Wire Size Calculator — AWG Gauge, Voltage Drop & Conduit

How to Determine Wire Size for Electrical Circuits

Choosing the correct wire size is critical for safety and performance. Undersized wire overheats, creating a fire hazard. Oversized wire wastes money. The right wire gauge depends on four factors: amperage (current draw), distance (wire run length), voltage, and acceptable voltage drop. This guide explains how electricians size wire for residential and commercial circuits.

AWG Wire Size Chart — Ampacity (75°C, THWN/THHN)

AWG Gauge	Diameter (in)	Copper Amps	Aluminum Amps	Typical Use
14	0.064	15A	—	15A lighting circuits
12	0.081	20A	15A	20A outlets, kitchen
10	0.102	30A	25A	Dryers, water heaters
8	0.128	40A	30A	Range, large AC units
6	0.162	55A	40A	Sub-panels, large appliances
4	0.204	70A	55A	Feeders, sub-panels
3	0.229	85A	65A	Large sub-panels
2	0.258	95A	75A	Service entrance (100A AL)
1	0.289	110A	85A	Large feeders
1/0	0.325	125A	100A	Service entrance (100A CU)
2/0	0.365	145A	115A	150A sub-panels
3/0	0.410	165A	130A	200A service (AL)
4/0	0.460	195A	150A	200A service (CU)
250 MCM	0.500	215A	170A	Large commercial feeders

Voltage Drop Reference Table (copper, 240V, 3% max)

Wire Size	20A	30A	40A	50A
14 AWG	34 ft	—	—	—
12 AWG	54 ft	36 ft	—	—
10 AWG	86 ft	57 ft	43 ft	34 ft
8 AWG	137 ft	91 ft	68 ft	55 ft
6 AWG	218 ft	145 ft	109 ft	87 ft
4 AWG	346 ft	231 ft	173 ft	138 ft
2 AWG	551 ft	367 ft	276 ft	220 ft

*Maximum one-way distance before exceeding 3% voltage drop at 240V. At 120V, halve these distances.

Common Circuit Sizing

Application	Amps	Wire (Cu)	Wire (Al)	Breaker
General lighting	15A	14 AWG	12 AWG	15A
Kitchen/bath outlets	20A	12 AWG	10 AWG	20A
Window AC (small)	20A	12 AWG	10 AWG	20A
Electric dryer	30A	10 AWG	8 AWG	30A
Water heater	30A	10 AWG	8 AWG	30A
Electric range/oven	40–50A	8–6 AWG	6–4 AWG	40–50A
Central AC (3 ton)	30–40A	10–8 AWG	8–6 AWG	30–40A
EV charger (Level 2)	40–50A	8–6 AWG	6–4 AWG	50–60A
Sub-panel (60A)	60A	6 AWG	4 AWG	60A
Sub-panel (100A)	100A	3 AWG	1 AWG	100A
Main service (200A)	200A	4/0 AWG	4/0 AWG	200A

Conduit Sizing Guide

Wire Size	2 Wires	3 Wires	4 Wires
14–12 AWG	1/2" EMT	1/2" EMT	1/2" EMT
10 AWG	1/2" EMT	1/2" EMT	3/4" EMT
8 AWG	1/2" EMT	3/4" EMT	3/4" EMT
6 AWG	3/4" EMT	1" EMT	1" EMT
4 AWG	1" EMT	1" EMT	1-1/4" EMT
2 AWG	1" EMT	1-1/4" EMT	1-1/2" EMT
1/0–4/0	1-1/4"–2" EMT	1-1/2"–2" EMT	2"–2-1/2" EMT

Copper vs. Aluminum Wire

Factor	Copper	Aluminum
Conductivity	100% (reference)	61% of copper
Weight	Heavier	30% lighter
Cost per foot	Higher ($$$)	Lower ($–$$)
Minimum gauge for 30A	10 AWG	8 AWG
Connectors	Standard	Must be AL/CU rated
Best for	Branch circuits, short runs	Service entrance, large feeders
Fire risk	Low	Higher if improperly connected

Wire Sizing Tips

• Continuous loads require 125% sizing. If a load runs for 3+ hours (water heater, AC, EV charger), the NEC requires the circuit to be sized at 125% of the load. A 30A continuous load needs a 40A circuit.
• Voltage drop matters more at 120V. The same wire at 120V has twice the percentage voltage drop compared to 240V. Long 120V runs often need upsized wire.
• Use THHN/THWN in conduit. NM-B (Romex) can only be used in dry locations inside framing. For conduit runs, garages, and exposed wiring, use individual THHN or THWN conductors.
• Derate for bundling. More than 3 current-carrying conductors in conduit require ampacity derating per NEC Table 310.15(C)(1). This can reduce ampacity by 20–50%.
• Ground wire can be smaller. Equipment grounding conductors follow NEC Table 250.122 and are typically 1–2 sizes smaller than the circuit conductors.