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Category Electronics

Load Current

Amperage draw of the circuit

System Voltage

Nominal voltage (120, 240, 208, 480)

One-Way Run Distance

Distance from panel to load

Conductor Material Copper is standard residential

Insulation Temperature Rating Check wire jacket markings

Phase Residential is typically single

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About

Undersized conductors cause resistive heating proportional to I² × R. This is not an inconvenience. It is the ignition mechanism behind thousands of residential electrical fires annually. NEC Table 310.16 specifies maximum allowable ampacities for insulated conductors rated 0 - 2000 V, and compliance is not optional. This calculator performs a direct ceiling-match lookup against that table for both copper and aluminum conductors across 60°C, 75°C, and 90°C insulation temperature ratings. It then validates the result against NEC’s recommended maximum 3% voltage drop threshold for branch circuits given your one-way run distance and system voltage.

The tool assumes no more than three current-carrying conductors in a raceway at an ambient temperature of 30°C. Derate factors for conduit fill, ambient temperature above 30°C, or continuous loads (125% rule) are the user’s responsibility. Pro tip: for continuous loads exceeding 3 hours, multiply your amperage by 1.25 before entering it here.

Formulas

Wire sizing follows a two-step process: ampacity matching and voltage drop verification.

Step 1 - Ampacity Match: Select the smallest AWG gauge from NEC Table 310.16 where:

I_rated ≥ I_load

Step 2 - Voltage Drop Check:

V_drop = 2 × L × I × ρA

V_drop% = V_dropV_source × 100

If V_drop% > 3%, upsize the conductor until the threshold is met.

Where L = one-way cable run length in ft, I = load current in A, ρ = conductor resistivity in Ω⋅ft, A = cross-sectional area in cmil, and V_source = system voltage. The factor of 2 accounts for the complete circuit (hot + neutral). For 3-phase circuits, replace 2 with 1.732 (√3).

Reference Data

AWG	Area (kcmil)	Cu 60°C (A)	Cu 75°C (A)	Cu 90°C (A)	Al 60°C (A)	Al 75°C (A)	Al 90°C (A)	Ω/1000ft Cu	Typical Use
18	1.62	7	10	14	-	-	-	6.385	Thermostats, doorbells
16	2.58	10	13	18	-	-	-	4.016	Extension cords (light)
14	4.11	15	20	25	-	-	-	2.525	15A branch circuits, lighting
12	6.53	20	25	30	15	20	25	1.588	20A branch circuits, outlets
10	10.38	30	35	40	25	30	35	0.999	Dryers, water heaters, 30A circuits
8	16.51	40	50	55	30	40	45	0.628	Ranges, large appliances
6	26.24	55	65	75	40	50	60	0.395	A/C units, sub-panels
4	41.74	70	85	95	55	65	75	0.249	Feeders, large A/C
3	52.62	85	100	115	65	75	85	0.197	Service entrance
2	66.36	95	115	130	75	90	100	0.156	Service entrance, feeders
1	83.69	110	130	145	85	100	115	0.124	Service entrance, sub-panels
1/0	105.6	125	150	170	100	120	135	0.098	200A residential service
2/0	133.1	145	175	195	115	135	150	0.078	200A service, feeders
3/0	167.8	165	200	225	130	155	175	0.062	Large feeders, commercial
4/0	211.6	195	230	260	150	180	205	0.049	200A+ service entrance
250 kcmil	250	215	255	290	170	205	230	0.042	Commercial feeders
300 kcmil	300	240	285	320	190	230	255	0.035	Large commercial service
350 kcmil	350	260	310	350	210	250	280	0.029	Industrial feeders
400 kcmil	400	280	335	380	225	270	305	0.026	Industrial service
500 kcmil	500	320	380	430	260	310	350	0.021	Heavy industrial

Frequently Asked Questions

The insulation temperature rating determines how much heat the conductor can dissipate before insulation degradation occurs. A wire rated at 90°C (e.g., THHN) can carry more current than the same gauge rated at 60°C (e.g., TW) because its insulation tolerates higher operating temperatures. However, NEC 110.14(C) restricts termination ratings: most residential terminals are rated 75°C, so you must use the 75°C column even with 90°C wire unless the equipment is specifically listed for higher temperatures.

NEC 210.20(A) requires that conductors supplying continuous loads (loads expected to run for 3 hours or more) be sized at 125% of the continuous load current. For example, a 40A continuous load requires wire rated for at least 50A. Enter 50 into this calculator if your 40A load is continuous.

Voltage drop increases linearly with distance per V_drop = 2 × L × I × R. NEC recommends no more than 3% drop for branch circuits and 5% total for feeder plus branch. A 20A circuit on 12 AWG copper is fine at 50ft, but at 150ft you may need to upsize to 10 AWG or even 8 AWG to stay within 3%.

Yes, but aluminum has approximately 61% the conductivity of copper, so you typically need to go up 2 AWG sizes. Aluminum also requires anti-oxidant compound at terminations and connectors rated for AL/CU. Aluminum is common and cost-effective for service entrance cables (e.g., SER) and large feeders above 4 AWG. NEC prohibits aluminum for 18, 16, and 14 AWG branch circuits.

This calculator uses NEC Table 310.16 baseline values assuming no more than 3 current-carrying conductors in a raceway at 30°C ambient. You must manually apply NEC Table 310.15(C)(1) for ambient temperatures above 30°C and Table 310.15(C)(1) for conduit fill with more than 3 conductors. For example, 4 - 6 conductors in a conduit require an 80% derating factor.

Yes. For single-phase circuits, the formula uses a multiplier of 2 (hot + neutral). For 3-phase circuits, the multiplier is √3 (1.732). This calculator supports both configurations. The 3% recommendation still applies per NEC 210.19(A) Informational Note No. 4, though the actual drop will be lower in a balanced 3-phase system.