually good for.
The order matters less than the principle: the correction and the adjustment both multiply the base value, and the termination cap is a ceiling applied at the end, not another multiplier. You derate from the 90 degree C value, then you check that the answer does not exceed what the 75 degree C termination allows. Whichever is lower wins.
Run the conductor count and the ambient against the load. If the derated ampacity is below the load the conductor carries, the conductor is too small for the conditions and you go up a size, then rerun the math, because the larger conductor has a larger base ampacity to derate from.
Aderated = A90°C × Ftemp × FadjustAallowed = min(Aderated, Aterm)- A90C
- Base ampacity from NEC Table 310.16, read in the 90 degree C column for 90 degree C insulation
- Ftemp
- Ambient temperature correction factor from NEC Table 310.15(B)(1)
- Fadjust
- Adjustment factor for more than three current-carrying conductors, NEC 310.15(C)(1)
- Aterm
- Ampacity in the column matching the lowest-rated termination, usually 75 degrees C, per NEC 110.14(C)
Why you start the derating at the 90C column
Start the derating math at the 90 degree C ampacity even when the termination is only 75 degrees C. This looks backward at first, but 110.14(C) allows it: a conductor with insulation rated higher than the termination may use that higher rating for the correction and adjustment math, then the termination caps the final result. You get the benefit of the 90 degree C insulation for the heat math, and you give it back at the termination.
Why it helps. The 90 degree C value is larger, so after you multiply by the correction and adjustment factors, the derated number is larger than it would be if you had started at 75. On a hot, crowded run that extra headroom can be the difference between a conductor that passes and one that forces an upsize. The insulation can take the heat. The termination cannot, so the cap brings the answer back to what the lug allows.
The mistake is to start at the 90 column and then forget the cap, walking away with the inflated derated value as if it were the final ampacity. The 90 column is the starting line for the heat math, never the finish line at the lug. Derate from 90, then cap at 75.
When does the neutral count as a current-carrying conductor?
The neutral counts as a current-carrying conductor when it carries harmonic current from nonlinear loads, under NEC 310.15(E). In a normal three-phase, four-wire wye system feeding balanced linear loads, the neutral carries only the imbalance and does not count toward the conductor-count adjustment. The phase conductors carry the load and cancel at the neutral, so the neutral runs cool.
Nonlinear loads break that. Switch-mode power supplies, LED drivers, variable-frequency drives, and electronic ballasts draw current in pulses, not smooth sine waves, and the third harmonic and its odd multiples do not cancel at the neutral. They add. The neutral on a panel full of computers or LED lighting can carry as much current as a phase, sometimes more, even when the phases look balanced. When the major portion of the load is nonlinear, 310.15(E) treats that neutral as a current-carrying conductor.
On the count, this is the conductor that pushes a four-wire feeder from three current-carrying conductors to four, which triggers the 80 percent adjustment. The equipment grounding conductor never counts, because it carries current only during a fault. So on a data center branch or an office lighting feeder full of electronics, count the neutral, and expect the bundle to derate even though a naive count said three.
Harmonics are their own subject, with their own measurement and mitigation. The point for ampacity is narrow: a heavily nonlinear load makes the neutral a heat source, so it counts.
Bundling drives both the conduit fill and the derating
The number of conductors in a raceway controls two separate code limits at once, and they pull in the same direction. Conduit fill caps how many conductors fit by cross-sectional area, to keep the pull clean and let heat escape. The conductor-count adjustment cuts the ampacity of each conductor for the heat the bundle traps. Add conductors and you push against both.
These are not the same check, and passing one does not pass the other. A raceway can be legal on fill and still need a heavy ampacity adjustment because the count crossed a threshold. A common move on a tight job is to split conductors across two raceways instead of stuffing one. That can keep each pipe at three current-carrying conductors and dodge the adjustment, and it eases the fill at the same time.
The fill rules, the conductor areas, and the percent-fill caps live in NEC Chapter 9 and its tables. The ampacity adjustment lives in 310.15(C). When you plan a raceway, run both: size the pipe for fill, then check whether the count derates the conductors enough to force a larger conductor or a second pipe.
Continuous loads and the 125 percent factor
A continuous load, one expected to run at its maximum for three hours or more, gets a separate factor that lives alongside derating, not inside it. The NEC requires the conductor and the overcurrent device for a continuous load to be sized for at least 125 percent of the continuous current. That 125 percent is about the breaker running warm for hours, and it stacks on top of the conditions-of-use derating, not in place of it.
Run both, in their own lanes. The conductor's ampacity, after ambient correction and conductor-count adjustment, has to carry the load. And the conductor and breaker have to meet the 125 percent continuous sizing. On a long continuous feeder in a hot, crowded raceway, both forces push the conductor larger, and the governing one is whichever lands you on the bigger size.
This is where the load calculation and the ampacity calculation meet. The load calc tells you the design current and whether it is continuous. The ampacity work tells you what a given conductor survives in the conditions it runs in. Size from both, because a conductor that satisfies the load calc on paper can still be undersized once the heat and the crowding take their cut.
Raceways in the sun on a rooftop
A raceway sitting in direct sun on a rooftop runs far hotter inside than the air temperature suggests, and the NEC handles it with a temperature adder applied before you read the correction factor. In recent editions the rule, found in 310.15(B), adds 33 degrees C (60 degrees F) to the outdoor ambient when the raceway or cable is exposed to sunlight on or above a roof and sits within about 7/8 in of the roof surface. You add that to the ambient, then apply the correction factor for the higher number.
The history matters because the section number and the detail have moved. Older editions, roughly 2008 through 2017, used a distance-based table with adders that shrank as the raceway sat higher off the roof, 33 degrees C close to the surface stepping down to smaller adders with more air gap. Newer editions simplified it to the single close-to-the-roof case. Confirm which version the jurisdiction has adopted before you cite a number, because this is exactly the kind of rule that changed between cycles.
The practical effect is brutal on a hot roof. A 40 degree C summer ambient plus the 33 degree C adder puts the conductor in a 73 degree C world, where the correction factor falls off a cliff and even 90 degree C insulation loses most of its margin. One common dodge is XHHW-2, which the rooftop adder has exempted, so a roof run in XHHW-2 escapes the adder where THHN would not. The other dodge is air: get the raceway up off the roof on standoffs so it is no longer in the close band.
Underground and duct bank ampacity
Underground conductors do not derate the way raceways in air do, because the soil, the burial depth, the spacing between ducts, and the heat the conductors dump into the ground all change the picture. The 310.16 table and its in-air adjustment factors are not the right tool for a loaded duct bank. The ampacity there is an engineered number.
The NEC gives ampacities for some underground arrangements directly, and for the general engineered case it points to Annex B, which is informational and based on the Neher-McGrath heat-transfer method. Annex B accounts for the things that actually drive underground ampacity: the thermal resistivity of the soil, the load factor over the day, the mutual heating between adjacent ducts, and the depth of burial. A tightly packed duct bank carrying many loaded circuits can derate hard, because the conductors in the middle of the bank cannot get their heat out.
On a real duct bank, the ampacity often comes from an engineering study rather than a table, and that study is part of the project documents. Use it. The in-air rules do not transfer underground, and the soil thermal resistivity the study assumed is a number the field can actually get wrong by backfilling with the wrong material.
Parallel conductors
Large feeders are often run as parallel sets, several conductors per phase, both to hit an ampacity a single conductor cannot and to make the pull manageable. The NEC permits paralleling at larger sizes, commonly 1/0 and up, and the sets have to match: same length, same metal, same insulation, same size, same terminations, so the current divides evenly between them.
Parallel runs interact with derating in a way that catches crews. When you split the parallel sets across separate raceways, each raceway carries its own count of current-carrying conductors and gets adjusted on that count. When you pull all the parallel conductors into one large raceway, the total count in that pipe drives the adjustment, and it climbs fast. Three phases in parallel, two conductors each, plus a neutral that counts, is a lot of current-carrying conductors in one pipe.
If the sets are not balanced, one carries more than its share, runs hotter, and is effectively beyond its derated ampacity while the others loaf. That is why the matching rules are strict. Unequal length is the usual culprit, and it is the one the field introduces by routing one set the long way around.
Field example: a 90C feeder in a hot, crowded raceway
Take a 175 A feeder pulled in copper THHN, 90 degree C insulation, landing on 75 degree C breaker and equipment terminations. The raceway carries six current-carrying conductors, and it runs through a space that sits at 40 degrees C. Start with 3/0 copper and run the math.
From Table 310.16, 3/0 copper reads 225 A in the 90 degree C column. The ambient correction for 40 degrees C in the 90 column is about 0.91. Six current-carrying conductors put the adjustment at 80 percent. So the derated ampacity is 225 times 0.91 times 0.80, which is about 163.8 A. Now the termination cap: the 75 degree C ampacity of 3/0 copper is 200 A, and the derated 163.8 A is the smaller of the two, so 163.8 A is what the 3/0 is good for. That is below the 175 A load. The 3/0 fails the conditions.
Go up to 4/0. From the table, 4/0 copper reads 260 A in the 90 column. Rerun: 260 times 0.91 times 0.80 is about 189.3 A. The 75 degree C termination ampacity of 4/0 is 230 A, so the derated 189.3 A still governs, and 189.3 A clears the 175 A load. The 4/0 works. The heat and the crowding, not the load alone, drove the conductor a full size larger than the ampacity table would suggest if you read it at face value.
| Step | 3/0 copper | 4/0 copper |
|---|---|---|
| Base ampacity (90C, Table 310.16) | 225 A | 260 A |
| Ambient correction (40C) | 0.91 | 0.91 |
| Adjustment (6 CCC) | 0.80 | 0.80 |
| Derated ampacity | 163.8 A | 189.3 A |
| Termination cap (75C) | 200 A | 230 A |
| Final allowable | 163.8 A | 189.3 A |
| Carries 175 A load? | No | Yes |
The breaker and the derated conductor ampacity
The overcurrent device protects the conductor, so its rating is tied to the conductor's ampacity after derating, not to the base table value. NEC 240.4 is the rule: the breaker or fuse generally cannot exceed the ampacity of the conductor it protects. Derate the conductor, get the real ampacity, and the protection follows that number.
There is a rounding allowance. Under 240.4(B), when the conductor ampacity does not land on a standard breaker size, you may go up to the next standard size, as long as the circuit is not a multi-receptacle branch for portable loads and the device is 800 A or less. So a conductor good for 189 A can take a 200 A breaker under that allowance. It is a small rounding step, not license to oversize.
Small conductors are the hard exception. NEC 240.4(D) fixes the maximum protection regardless of the table or any rounding: 15 A on 14 AWG copper, 20 A on 12 AWG copper, 30 A on 10 AWG copper. Those caps do not move, because undersized protection on small wire is one of the fastest paths to a fire. The blunt rule: protect the conductor to its derated ampacity, take the next-size-up allowance only where 240.4(B) permits, and never run past the 240.4(D) caps on small wire.
Sizing from load, derating, termination, and voltage drop
A conductor has to clear four tests, and the governing one is whichever forces the largest size. It has to carry the load, including the 125 percent factor if the load is continuous. It has to survive its conditions of use after ambient correction and conductor-count adjustment. It has to fit the termination temperature. And on a long run, it has to hold the voltage drop the equipment can tolerate.
These do not always point the same way, and the trap is solving one and forgetting the rest. A conductor sized for ampacity in a cool, uncrowded pipe can still be too small once you correct for a hot attic and adjust for a packed raceway. A conductor that clears all the heat math can still leave the equipment low on a 300 ft run, because voltage drop is a distance problem the ampacity tables do not see.
Run all four, then take the biggest conductor any of them demands. On short feeders, the ampacity and termination usually govern. On long feeders, voltage drop often governs and pushes the size past what the heat math alone would call for. Size for the worst case, because the conductor only has to fail one test to be the wrong conductor.
High-density and bundled feeders in data centers
Data center power is where derating stops being academic. Rows of cabinets pull through dense feeder runs to power distribution units, many circuits share raceways and cable trays, and the loads are almost entirely nonlinear, which means the neutrals carry harmonic current and count toward the conductor total. The ambient inside a hot aisle or a packed electrical room is well above the 30 degree C table base, and ASHRAE TC 9.9 thermal guidelines push the white space warmer to save cooling energy, which raises the conductor ambient at the same time.
Stack those and the derating is severe. A tray loaded with branch circuits to a PDU can carry dozens of current-carrying conductors once you count the harmonic neutrals, which drives the adjustment factor down into the 50 percent range or worse, and the hot room ambient takes another cut on top. The conductor that the load current alone would allow ends up a size or two larger by the time the heat math is done.
The field discipline is to count the harmonic neutrals honestly, use the real room ambient rather than a comfortable assumption, and treat the densest tray or raceway as the design case. The conductors in the middle of a packed bundle are the ones that cook, and they are the ones nobody can inspect once the cabinets are in and energized.
What to document
Bury the ampacity calculation where no one can find it and it is worth nothing to the next person who has to trust the conductor size. When a feeder runs hot or an inspector asks why a conductor is the size it is, the record is what answers it. Capture the inputs and the cap, not just the final size.
Record the circuit, the conductor size and insulation type, the base ampacity from the 90 degree C column, the ambient used and the correction factor, the current-carrying conductor count and the adjustment factor, the derated ampacity, the termination temperature and its column ampacity, and the final allowable number against the load. If the conductor went up a size, write down which test drove it, the heat math or the voltage drop, because the next person will wonder why the feeder is bigger than the table demands.
| Field to record | Why it matters |
|---|---|
| Conductor size and insulation | Sets the base ampacity and the column |
| Base ampacity (90C) | The starting value for the derating math |
| Ambient and correction factor | Heat from the space cuts the ampacity |
| CCC count and adjustment factor | Crowding cuts the ampacity again |
| Derated ampacity | Base times correction times adjustment |
| Termination rating and column ampacity | The 110.14(C) cap on the final |
| Final allowable vs load | Proves the conductor carries the load |
| What drove an upsize | Explains a conductor larger than the table |
Common mistakes
- Skipping the derating entirely and sizing straight off the table for a bundle or a hot space.
- Reading the final ampacity from the 90 degree C column at a 75 degree C termination instead of capping at the termination.
- Starting the derating at the 75 column when the 90 column is allowed for the heat math, leaving headroom on the table.
- Not counting the neutral as a current-carrying conductor on a heavily nonlinear or harmonic load.
- Counting the equipment grounding conductor toward the conductor-count adjustment, which it never joins.
- Ignoring the rooftop temperature adder on a raceway sitting in the sun close to the roof.
- Setting the overcurrent device above the conductor's derated ampacity, or past the 240.4(D) caps on small wire.
- Using the in-air adjustment factors on an underground duct bank that needs an engineered ampacity.
Field checklist
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Standards and references
The NEC, NFPA 70, is the framework. Base ampacities for the common case come from Table 310.16, for insulated conductors up to 2000 V in a raceway, cable, or earth, based on 30 degrees C ambient and not more than three current-carrying conductors, in 60, 75, and 90 degree C columns for copper and aluminum. The ambient temperature correction factors are in Table 310.15(B)(1), and the rooftop temperature adder for raceways in sunlight is in 310.15(B). The adjustment for more than three current-carrying conductors is in 310.15(C)(1), and the rule that a harmonic neutral counts is in 310.15(E).
The termination temperature limit, the 75 degree C rule that usually caps the final ampacity, is 110.14(C). Overcurrent protection of conductors, including the next-size-up allowance and the fixed caps on small conductors, is in 240.4, with the small-conductor limits in 240.4(D). Engineered underground ampacity is informational in Annex B, based on the Neher-McGrath method.
Section numbers and the rooftop detail have moved between code cycles, so confirm them against the edition the jurisdiction has actually adopted and any local amendments before you cite them on a submittal. Equipment listings under UL and the manufacturer's instructions can impose a termination temperature or a conductor requirement that is stricter than the table, and where they do, the listing governs. The AHJ has the final say on the adopted edition and its interpretation.
Units, terms, and conversions
Ampacity and its inputs show up in a few unit systems and a few names, so the same value can read differently across a drawing set, a manufacturer sheet, and the code.
Ampacity is in amps, sometimes written A or amperes. Conductor size is AWG for smaller conductors and kcmil (thousands of circular mils) for larger ones, while metric drawings use mm squared. Insulation and termination temperatures are in degrees C in the code, often degrees F in the field, so 60, 75, and 90 degrees C read as roughly 140, 167, and 194 degrees F. Correction and adjustment factors are unitless multipliers applied to the base ampacity. Derating is the general term for both the ambient correction and the conductor-count adjustment, even though the NEC keeps them as two separate steps.
- Ampacity
- The current a conductor can carry continuously without exceeding its temperature rating
- Correction factor
- The ambient temperature multiplier from Table 310.15(B)(1)
- Adjustment factor
- The more-than-three-conductor multiplier from 310.15(C)(1)
- CCC
- Current-carrying conductor; the count that drives the adjustment, excluding the EGC
- Termination rating
- The temperature rating of the lug or terminal that caps the ampacity under 110.14(C)
- AWG / kcmil
- American Wire Gauge for smaller sizes, thousand circular mils for larger conductors
FAQ
What is conductor ampacity?
Conductor ampacity is the current a wire carries continuously without driving its insulation past its temperature rating. It is set by the balance between the heat the conductor makes and the heat it sheds to its surroundings, so it changes with ambient temperature and with how many conductors are bundled together, not just the size of the metal.
Why do you derate conductors?
You derate because the base ampacity table assumes one set of conditions: 30 degrees C ambient and not more than three current-carrying conductors. A hotter space and a crowded raceway both leave the conductor less able to shed heat, so you cut the ampacity to match. Skip it and the insulation runs hot and fails early.
What is the 75C termination rule?
NEC 110.14(C) limits the ampacity to the column matching the lowest-rated termination in the circuit, usually 75 degrees C. Even with 90 degree C wire, the final ampacity reads from the 75 column because the lugs are only listed for 75. The 90 column is for the derating math, then the termination caps the result.
When does the neutral count as a current-carrying conductor?
The neutral counts when it carries harmonic current from nonlinear loads, under NEC 310.15(E). On a three-phase wye feeder serving computers, LED lighting, or drives, the third harmonic adds in the neutral instead of canceling, so it carries real current and counts toward the adjustment. On balanced linear loads, the neutral does not count.
Do you start derating at the 90C or the 75C column?
Start the correction and adjustment math at the 90 degree C ampacity when the conductor has 90 degree C insulation, even if the termination is 75 degrees C. NEC 110.14(C) allows the higher rating for the heat math. After multiplying by the factors, cap the result at the 75 degree C termination ampacity and take the smaller number.
How much do you derate for six conductors in a raceway?
Six current-carrying conductors in a raceway derate to 80 percent under NEC Table 310.15(C)(1), so each conductor's ampacity is multiplied by 0.80. The step starts at four conductors. Adding a single circuit to a pipe that already had three drops every conductor in it to 80 percent, then any ambient correction applies on top of that.
What if the derated ampacity is below my load?
Go up a conductor size and rerun the math, because the larger conductor has a larger base ampacity to derate from. You can also split conductors into a second raceway to cut the count back to three, or drop the ambient by moving or shading the run. Rerun the heat math on whatever you change.
Does the equipment grounding conductor count toward the conductor count?
No. The equipment grounding conductor carries current only during a fault, not in normal operation, so it never counts as a current-carrying conductor for the more-than-three adjustment. The phase conductors always count, and the neutral counts only when it carries harmonic current from a heavily nonlinear load under NEC 310.15(E).
How does the rooftop adder change conductor ampacity?
A raceway in direct sun close to a roof gets a temperature adder before the correction factor, commonly 33 degrees C (60 degrees F) added to the outdoor ambient under NEC 310.15(B) in recent editions. That pushes the conductor into a hotter band where the correction factor falls hard. XHHW-2 has been exempt from the adder.
Can the breaker exceed the derated conductor ampacity?
No, the overcurrent device protects the conductor at its derated ampacity, not the base table value, under NEC 240.4. You may round up to the next standard size under 240.4(B) when the ampacity does not land on one and the device is 800 A or less. The 240.4(D) caps on 14, 12, and 10 AWG never move.
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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.