NEC 310.16
Conductor Capacity Check
You are at a panel looking at a wire that is already in the wall. This works out what it carries in your conditions, which breaker it takes, the ground wire that goes with it, and what to pull if the answer is no.
The temperature buttons are the INSULATION rating printed on the jacket: THHN, THWN-2 and XHHW-2 are 90°C, THW and THWN are 75°C, TW and UF are 60°C. It is the column you derate from, and it is not the terminal rating that caps the answer.
30.0°C. Use the air the conductor actually sits in. An attic, an equipment room or a sunlit wall is hotter than the forecast, and 86°F (30°C) is the ambient Table 310.16 itself assumes.
Six of these come from NEC Table 310.15(C)(1), for more than three current-carrying conductors bundled longer than 24 inches. The 1–3 entry is our own “no adjustment” option and not a row of that table. Terminations are the breaker and lug rating at each end, not the wire: most modern breakers are marked 75°C.
Noncontinuous
Continuous (3 hours or more)
Everything the circuit will carry once the new load is on it, not just the addition. NEC 210.19(A)(1) and 210.20(A) size both the conductor and the device at 1.25 times the continuous part plus all of the rest.
Leave this alone and the answer names the device to install. Set it and the answer also says whether what is in there now is right for this conductor in these conditions.
Add the run length (optional, for voltage drop)
One-way length
Volts
Drop is computed from NEC Chapter 9 Table 9 resistance and reactance for the raceway you pick, at 0.85 power factor. The raceway metal changes the answer, which is why it is asked rather than assumed.
Common situations
governing
75 A at 90°C, times 1.000 for ambient, is 75.0 A. Your 75°C terminations cap it at 65 A under NEC 110.14(C), and that is the lower of the two, so the equipment is what limits this run.
Type what the circuit will carry, above, and this becomes a verdict: whether the load goes on this wire, which device it lands on, the ground wire that goes with it, and the size to pull if the answer is no.
Devices are the standard ratings of NEC 240.6(A) and this rounds down to one. NEC 240.4(B) permits the next size up where a conductor’s ampacity does not correspond to a standard rating, on circuits not over 800 A, and not where the conductors are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. This screen cannot tell whether yours qualifies, so it does not assume it. This tool sizes ordinary branch and feeder conductors. Motor and hermetic-motor circuits are sized under NEC Article 430 and 440 instead, and a compressor or motor current typed into the load fields above will be judged by the wrong rule.
Right now this page states facts about the wire: what it carries here and the largest device it takes. Enter the load and it becomes a verdict about your circuit, with the device to fit, the ground wire that goes with it, and the size to pull if the answer is no. It stays blank on purpose, because a verdict computed from a load nobody described is a verdict about somebody else's circuit.
Neither the conditions nor the terminations are holding this conductor below what the circuit needs, so there is no limit here to relieve and nothing on this screen to change for capacity. Anything left to decide is about length, not ampacity.
Show the math
75 A x 1.0000 = 75.00 A, min(75.00, 65) = 65.00 A
The correction comes from NEC Equation 310.15(B)(1), which the code prints and permits in place of its banded table. The conductor count adjustment is a value NEC Table 310.15(C)(1) sets rather than computes, so it is cited and applied, not derived.
Where this number comes fromIntry Verified
- Calculated from
- NEC Table 310.16 with Equation 310.15(B)(1) and Table 310.15(C)(1), held to 110.14(C), 240.4(D) and 240.6(A), with Table 250.122 and Chapter 9 Table 9. Computed from the NEC Table 310.16 ampacity with Equation 310.15(B)(1) and the Table 310.15(C)(1) adjustment, held to the 110.14(C) termination column, the 240.4(D) small-conductor caps and the 240.6(A) standard ratings, with the Table 250.122 grounding conductor and Chapter 9 Table 9 impedance. See the full receipt ↓
- Run with
- Conductor#6 AWG copper, 90 C
- Ambient86 F (30.0 C)
- Current-carrying conductors1-3 (1.00)
- Terminations75 C
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- Your AHJ and local amendments have final say. Confirm before rough-in.
Intry Verified · NEC 2023 · Build AE2FC52 · 2026-08-21
| Device | Smallest conductor | Usable | Headroom | Ground | 3% run |
|---|---|---|---|---|---|
| 15 A | #14 AWG | 20.0 | 5.0 | #14 AWG | 90 ft |
| 20 A | #12 AWG | 25.0 | 5.0 | #12 AWG | 104 ft |
| 25 A | #10 AWG | 35.0 | 10.0 | #10 AWG | 137 ft |
| 30 A | #10 AWG | 35.0 | 5.0 | #10 AWG | 114 ft |
| 35 A | #8 AWG | 50.0 | 15.0 | #10 AWG | 148 ft |
| 40 A | #8 AWG | 50.0 | 10.0 | #10 AWG | 130 ft |
| 45 A | #8 AWG | 50.0 | 5.0 | #10 AWG | 115 ft |
| 50 A | #8 AWG | 50.0 | 0.0 | #10 AWG | 104 ft |
| 60 A | #6 AWG | 65.0 | 5.0 | #10 AWG | 135 ft |
| 70 A | #4 AWG | 85.0 | 15.0 | #8 AWG | 178 ft |
| 80 A | #4 AWG | 85.0 | 5.0 | #8 AWG | 155 ft |
| 90 A | #3 AWG | 100.0 | 10.0 | #8 AWG | 168 ft |
| 100 A | #3 AWG | 100.0 | 0.0 | #8 AWG | 151 ft |
| 110 A | #2 AWG | 115.0 | 5.0 | #6 AWG | 176 ft |
| 125 A | #1 AWG | 130.0 | 5.0 | #6 AWG | 189 ft |
| 150 A | #1/0 AWG | 150.0 | 0.0 | #6 AWG | 191 ft |
| 175 A | #2/0 AWG | 175.0 | 0.0 | #6 AWG | 191 ft |
| 200 A | #3/0 AWG | 200.0 | 0.0 | #6 AWG | 205 ft |
Headroom is what the conductor has left over the device, in amps. The 3 percent run is the one-way distance that conductor reaches at the device rating before drop passes 3 percent, which NEC 210.19 recommends and NEC 90.5(C) makes advisory rather than enforceable. Ground wire is the equipment grounding conductor of NEC Table 250.122, sized from the device and not from the conductor beside it. The Usable figure is the same number NEC Table 310.16 prints wherever your terminations are what govern, because 110.14(C) makes that printed cell the ceiling; it diverges from the book exactly when heat and conductor count take the conductor below it.
Why the Table Value Is Not the Answer
NEC Table 310.16 gives #6 copper as 65A at the 75°C column. That figure assumes two things almost no real run satisfies: an ambient of 30°C (86°F), and no more than three current-carrying conductors in the raceway or cable. Put the same conductor in a 108°F attic with 4-6 conductors beside it and it carries 53.5A, which is a 50A breaker rather than a 60A one. The wire did not change. What it runs through did, and that difference is the number the book cannot print because it does not know your attic.
Three limits then compete for the answer and only one of them wins on any given circuit. The derated ampacity is heat and conductor count. The termination column is NEC 110.14(C): the corrected result may never exceed what the conductor is rated at the temperature rating of the equipment at each end, which is 75°C on most modern breakers and lugs. And NEC 240.4(D) caps the overcurrent device outright at 15A for 14 AWG copper, 20A for 12 AWG and 30A for 10 AWG, whatever the ampacity says. The check above names which of the three is governing, because that is the sentence that tells you whether a cheaper fix exists: heat you can sometimes route around, terminations you can sometimes upgrade, and 240.4(D) you cannot do anything about at all.
Worked Examples
Adding a 45A load to #6 copper in a 108°F attic
75A at 90°C × 0.892 ambient × 0.80 conductor count = 53.5A
That is below the 65A the 75°C terminations allow, so the conditions govern here and the usable figure is 53.5A. The 45A load needs 45A and a 45A device, both of which fit. But the largest device this conductor may be protected at here is 50A, so the 60A breaker in the panel has to come down to 50A. The ground wire for that device is #10 copper, from NEC Table 250.122. In easy conditions the same conductor answers 65.0A on a 60A device, and the 60A breaker would have been fine.
A 24A load on #12 copper, in perfect conditions
30A at 90°C, held to 25A by 110.14(C), capped at 20A by 240.4(D)
The ampacity covers the load and the circuit still does not fit, which is the case people get wrong. #12 copper carries 25A at 86°F with nothing else in the pipe, and 24A is inside that. But the device has to cover the load too, and the smallest standard rating that does is 25A, while NEC 240.4(D)(6) will not allow more than 20A in front of a #12 copper conductor. Nothing about the installation changes that ceiling, so the answer is to pull #10 copper, which is the smallest size that both carries the load and takes the device.
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The Four Answers, and What Each One Costs
It fits. The conductor carries the load in your conditions and the device that covers the load is one it may be protected at. Land it and go. This is the cheapest outcome and it is also the one most often missed, because a table read at 86°F says nothing about a run at 108°F in either direction.
Change the breaker. Two different jobs share this answer. Going up is ordinary: the wire already supports more than the device in front of it. Coming down is a defect that was there before you arrived, because the conductor in these conditions may not be protected at what is in the panel, and it is worth knowing that the fix is a device and not a re-pull.
Pull a bigger conductor. The check names the smallest size of the same material that both carries the load and takes the device, in the same conditions, so the answer is a size you can buy rather than a direction. Confirm it fits the raceway it has to go into before you order it: NEC Chapter 9 fill is a separate check and the conduit fill calculator runs it.
This circuit is done. No single conductor in NEC Table 310.16 reaches the load in these conditions. This is the most expensive outcome on the page, so the check reaches it only after searching every size in the table, and it always says which limit forced it. ⚠ It is not the end of the options: parallel sets under NEC 310.10(G) are permitted at 1/0 and larger and are a re-pull rather than a new service, and where the conditions are not already at the table’s own basis a cooler route or fewer current-carrying conductors is a different answer. The check recomputes on both and names the parallel option itself. A load past the largest device NEC 240.6(A) prints lands here too, and there the answer is a service design rather than anything about the wire.
Voltage Drop Is Reported, Not Enforced
Add a run length and the check computes the drop from NEC Chapter 9 Table 9 resistance and reactance for the raceway you pick, and the distance that conductor reaches before drop passes 3 percent. It reports both beside the verdict and neither one changes it. The 3 percent branch-circuit figure and the 5 percent total figure are Informational Notes under NEC 210.19 and 215.2, and NEC 90.5(C) states that such notes are informational only and are not enforceable as requirements. Many inspectors treat them as practical requirements and some jurisdictions adopt them by local amendment, which is a good reason to design to them and not a reason for a tool to tell you a circuit fails code when it does not. Drop is computed at the current actually flowing, not at the 1.25 continuous multiplier, which is a conductor-sizing allowance no conductor ever carries.
Frequently Asked Questions
How do I know if I can add a load to an existing circuit?
Three things have to hold. The load figure both halves are sized against is 125 percent of the continuous load plus all of the noncontinuous load (NEC 210.19(A)(1) and 210.20(A)). The overcurrent device has to cover that figure. And that device has to be one the conductor may legally be protected at, which is its ampacity in your actual conditions, rounded down to a standard NEC 240.6(A) rating and then capped by NEC 240.4(D) for 14, 12 and 10 AWG. The check runs all three and tells you which one failed, because that is what decides whether a cheaper fix exists. Note that 210.19(A)(1) states its requirement as the larger of two comparisons, one of which is measured before the correction and adjustment factors are applied; this tool applies the single conservative test above and does not attempt to split them.
What is the ampacity of #6 copper wire?
#6 copper is 65A at the 75°C column of NEC Table 310.16, which is the value most terminations are held to. That is the starting point, not the answer: in a 108°F attic with 4-6 current-carrying conductors the same wire carries 53.5A and takes a 50A breaker. The table value assumes 86°F and no more than three conductors, and almost no real run is both.
How many amps can 12 gauge THHN copper wire carry?
12 AWG THHN copper has a 90°C insulation rating, so NEC Table 310.16 lists 30A. That figure is a starting point for derating and never the usable circuit current: NEC 110.14(C) holds it to the 75°C column at 25A, and NEC 240.4(D)(6) caps the overcurrent device at 20A regardless of the ampacity. So a 24A load on #12 copper does not fit, and the smallest copper that carries it is #10.
Why does my breaker have to come down when nothing changed?
Because the conditions are part of the rating. Ampacity in NEC Table 310.16 assumes 30°C (86°F) ambient and no more than three current-carrying conductors. Add heat or add conductors to the raceway and the same wire carries less: #6 copper goes from 65.0A on a 60A device in easy conditions to 53.5A on a 50A device at 108°F with 4-6 conductors. Nothing about the wire changed; what it runs through did.
What size ground wire goes with the circuit?
The equipment grounding conductor comes from NEC Table 250.122 and is sized from the rating of the overcurrent device, not from the circuit conductor beside it. A 50A device takes a #10 copper EGC. Do not confuse this with the grounding electrode conductor of NEC Table 250.66, which is sized from the service-entrance conductor and answers a different question. Where the ungrounded conductors are increased in size beyond the minimum the load requires, NEC 250.122(B) requires the grounding conductor to be increased proportionally as well.
What is the difference between the 60, 75, and 90 degree columns?
The columns are the insulation temperature rating: 60°C is TW and UF, 75°C is THW, THWN and XHHW in wet locations, and 90°C is THHN, THWN-2 and XHHW-2. You derate from the column that matches your insulation, then hold the result to the terminal rating of the equipment at each end, which NEC 110.14(C)(1) makes a ceiling on the final answer. For most modern breakers and lugs that ceiling is the 75°C column.
Can I use the 90 degree column for ampacity?
As a starting point for derating, yes, and that is exactly what NEC 110.14(C)(1) contemplates. As the final rating, no: after correction and adjustment the result still has to land at or below the 60°C or 75°C terminal rating of the equipment. Starting at 90°C is what lets a conductor keep useful capacity in a hot or crowded run; it does not raise the ceiling.
Is aluminum wire one size larger than copper?
At the 75°C column aluminum carries roughly one to two AWG sizes less current than the same-size copper: #2 copper is 115A against 90A for #2 aluminum. The gap is not a constant, so a rule of thumb will eventually cost you a size. Switch the material on the check above and it recomputes the whole answer, device and ground wire included.
Related Calculators
Wire Size Calculator
The other direction. Start from a load and distance and get the conductor, when you are specifying a circuit rather than checking one that already exists.
Ampacity Derating Calculator
The correction factor on its own, computed at any ambient from NEC Equation 310.15(B)(1) rather than rounded into a 5°C band.
Voltage Drop Calculator
The full drop picture: size for a target, check a run, and see the effective impedance per conduit type from NEC Chapter 9 Table 9.
Wire checked. Now check what is feeding it.
A circuit that fits is only half the question when the panel is already full. The load calculator works out what the service has left before you add anything to it.
Keep going