Wire and Wire Gauge
Separate the conductor from its insulation, and understand what a wire gauge does and does not tell you.
The conductor carries current; insulation separates conductors
The conductor carries current; insulation separates conductors
The metal inside a wire is the conductor. Its resistance depends on material, length, cross-sectional area and temperature. The covering is insulation: it keeps the conductor from contacting other conductors or surrounding objects. The outside diameter includes that covering and does not by itself tell you how much metal is inside.
Solid wire has one solid conductor; stranded wire has several smaller strands forming a conductor. Insulation type and condition matter separately from the metal size. See Electrical Safety for use and inspection guidance.
A smaller AWG number means a thicker conductor
A smaller AWG number means a thicker conductor
American Wire Gauge (AWG) uses smaller numbers for larger conductor cross sections. For the same material, length and temperature, a thicker conductor has less resistance. See Adafruit’s wire-gauge explanation. Under comparable installation and temperature conditions, it can carry more current without exceeding its temperature limit.
Ampacity means the allowable current under specified conditions. Gauge alone is not an ampacity rating: material, insulation temperature rating, ambient temperature, bundling and installation all matter. Use the applicable manufacturer data and installation requirements for a real selection; this page is not a wire-sizing table. IEWC explains these ampacity factors. Choosing a wire involves voltage drop as well as allowable current.
Why a long wire can matter even below its current limit
A wire can stay within its temperature limit yet drop too much voltage for its load. Length increases resistance, and both voltage drop and heating follow from that resistance. The worked cord example in Current, Voltage, and Resistance shows the calculation.
Count the return wire: explore voltage drop and heating; Interactive; Practice
Count the return wire: explore voltage drop and heating
A two-conductor circuit carries current out and back. Use twice the one-way run for its wire resistance. At the same current, longer or thinner wire drops more voltage and wastes more power as heat. Voltage drop is a separate check from ampacity: get the current rating from the applicable code table or manufacturer for the insulation and installation.
| Single wire: Ω / 1000 ft | 6.385 |
|---|---|
| Single wire: Ω / km | 20.95 |
| Loop resistance (Ω) | 0.6385 |
| Voltage drop (V) | 3.192 |
| Source voltage lost (%) | 13.3 |
| Load voltage (V) | 20.81 |
| Wire heating loss (W) | 15.96 |
Worked example: 50 ft out + 50 ft back = 30.48 m of copper. R = ρL/A = 0.6385 Ω. At 5 A, IR = 3.192 V and I²R = 15.96 W.
Outline = baseline; filled = now. Each pair shares its own scale; compare within a pair. Loss is heating power, not wire temperature.
Doubling only current doubles the drop and quadruples heating loss. Doubling only length doubles both; a smaller AWG number lowers both at the same current.
This model assumes solid copper at 20 °C, equal outgoing and return wires, and DC/resistive behavior. Current is an input, not a prediction of how a particular load responds. Heating loss is not a temperature prediction; copper resistance rises with temperature (approx. 0.393%/°C near 20 °C). Connections and AC reactance are omitted.
Definitions used to calculate every result
For AWG number n, diameter d = 0.127 × 92(36 − n)/39 mm. Convert d to meters, then A = πd²/4 in m². Using annealed-copper resistivity ρ = 1.724 × 10−8 Ω·m at 20 °C, R = ρL/A. One foot is exactly 0.3048 m.
With one-way run ℓ, L = 2ℓ and Rloop = ρ(2ℓ)/A. Vdrop = IRloop; percent drop = 100Vdrop/Vsource; Vload = Vsource − Vdrop; wire loss P = I²Rloop. A two-wire single-phase resistive approximation uses the same return path, but this model does not include AC effects.
Practice: Loop resistance · Voltage drop · Percent drop · Load voltage · Choose a gauge.