NEC 310.16 · 310.15 · 110.14(C) · 2023 Code
Secondary Conductor Sizing Calculator
Pick the transformer rating and the installation conditions. The tool sizes the conductors and shows which constraint decided it.
1Transformer secondary
2Conductor
Installation conditionsTerminals, ambient temperature, conductor count
30 °C is the baseline for Table 310.16.
1–3 band. Equipment grounding conductors never count; whether a neutral counts depends on the circuit — see the FAQ below.
Each must be at least 1/0 AWG under NEC 310.10(H).
Conductor per phase
Single copper conductor per phase
Each 4/0 conductor carries 225.5 A against an allowable 230 A.
How that size was reached
- 1Load to be carried180.4 A total
One conductor per phase
- 2Continuous-load factor× 1.25
NEC 215.2(A)(1): 180.4 A × 1.25 = 225.5 A design current
- 3Required per conductor225.5 A
Each conductor must carry at least this after all correction
- 4Base ampacity — 4/0 Cu, 90 °C column260 A
NEC 310.16, at 30 °C ambient with up to 3 current-carrying conductors
- 5Ambient temperature correction× 1.00
NEC 310.15(B)(1), 26–30 °C ambient on a 90 °C conductor
- 6Adjustment for conductor count× 1.00
3 current-carrying conductors — no adjustment
- 7Corrected ampacity260 A
260 A × 1.00 × 1.00
- 8Terminal limit — 75 °C column230 A
NEC 110.14(C): the ampacity cannot exceed the terminal temperature column, even with better insulation
- 9Allowable ampacity230 A
Governed by the terminal rating
Key figures
- Secondary load current
- 180.4 A
- Design current (125%)
- 225.5 A
- Required per conductor
- 225.5 A
- Corrected ampacity
- 260 A
- Terminal limit
- 230 A
- Allowable ampacity
- 230 A
- Limited by
- terminal rating
Code references
- — NEC 215.2(A)(1) — conductors sized for 125% of the continuous load
- — NEC 310.16 — allowable ampacities, 30 °C ambient, 3 or fewer conductors
- — NEC 310.15(B)(1) — ambient temperature correction factors
- — NEC 310.15(C)(1) — adjustment for more than three current-carrying conductors
- — NEC 110.14(C) — conductor temperature rating limits at equipment terminations
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Conductor per phase · 150 kVA
4/0Cu
Why the order of operations matters
Sizing is not one multiplication. Correction and adjustment run against theinsulation rating, then the result is capped by theterminal rating. Applying every factor to the terminal column — the most common shortcut — under-derates and pushes you to a larger, more expensive conductor.
Step 1
Start at 90 °C
Look up the size in the column matching the conductor insulation, not the terminals. THHN in conduit gives you the 90 °C numbers.
Step 2
Apply both factors
Multiply by the ambient correction and by the 310.15(C)(1) conductor-count adjustment. Both come off the same starting ampacity.
Step 3
Cap at the terminal
NEC 110.14(C) limits the answer to the terminal column. Whichever of the two is lower is the ampacity you may actually use.
NEC Table 310.16 ampacities at a glance
Base values at 30 °C ambient with not more than three current-carrying conductors. The calculator applies correction and adjustment on top of these.
Copper
| Size | 60 °C | 75 °C | 90 °C | Adjustment 4–6 | 7–9 | 10–20 |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | 20 | 18 | 13 |
| 12 AWG | 20 | 25 | 30 | 24 | 21 | 15 |
| 10 AWG | 30 | 35 | 40 | 32 | 28 | 20 |
| 8 AWG | 40 | 50 | 55 | 44 | 39 | 28 |
| 6 AWG | 55 | 65 | 75 | 60 | 53 | 38 |
| 4 AWG | 70 | 85 | 95 | 76 | 67 | 48 |
| 3 AWG | 85 | 100 | 115 | 92 | 81 | 58 |
| 2 AWG | 95 | 115 | 130 | 104 | 91 | 65 |
| 1 AWG | 110 | 130 | 145 | 116 | 102 | 73 |
| 1/0 | 125 | 150 | 170 | 136 | 119 | 85 |
| 2/0 | 145 | 175 | 195 | 156 | 137 | 98 |
| 3/0 | 165 | 200 | 225 | 180 | 158 | 113 |
| 4/0 | 195 | 230 | 260 | 208 | 182 | 130 |
| 250 kcmil | 215 | 255 | 290 | 232 | 203 | 145 |
| 300 kcmil | 240 | 285 | 320 | 256 | 224 | 160 |
| 350 kcmil | 260 | 310 | 350 | 280 | 245 | 175 |
| 400 kcmil | 280 | 335 | 380 | 304 | 266 | 190 |
| 500 kcmil | 320 | 380 | 430 | 344 | 301 | 215 |
| 600 kcmil | 350 | 420 | 475 | 380 | 333 | 238 |
| 700 kcmil | 385 | 460 | 520 | 416 | 364 | 260 |
| 750 kcmil | 400 | 475 | 535 | 428 | 375 | 268 |
| 800 kcmil | 410 | 490 | 555 | 444 | 389 | 278 |
| 900 kcmil | 435 | 520 | 585 | 468 | 410 | 293 |
| 1000 kcmil | 455 | 545 | 615 | 492 | 431 | 308 |
| 1250 kcmil | 495 | 590 | 665 | 532 | 465 | 333 |
| 1500 kcmil | 525 | 625 | 705 | 564 | 493 | 353 |
| 1750 kcmil | 545 | 650 | 735 | 588 | 515 | 368 |
| 2000 kcmil | 555 | 665 | 750 | 600 | 525 | 375 |
The last three columns are the 90 °C value with only the 310.15(C)(1) conductor-count adjustment applied. Ambient temperature correction and the terminal limit must be checked separately; these adjusted columns are not final allowable ampacities.
Aluminium
| Size | 60 °C | 75 °C | 90 °C | Adjustment 4–6 | 7–9 | 10–20 |
|---|---|---|---|---|---|---|
| 12 AWG | 15 | 20 | 25 | 20 | 18 | 13 |
| 10 AWG | 25 | 30 | 35 | 28 | 25 | 18 |
| 8 AWG | 35 | 40 | 45 | 36 | 31 | 23 |
| 6 AWG | 40 | 50 | 55 | 44 | 39 | 28 |
| 4 AWG | 55 | 65 | 75 | 60 | 53 | 38 |
| 3 AWG | 65 | 75 | 85 | 68 | 59 | 43 |
| 2 AWG | 75 | 90 | 100 | 80 | 70 | 50 |
| 1 AWG | 85 | 100 | 115 | 92 | 81 | 58 |
| 1/0 | 100 | 120 | 135 | 108 | 95 | 68 |
| 2/0 | 115 | 135 | 150 | 120 | 105 | 75 |
| 3/0 | 130 | 155 | 175 | 140 | 122 | 88 |
| 4/0 | 150 | 180 | 205 | 164 | 144 | 103 |
| 250 kcmil | 0 | 0 | 0 | |||
| 300 kcmil | 0 | 0 | 0 | |||
| 350 kcmil | 0 | 0 | 0 | |||
| 400 kcmil | 0 | 0 | 0 | |||
| 500 kcmil | 0 | 0 | 0 | |||
| 600 kcmil | 0 | 0 | 0 | |||
| 700 kcmil | 0 | 0 | 0 | |||
| 750 kcmil | 0 | 0 | 0 | |||
| 800 kcmil | 0 | 0 | 0 | |||
| 900 kcmil | 0 | 0 | 0 | |||
| 1000 kcmil | 0 | 0 | 0 | |||
| 1250 kcmil | 0 | 0 | 0 | |||
| 1500 kcmil | 0 | 0 | 0 | |||
| 1750 kcmil | 0 | 0 | 0 | |||
| 2000 kcmil | 0 | 0 | 0 |
This chart covers aluminum and copper-clad aluminum from 12 AWG upward. It does not include the smaller copper-clad-only entries shown in the supplied 2026 table.
Ambient temperature correction factors
NEC 2023 Table 310.15(B)(1), based on 30 °C. A dash means the code lists no factor for that combination, which usually means the insulation is not adequate there. The 2026 edition separates the 30 °C and 40 °C bases into Tables 310.15(B)(1)(1) and (2); the 40 °C table is not interchangeable with the data shown here.
| Ambient °C | 60 °C conductor | 75 °C conductor | 90 °C conductor |
|---|---|---|---|
| 10 and below | 1.29 | 1.2 | 1.15 |
| 11–15 | 1.22 | 1.15 | 1.12 |
| 16–20 | 1.15 | 1.11 | 1.08 |
| 21–25 | 1.08 | 1.05 | 1.04 |
| 26–30 | 1 | 1 | 1 |
| 31–35 | 0.91 | 0.94 | 0.96 |
| 36–40 | 0.82 | 0.88 | 0.91 |
| 41–45 | 0.71 | 0.82 | 0.87 |
| 46–50 | 0.58 | 0.75 | 0.82 |
| 51–55 | 0.41 | 0.67 | 0.76 |
| 56–60 | — | 0.58 | 0.71 |
| 61–65 | — | 0.47 | 0.65 |
| 66–70 | — | 0.33 | 0.58 |
| 71–75 | — | — | 0.5 |
| 76–80 | — | — | 0.41 |
| 81–85 | — | — | 0.29 |
Adjustment for more than three current-carrying conductors
NEC Table 310.15(C)(1). Note the cliff between nine and ten conductors — the factor drops from 70% to 50% in one step.
1–3 conductors
100%no adjustment
4–6 conductors
80%of table ampacity
7–9 conductors
70%of table ampacity
10–20 conductors
50%of table ampacity
21–30 conductors
45%of table ampacity
31–40 conductors
40%of table ampacity
41 and above conductors
35%of table ampacity
Data sources
- NEC 2023 Table 310.16 — allowable ampacities, 30 °C, three or fewer conductors
- NEC 2023 Table 310.15(B)(1) — ambient temperature correction
- NEC 2023 Table 310.15(C)(1) — conductor-count adjustment
- NEC 2023 110.14(C), 240.4(D), 310.10(H), 310.15(E)
- 2026 comparison. The supplied 2026 pages confirm the corresponding provisions as 110.14(C), 240.4(D), 310.10(G), 310.15(E), and 215.4(A)(1). Table 310.16 also includes 16 AWG copper and 14 AWG copper-clad aluminum values; the latter are not general aluminum ratings. The 2026 Table 310.15(B)(1)(1) 30 °C correction factors were checked against the supplied page and match the values used in this 2023 data set. The result still remains a 2023 calculation because the 2026 small-conductor rows are not mixed into it.
- Table values cross-checked against the Schneider Electric conductor ampacity data bulletin and the NFPA code text. This tool sizes for ampacity only — it does not check voltage drop.
- Two cases are deliberately out of scope, and both are common enough to call out.
- 310.15(B)(2) rooftop adder. A raceway on or above a roof needs the ambient raised before the correction factor is applied. Add it yourself; this tool takes your ambient number as given.
- 334.80 non-metallic cable. NM-B ampacity must not exceed the 60 °C column. Its 90 °C rating may be used only as the starting point for derating, and the final derated value still cannot exceed the 60 °C ampacity. An NM-B feed sizes differently from the same conductors in conduit.
Questions people actually ask
Why does the tool size for 125% of the transformer current?
Because NEC 215.2(A)(1) sizes feeder conductors for the continuous load at 125% plus 100% of any non-continuous load. A continuous load is one expected to run for three hours or more. The calculator defaults to Continuous because it is the conservative choice. Switch to Non-continuous only if you know the load is intermittent, or if the equipment is listed for operation at 100% of its rating.
Do I use the 90 °C ampacity when the terminals are only rated 75 °C?
Yes — for correction and adjustment you start from the insulation rating, then check the terminal limit separately. NEC 110.14(C) caps the final allowable ampacity at the terminal column, but it does not stop you using the higher insulation column to survive derating. The calculator does both steps in order, which is where most sizing tools get it wrong by applying every factor to the 75 °C column.
Why is the 75 °C or 60 °C column still the limit if I have THHN?
Because the weakest link governs. THHN is rated 90 °C, but the heat has to leave the conductor at the lug. If the busbar is marked 75 °C, the conductor is limited to the 75 °C ampacity of its size, no matter what insulation you pulled in.
Does the neutral count as a current-carrying conductor?
A neutral that carries only the imbalance from the other conductors of the same circuit need not count. Do count the neutral in a 2-wire circuit, in a 3-wire circuit using two phases and the neutral of a 4-wire three-phase wye system, and in a 4-wire three-phase circuit where the major portion of the load is nonlinear. The supplied 2026 text states these cases in 310.15(E)(1) and (2). Enter the count for each raceway or cable, not the total across separate raceways.
What ambient temperature should I use?
30 °C is the Table 310.16 baseline, so no factor applies there. Use the actual air temperature at the hottest part of the run: roughly 40 °C for a shaded outdoor raceway, and 45–50 °C for rooftop or attic work. Above 30 °C the factor eats into your margin quickly — a 90 °C conductor at 50 °C keeps only 82% of its table ampacity.
When do I need parallel conductors?
When no single size in Table 310.16 carries the load after correction. Each parallel conductor must be at least 1/0 AWG under NEC 310.10(H), and every conductor in the paralleled set counts toward the 310.15(C)(1) adjustment.
Does this check the secondary protective device or the 240.21(C) tap rules?
No. It sizes the conductors for ampacity only. Secondary conductor protection under NEC 240.21(C) and the primary protection from the fuse size chart are separate checks, so run both before you finalize a design.
Does this cover voltage drop?
No. Table 310.16 is an ampacity table and the code note says its values are based on temperature alone. Voltage drop is a separate check under Informational Note 210.19(A) and has no code limit — design to it, but do not expect this tool to size for it.
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Disclaimer. Based on the 2023 NEC. Not engineering advice and not a substitute for the code text. Verify against the edition adopted in your jurisdiction and with your Authority Having Jurisdiction — protection decisions affect life safety.