Cable ampacity and derating
The ampacity table assumes 30 °C ambient and three current-carrying conductors. The calculator applies temperature and conductor-count corrections, the terminal temperature cap, continuous-load sizing, a selected breaker/fuse check including supported NEC 240.4(D) limits, and voltage drop.
| Step | Value | Note |
|---|---|---|
| Conductor | 10 AWG/kcmil copper | 75 °C insulation |
| Table 310.16 ampacity | 35 A | 30 °C ambient, three conductors |
| Ambient correction | 1.000 | Table 310.15(B)(1) at 30 °C |
| Conductor adjustment | 1.000 | Table 310.15(C)(1), 3 current-carrying |
| Corrected ampacity | 35.0 A | after ambient correction and conductor adjustment |
| Termination limit | 30 A | 60 °C table ampacity per §110.14(C) |
| Permitted ampacity | 30.0 A | lower of corrected ampacity and termination limit |
| Load current | 24.0 A | continuous |
| Design load | 30.0 A | 125% per §210.19 |
| Ampacity check | Pass | 100% utilised |
| Overcurrent device | 30 A | selected circuit-breaker/fuse rating |
| Maximum device rating | 30.0 A | includes §240.4(D) 30 A ceiling |
| Protection check | Pass | device must cover design load and protect conductor |
| DC resistance | 1.2400 Ω/kFT | Chapter 9, Table 8 at 75 °C |
| Voltage drop | 5.95 V | 2.48% of 240 V |
| Drop limit | 3% | branch circuit, §210.19 informational note |
| Recommended size | 10 | selected size passes ampacity, protection and drop checks |
The 125% rule applies to the load, not the conductor: a continuous load is taken at 125% and then compared with the derated ampacity. Applying it twice is the most common error in a panel schedule.
I′ = I_table × f_temp × f_bundle · Both factors from Article 310.
I_allow = min(I′, I_terminal) · The terminal temperature column caps the corrected value under §110.14(C).
I_design = 1.25 × I_load · §210.19(A)(1) for branch circuits.
VD = 2·L·I·R/1000 · Single phase; √3 replaces 2 for three phase.
I_design ≤ I_OCPD ≤ min(I_allow, I_240.4(D)) · Small-conductor ceiling applies where relevant; exceptions are not evaluated.
I_allow ≥ I_design; I_OCPD passes; VD ≤ 3% · All selected checks must hold.
A clear path from inputs to results.
The ampacity table assumes 30 °C ambient and three current-carrying conductors. The calculator corrects for temperature and conductor count, caps ampacity at the selected terminal rating, checks the selected fuse or breaker against the conductor and NEC 240.4(D) where applicable, and checks voltage drop.
How to use
- Pick the conductor size, material and insulation rating.
- Set the terminal temperature, ambient temperature and number of current-carrying conductors.
- Enter the load current and say whether it is continuous.
- Select the installed overcurrent-device rating, then read ampacity, protection, voltage drop and recommended size checks.
The formula
Reference: NFPA 70 (NEC) Tables 310.16, 310.15(B)(1), 310.15(C)(1), §§110.14(C), 210.19(A)(1), 240.4(D); resistance from Chapter 9, Table 8.
Worked example
10 AWG copper at 24 A, 100 ft, 240 V
- 10 AWG copper, 75 °C insulation.
- 60 °C terminations and a selected 30 A circuit breaker.
- 30 °C ambient, three current-carrying conductors.
- 24 A continuous load, 100 ft one-way, 240 V single phase.
- 1
Table 310.16 gives 35 A for 10 AWG copper at 75 °C.
- 2
At 30 °C the ambient factor is 1.00 and three conductors need no adjustment, so corrected ampacity is 35 A before the terminal cap.
- 3
The 60 °C terminal column limits this conductor to 30 A. A continuous load is taken at 125%, so the design load is 24 × 1.25 = 30 A.
- 4
NEC 240.4(D) limits 10 AWG copper protection to 30 A; the selected 30 A breaker meets the calculator's protection check.
- 5
The conductor resistance is 1.24 Ω per 1000 ft, so the drop is 2 × 100 × 24 × 1.24 / 1000 = 5.95 V.
- 6
That is 2.48% of 240 V, inside the 3% branch-circuit limit.
- 7
The selected 30 A device covers the 30 A design load and meets the small-conductor limit; ampacity is fully used and voltage drop passes.
30 A permitted · 30 A design · 30 A OCPD · 2.48% drop · passes.
Interpreting the result
Ampacity, overcurrent protection and voltage drop are separate checks. Upsizing the conductor can address ampacity, a too-large breaker or fuse, or voltage drop; an overcurrent device below the design load needs a different device selection, not wire upsizing alone.
Assumptions & limitations
- Ampacities are the NEC Table 310.16 values for not more than three current-carrying conductors in a raceway or cable at 30 °C.
- Ambient correction and conductor adjustment multiply, and both are applied to the table value.
- Resistance is the DC value at 75 °C from Chapter 9, Table 8; reactance is neglected.
- Voltage drop uses 2LI R for single phase and √3LI R for three phase.
- The selected terminal temperature caps ampacity; follow equipment markings/listing instructions. The selected overcurrent device is checked against corrected ampacity and the §240.4(D) ceilings supported by this calculator (14/12/10 AWG).
- §240.4(B), (E) and (G) exceptions, motor circuits and other special overcurrent rules are not evaluated; the protection check is conservative and may reject a permitted exception.
Frequently asked questions
Why can't I use the 90 °C column?
Use the insulation column for ambient/conductor-count corrections, then cap the result at the terminal temperature shown on listed equipment. A 90 °C conductor with 75 °C terminations is therefore limited to the lower 75 °C ampacity; the selected rating must match equipment markings and applicable code rules.
Why 125% for a continuous load?
Because a load that runs for three hours or more heats the conductor to its steady-state temperature, and the code wants margin against that. The 125% applies to the load, not to the conductor, and applying it twice is a common error.
Does conduit fill affect ampacity?
Not directly — ampacity depends on the number of current-carrying conductors, not on how full the conduit is. Fill is a separate requirement in Chapter 9, and both have to be satisfied.
What counts as a current-carrying conductor?
The ungrounded conductors and, in some cases, the neutral. A neutral that carries only the unbalanced current of a three-wire circuit does not count; one that carries the full load current, as on a 120 V circuit, does.
What is the voltage-drop limit?
3% for a branch circuit and 5% overall including the feeder, from the informational notes to §210.19 and §215.2. It is advisory rather than mandatory in the code itself, but equipment manufacturers specify it and inspectors enforce it.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.