Wire Sizing & Ampacity Calculator (NEC 310.16)
Size copper or aluminum conductors for any load — NEC 310.16 ampacity, 310.15(B) ambient correction, and the 125% continuous-load rule built in.
How this is calculated
Base ampacity comes from NEC Table 310.16 (75°C column by default — most terminations rated ≤100 A are 75°C per 110.14(C)).
Continuous loads (3 hours or more) are sized at 125% of load current per NEC 210.19(A)(1) — the conductor's base ampacity must meet or exceed that value.
Ambient correction: ampacity is multiplied by the 310.15(B)(1) factor for your ambient temperature (standard basis is 30°C).
Small conductors: 240.4(D) limits overcurrent protection to 15 A (14 AWG), 20 A (12 AWG), and 30 A (10 AWG) — enforced automatically.
This tool covers ampacity only. Check voltage drop on long runs with our Voltage Drop Calculator. Always verify against the adopted NEC edition and local amendments.
Wire sizing FAQ
What size wire do I need for a 30-amp circuit?
For a non-continuous 30 A load on 75°C copper, 10 AWG (35 A ampacity) is sufficient. If the load is continuous, NEC 210.19(A)(1) requires sizing at 125% — 37.5 A — which pushes you to 8 AWG copper (50 A). Aluminum for the same continuous load requires 6 AWG.
Why does a continuous load require 125% sizing?
NEC 210.19(A)(1) requires conductors supplying continuous loads (3+ hours) to have an ampacity of at least 125% of the load. Heat builds up in conductors and terminations during sustained operation; the 25% headroom keeps insulation and breaker terminations within their temperature ratings. Solar PV output circuits are always treated as continuous.
Which temperature column of Table 310.16 should I use?
Use the column matching the weakest link in the circuit — usually the termination, not the wire. Per NEC 110.14(C), equipment rated 100 A or less generally uses the 75°C column; above 100 A, 75°C as well unless the equipment is marked otherwise. The 90°C column is mainly used as the starting point for derating calculations (ambient and conduit-fill corrections), not final termination ampacity.
Copper or aluminum — which should I choose?
Copper carries more current per size and is easier to terminate, so it's standard for branch circuits. Aluminum is roughly 60% of copper's ampacity but costs far less per amp in large feeders (2 AWG and up), which is why it's common for service entrances, subpanels, and PV array feeders. Aluminum terminations must be rated AL/CU and torqued to spec with antioxidant compound.
Related tools: All Solar & Electrical Calculators · Wire Gauge Amp Chart — and check NEC 3% drop with our Voltage Drop Calculator.
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