Wire ampacity & derating calculator

NEC Table 310.16 — temperature correction, bundling, terminal limits, NM-B rule, continuous loads

Check what a given conductor is good for after derating, or find the smallest wire that meets your load. Uses the same Table 310.16 bases and factors as the live calculator.

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NEC Table 310.16 — Copper ampacity (30°C, ≤3 CCC)

Size	60°C / TW / NM-B final	75°C / THWN / THW	90°C / THHN start
14 AWG	15A	20A	25A
12 AWG	20A	25A	30A
10 AWG	30A	35A	40A
8 AWG	40A	50A	55A
6 AWG	55A	65A	75A
4 AWG	70A	85A	95A
3 AWG	85A	100A	115A
2 AWG	95A	115A	130A
1 AWG	110A	130A	145A
1/0	125A	150A	170A
2/0	145A	175A	195A
3/0	165A	200A	225A
4/0	195A	230A	260A
250 kcmil	215A	255A	290A
300 kcmil	240A	285A	320A
350 kcmil	260A	310A	350A
400 kcmil	280A	335A	380A
500 kcmil	320A	380A	430A

NEC Table 310.16 — Aluminum ampacity

Size	60°C / TW	75°C / THWN	90°C / THHN start
12 AWG*	15A	20A	25A
10 AWG	25A	30A	35A
8 AWG	35A	40A	45A
6 AWG	40A	50A	55A
4 AWG	55A	65A	75A
3 AWG	65A	75A	85A
2 AWG	75A	90A	100A
1 AWG	85A	100A	115A
1/0	100A	120A	135A
2/0	115A	135A	150A
3/0	130A	155A	175A
4/0	150A	180A	205A
250 kcmil	170A	205A	230A
300 kcmil	195A	230A	260A
350 kcmil	210A	250A	280A
400 kcmil	225A	270A	305A
500 kcmil	260A	310A	350A

*Minimum aluminum branch circuit = 12 AWG per NEC 310.106(B).

Per NEC Table 310.16, 310.15(B)(1), 310.15(C)(1), 110.14(C), 210.19(A), and 334.80. Verify with the current NEC and local amendments.

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Wire, clamps, and verification tools

After you size conductors, verify loads in the field and strip terminations cleanly.

Clamp

Klein Tools CL800 Digital AC/DC Clamp Meter

Measure running current without breaking the circuit
AC/DC and True RMS for nonlinear loads
CAT III 600 V — panel and feeder friendly

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Meter

Fluke 117 Electricians True RMS Multimeter

Voltage and continuity checks at devices
Non-contact voltage detection
Pair with ampacity results before energizing

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Copper

Southwire 10 AWG Stranded THHN (500 ft)

Common 30 A branch sizing reference
THHN/THWN-2 — aligns with 90°C start column math
Stranded for pulls in conduit

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Strip

Gardner Bender GESP-70 Wire Stripper

10–20 AWG stripping slots
ArmorEdge blades for clean strips
Built-in gauge reference on the handle

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Multimeter

Klein Tools MM400 Multimeter

Manual-ranging DMM for circuits and devices
Temp probe useful for ambient checks
Budget-friendly second meter in the bag

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Rules this calculator applies

NM-B / Romex: NEC 334.80 limits final ampacity to the 60°C column even though individual conductors may be 90°C rated. The tool still starts derating from the correct base column per insulation selection, then caps at 60°C for NM-B.

Terminals: NEC 110.14(C) caps the usable ampacity at the terminal temperature column unless equipment is listed and marked for a higher temperature.

Both factors: Ambient correction (Table 310.15(B)(1) style factors embedded as live) and bundling (Table 310.15(C)(1) style) multiply together before the terminal/NM-B cap.

Continuous loads: NEC 210.19(A) — when continuous is selected, required ampacity is 125% of load for sizing in find mode; in check mode the “max continuous load” line divides final ampacity by 1.25.

CCC count Equipment grounding conductors do not count. Neutrals on 2-wire circuits count; MWBC neutrals that only carry unbalanced current typically do not — when in doubt, ask your AHJ.

Step-by-step sizing (overview)

Step 1: Material and insulation type.
Step 2: Base ampacity from the column the live tool uses for math (90°C for THHN/NM-B start, 75°C for THW, 60°C for TW).
Step 3: Multiply by ambient correction.
Step 4: Multiply by bundling factor.
Step 5: Cap at terminal column (and NM-B 60°C overall cap).
Step 6: Apply 125% when continuous is selected.

Frequently asked questions

What size wire for a 30 A circuit?

Non-continuous 30 A with 75°C terminals often lands on 10 AWG copper at 35 A base in the 75°C column. Continuous 30 A needs 37.5 A minimum after derating chain — the calculator’s Mode B walks sizes until the final derated ampacity meets that requirement. NM-B uses the 60°C column cap per 334.80.

Why does NM-B use 60°C for the final answer?

NEC 334.80 requires NM and NMC ampacity not exceed the 60°C column. You may still start derating math from 90°C for THWN-2 conductors inside cable, but the cap is enforced in this tool the same way as live.

Do equipment grounding conductors count for bundling?

No. EGCs are not current-carrying for 310.15(C)(1) adjustment.

Wire ampacity & derating calculator