Sizing a conductor correctly under the National Electrical Code means more than reading a single column of Table 310.16. You have to account for the conductor's insulation temperature rating, the terminal temperature rating of the equipment, ambient temperature, how many current-carrying conductors share the raceway, and the 80% continuous-load rule. Get any one of those wrong and the conductor is either undersized (a fire risk) or needlessly expensive. This guide walks through the whole process step by step.
Start with the load and ampacity
Ampacityis the maximum current a conductor can carry continuously without exceeding its temperature rating. The governing table for most installations rated 0–2000 V is NEC Table 310.16(formerly 310.15(B)(16)), which lists allowable ampacities based on conductor size, material, and insulation temperature rating, with not more than three current-carrying conductors in a raceway at a 30 °C ambient.
Begin by establishing the load current the circuit must carry, then find a conductor whose ampacity - after all the adjustments below that equals or exceeds it.
Step 1: Pick the right temperature column
Table 310.16 has three columns for copper: 60 °C (e.g. TW), 75 °C (e.g. THW, THWN), and 90 °C(e.g. THHN, XHHW-2). It is tempting to use the 90 °C column because it shows the highest ampacity, but NEC 110.14(C) limits you to the lowest temperature rating of any part of the circuit, including the terminals:
- Equipment terminations rated for circuits 100 A or less, or 14 AWG through 1 AWG, are generally rated 60 °C unless marked otherwise.
- Terminations on circuits over 100 A, or larger than 1 AWG, are generally rated 75 °C.
The practical rule: you may use the higher-rated insulation, but you must select the final conductor size from the column matching the terminal rating. A 90 °C conductor on a 75 °C terminal is sized from the 75 °C column - the 90 °C column is only used as the starting point for the derating calculations in steps 2 and 3.
Step 2: Apply ambient temperature correction
Table 310.16 is based on a 30 °C ambient. When the ambient is higher (an attic, a boiler room, direct sun), the conductor sheds heat less readily, so its ampacity must be reduced using the correction factors in Table 310.15(B)(1). For example, a 90 °C conductor in a 41–45 °C ambient is multiplied by 0.87.
Step 3: Apply conduit-fill (bundling) adjustment
When more than three current-carrying conductors run together in a raceway or cable, mutual heating reduces their ampacity. The adjustment factors of Table 310.15(C)(1) apply:
- 4–6 conductors: 80%
- 7–9 conductors: 70%
- 10–20 conductors: 50%
Note that the neutral of a balanced multiwire circuit and the equipment grounding conductor are generally notcounted as current-carrying. Both the ambient correction and the bundling adjustment are applied to the ampacity from the conductor's own temperature column (usually 90 °C), and they multiply together.
Step 4: Apply the 80% continuous-load rule
A continuous load is one expected to run for three hours or more. Per NEC 210.19(A)(1) and 215.2(A)(1), the conductor (and the overcurrent device) must be rated for at least 125% of the continuous load plus 100% of the non-continuous load. The same idea stated in reverse: a conductor and breaker may only be loaded to 80% of their rating by a continuous load.
Worked example
A continuous 80 A load is fed by THHN copper (90 °C insulation) on equipment with 75 °C terminals. The four current-carrying conductors run through an attic where the ambient reaches 40 °C.
- Minimum conductor rating= 80 A × 1.25 = 100 A (continuous-load rule).
- Start from the 90 °C ampacity of a candidate size. For 3 AWG THHN copper that is 110 A.
- Ambient correction at 40 °C for 90 °C wire = 0.91 → 110 × 0.91 = 100.1 A.
- Bundling adjustment for 4 conductors = 0.80 → 100.1 × 0.80 ≈ 80.1 A - this falls short of the 100 A requirement.
- Step up to 1 AWG THHN (150 A at 90 °C): 150 × 0.91 × 0.80 ≈ 109 A, which clears 100 A.
- Finally, confirm the 75 °C terminal ampacity of 1 AWG copper (130 A) is also ≥ 100 A. It is, so 1 AWG is the answer.
This is the order that trips people up: derating is done from the insulation column, but the final size must still satisfy the terminal-temperature column.
Don't forget voltage drop
Ampacity sizing keeps the conductor from overheating, but it says nothing about voltage delivered at the load. On long runs, voltage drop often dictates a larger conductor than ampacity alone would. The NEC recommends keeping branch-circuit voltage drop at or below 3%. Check it with our voltage drop calculator - and read voltage drop explained for the full method.
Tools that do the work for you
The free Voltix wire size calculator (NEC) applies Table 310.16, the temperature columns, ambient correction and bundling in one step. Pair it with the circuit breaker size calculator to size overcurrent protection, the conduit fill calculator to confirm your raceway is legal, and the wire gauge calculator (AWG) to look up resistance and circular-mil area. For the UK/AU-NZ equivalent of this process, see our BS 7671 cable sizing guide.