How do I choose the correct wire size for a circuit?

Wire size must satisfy two criteria: ampacity (current-carrying capacity) and voltage drop. First, find the minimum AWG from NEC Table 310.16 that meets the breaker or load current at the appropriate temperature rating (60 °C, 75 °C, or 90 °C). Then check that voltage drop over the run length stays within 3% (NEC recommendation). The calculator evaluates both and reports the larger required size.

What is the difference between conductor ampacity and voltage-drop sizing?

Ampacity sizing ensures the wire does not overheat under load — it is a safety requirement governed by NEC Table 310.16 (or BS 7671 Table 4D for UK). Voltage-drop sizing ensures adequate voltage reaches the load — it is a performance recommendation. A long run at moderate current may require a larger conductor for voltage drop even though a smaller wire would be thermally safe.

Does wire size change with conductor temperature rating?

Yes. NEC Table 310.16 lists separate ampacity columns for 60 °C, 75 °C, and 90 °C insulation. The usable ampacity is limited by the lowest-rated termination in the circuit (typically 60 °C or 75 °C for most devices). The calculator applies the correct column based on the temperature rating you select.

How does ambient temperature affect wire sizing?

Higher ambient temperatures reduce a conductor's ability to dissipate heat, requiring a derating correction factor. NEC Table 310.15(B)(1) provides correction factors for temperatures above 30 °C. The calculator applies this derating automatically when you enter an ambient temperature.

What conduit fill or bundling derating applies?

When four or more current-carrying conductors are bundled together in a conduit or cable, NEC Table 310.15(C)(1) requires derating: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20, and so on. The wire-size calculator applies bundling derating when you specify more than three conductors.

Is this wire size calculator free?

Yes — the wire size calculator is free with no signup required. Enter your load current, circuit length, voltage, conductor material, and temperature rating, and the recommended AWG or mm² size is shown immediately.

What is the minimum wire size for a 20-amp circuit?

Per NEC Table 310.16, a 20-amp, 120/240 V branch circuit requires at minimum 12 AWG copper (60 °C column) or 10 AWG aluminium. In practice most electricians use 12 AWG copper with a 20 A breaker for standard household circuits.

Does the calculator support metric mm² wire sizes?

Yes. Toggle the unit selector to mm² and the calculator uses IEC conductor sizes (1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², etc.) with BS 7671 current-carrying capacity tables.

Wire Size Calculator — Free AWG & mm² Sizing Tool

Wire Gauge Amp Chart (NEC Table 310.16) — Copper & Aluminum Ampacity

A 12 AWG copper wire is rated for 20 A, 10 AWG for 30 A, and 8 AWG for 40 A at 60 °C per NEC Table 310.16. The full chart below lists allowable ampacities for copper and aluminum conductors from 14 AWG to 500 kcmil at 60 °C, 75 °C, and 90 °C insulation temperature ratings, followed by the ambient-temperature correction factors and conductor-bundling adjustment factors needed to derate correctly. The calculator above applies all of these tables automatically.

NEC 2023 Table 310.16 — Allowable Conductor Ampacities

Conditions of use for this table: insulated conductors rated up to 2,000 V, no more than three current-carrying conductors in a raceway, cable, or directly buried, at a 30 °C (86 °F) ambient temperature. Source: NEC 2023 Table 310.16.

Wire size (AWG / kcmil)	Copper — Ampacity (A)			Aluminum — Ampacity (A)
Wire size (AWG / kcmil)	60 °C (TW, UF)	75 °C (THWN, XHHW)	90 °C (THHN, THWN-2, XHHW-2)	60 °C	75 °C	90 °C
14 AWG	15	20	25	—	—	—
12 AWG	20	25	30	15	20	25
10 AWG	30	35	40	25	30	35
8 AWG	40	50	55	35	40	45
6 AWG	55	65	75	40	50	55
4 AWG	70	85	95	55	65	75
3 AWG	85	100	115	65	75	85
2 AWG	95	115	130	75	90	100
1 AWG	110	130	145	85	100	115
1/0 AWG	125	150	170	100	120	135
2/0 AWG	145	175	195	115	135	150
3/0 AWG	165	200	225	130	155	175
4/0 AWG	195	230	260	150	180	205
250 kcmil	215	255	290	170	205	230
300 kcmil	240	285	320	195	230	260
350 kcmil	260	310	350	210	250	280
400 kcmil	280	335	380	225	270	305
500 kcmil	320	380	430	260	310	350

NEC Table 310.16 continues beyond 500 kcmil (600, 700, 750, 800, 900, 1000, 1250, 1500, 1750, and 2000 kcmil); the sizes above cover residential and light-commercial work. Aluminum conductors smaller than 12 AWG are not listed in the table.

Ampacity by wire size — quick facts

14 AWG copper is rated 15 A at 60 °C and 20 A at 75 °C per NEC Table 310.16.
12 AWG copper is rated 20 A at 60 °C and 25 A at 75 °C per NEC Table 310.16.
10 AWG copper is rated 30 A at 60 °C and 35 A at 75 °C per NEC Table 310.16.
8 AWG copper is rated 40 A at 60 °C and 50 A at 75 °C per NEC Table 310.16.
6 AWG copper is rated 55 A at 60 °C and 65 A at 75 °C per NEC Table 310.16.
4 AWG copper is rated 70 A at 60 °C and 85 A at 75 °C per NEC Table 310.16.
3 AWG copper is rated 85 A at 60 °C and 100 A at 75 °C per NEC Table 310.16.
2 AWG copper is rated 95 A at 60 °C and 115 A at 75 °C per NEC Table 310.16.
1/0 AWG copper is rated 125 A at 60 °C and 150 A at 75 °C per NEC Table 310.16.
2/0 AWG copper is rated 145 A at 60 °C and 175 A at 75 °C per NEC Table 310.16.

Which temperature column do I use?

Use the column that matches the lowest temperature rating in the circuit — usually the terminations, not the wire insulation (NEC 110.14(C)):

NM-B cable (Romex): always use the 60 °C column, even though the conductors inside are 90 °C rated (NEC 334.80). This is why 12 AWG NM-B is a 20 A conductor.
THHN/THWN-2 in conduit: use the 75 °C column in most cases — circuit breakers and lugs on circuits over 100 A, or marked accordingly, are rated 75 °C; terminations on circuits 100 A and less default to 60 °C unless listed and marked 75 °C (NEC 110.14(C)(1)). Modern breakers are almost universally marked 60/75 °C, so 75 °C applies.
90 °C column: in practice this column is not used for final sizing against terminations — it is the starting point for applying correction and adjustment factors, after which the result must still not exceed the 60/75 °C termination ampacity.

Ambient-temperature correction factors (NEC 310.15(B)(1))

Table 310.16 assumes a 30 °C (86 °F) ambient. For other ambient temperatures, multiply the table ampacity by the factor below (NEC 2023 Table 310.15(B)(1)(1)):

Ambient temperature (°C)	Factor — 60 °C conductors	Factor — 75 °C conductors	Factor — 90 °C conductors
10 or less	1.29	1.20	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	1.00	1.00	1.00
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

More than three current-carrying conductors (NEC 310.15(C)(1))

When more than three current-carrying conductors are bundled in the same raceway or cable, multiply the ampacity by the adjustment factor below:

Current-carrying conductors	Adjustment factor (% of Table 310.16 ampacity)
4–6	80%
7–9	70%
10–20	50%
21–30	45%
31–40	40%
41 and above	35%

Copper vs aluminum

Aluminum has roughly 61% of copper's conductivity, so an aluminum conductor must typically be two AWG sizes larger to carry the same current: where 12 AWG copper carries 20 A at 60 °C, you need 10 AWG aluminum (25 A at 60 °C) for the same 20 A circuit. Aluminum is common for large feeders and service entrances (4/0 AWG aluminum at 75 °C is rated 180 A) where its lower cost and weight outweigh the larger size; branch circuits are almost always copper.

Common circuits — quick reference

Breaker size (A)	Minimum copper wire (60 °C basis)	Typical uses
15 A	14 AWG	Lighting circuits, general receptacles
20 A	12 AWG	Kitchen, bathroom, and garage receptacles
30 A	10 AWG	Electric dryer, water heater
40 A	8 AWG	Electric range, heat pump
50 A	6 AWG	Range, hot tub, EV charger (40 A continuous)
60 A	4 AWG	Subpanel feeder, large EV charger (48 A continuous)

Remember that 14, 12, and 10 AWG copper are capped at 15 A, 20 A, and 30 A overcurrent protection respectively regardless of the table column (NEC 240.4(D)), and long runs may need a larger conductor for voltage drop even when the ampacity is sufficient.

Last verified against NEC 2023 Table 310.16, Table 310.15(B)(1)(1), and Table 310.15(C)(1). Results are for planning; always confirm with the current NEC edition and your local authority having jurisdiction.