Wire Gauge Size Calculator (AWG)

What Is AWG (American Wire Gauge)? AWG is the standardized system used in North America to specify wire diameter. Counterintuitively, a lower AWG number means a thicker wire, so AWG 4 is much thicker than AWG 14. Thicker wire carries more current and has lower resistance per foot.

This is the full-range version: copper or aluminium, 12V and 24V direct current alongside 120V and 240V, a drop limit you choose, an outdoor derate, and sizes to 4/0 with a pass or fail printed beside every gauge. If your job is an ordinary indoor copper branch circuit and you would rather type the run in metres, the electrical wire gauge calculator is the shorter road to the same answer. The two share their ampacity and resistance tables.

The Two Selection Criteria Correct wire sizing must satisfy two independent requirements:

1. Ampacity, the wire must safely carry the load current without overheating.
2. Voltage drop, the wire must not drop too much voltage along its length.

Failing either test means the selected gauge is too small.

Ampacity (Current Capacity) The NEC sets a maximum current for each AWG size, and which column applies depends on the cable. This calculator uses the 60°C copper column, because NM-B cable (the Romex in most houses) is limited to that under NEC 334.80 no matter what its conductors are rated for. THHN in conduit may use the higher 75°C column, so a commercial job can legitimately run a smaller wire than the answer here. Aluminum wire has roughly 80% the ampacity of same-gauge copper wire, requiring one or two gauge steps up for equivalent capacity.

Voltage Drop Calculation Voltage drop is caused by the resistance of the wire itself: VD = 2 × L × I × R_per_foot Where L is the one-way circuit length, I is the current, and R_per_foot is the wire resistance in ohms per foot. The factor of 2 accounts for both the hot and neutral (or return) conductors. Resistance per 1000 ft for uncoated stranded copper, from NEC Chapter 9 Table 8: AWG 14 = 3.14 Ω, 12 = 1.98, 10 = 1.24, 8 = 0.778, 6 = 0.491, 4 = 0.308, 3 = 0.245, 2 = 0.194, 1 = 0.154, 1/0 = 0.122, 2/0 = 0.0967, 3/0 = 0.0766, 4/0 = 0.0608. The 60°C copper ampacities that go with them, from NEC 310.16: 15, 20, 30, 40, 55, 70, 85, 95, 110, 125, 145, 165 and 195 amps in the same order. AWG 3 and AWG 1 are the two everyone forgets, and forgetting them costs money: an 80 amp load only needs AWG 3, and a jump straight to AWG 2 buys copper you did not have to. Table 8 assumes a 75°C conductor. Work the same wire from its 20°C resistivity and you get numbers about 20% lower, which is why a physics-based voltage drop calculator and a code-based one disagree on identical inputs. This page uses the code figures. Aluminum runs about 1.6 times the resistance of copper at the same gauge.

NEC Voltage Drop Limits For branch circuits: maximum 3% voltage drop is recommended (NEC 210.19). For feeders, designers usually aim at 2% so that feeder and branch together stay inside the 5% the code suggests. The 2% itself is convention rather than code. A 120V circuit with 3% max drop allows at most 3.6V drop (120 × 0.03).

Why Material and Application Matter Copper is preferred for most residential wiring: easier to terminate and less prone to oxidation. Aluminum is used for large feeders (200A service entrance, sub-panel feeds) to reduce cost and weight. Outdoor and direct-burial applications require cables with appropriate insulation ratings (USE-2, UF-B). Interior wiring uses NM-B (Romex) or THHN wire in conduit.

Picking Outdoor here also trims the ampacity by 10%. Treat that as a stand-in for a warm-ambient correction rather than a code figure. NEC 310.15(B) works from the actual ambient temperature, and at 31 to 35°C around a 60°C conductor the real factor is 0.82, appreciably harsher than 0.90. Direct burial has its own tables on top of that. A run sitting in full sun on a roof deserves the code tables, not this page.

Practical Rule of Thumb For runs under 100 feet at typical household loads, ampacity usually governs the selection. For long runs (over 50 feet on 120V, over 100 feet on 240V), voltage drop often requires a larger gauge than ampacity alone demands. Always round UP to the next available gauge, never down.