Power & Electrical calculator
Cable Gauge & Voltage Drop Calculator
Voltage drop on a cable run is 2 × current × conductor resistance × one-way length (current flows out and back). Pick the mode for an AC power run or a speaker line, choose the gauge, and this calculator returns the drop against the common 3% planning guideline.
Formulas
AC voltage drop (single phase)
Vd = 2 × I × R × L- I:
- load current in amps
- R:
- conductor resistance in ohms per foot
- L:
- one-way run length in feet
Speaker power loss vs ideal cable
loss% = (1 - (Rload / (Rcable + Rload))²) × 100- Rcable:
- round-trip cable resistance
- Rload:
- speaker nominal impedance
Speaker level loss
loss(dB) = 20 × log10(Rload / (Rcable + Rload))How it works
Every foot of copper has resistance, and the current has to travel down and back, so the effective length is double the run. The values in this tool are the NEC Chapter 9 Table 8 figures for stranded copper, the same numbers electricians use.
For AC power the common design guideline is to keep branch-circuit drop under 3% (about 3.6 V on a 120 V circuit) and total drop to the load under 5%. It is a performance recommendation, not a code mandate, but low voltage at the end of a 200 ft stinger run makes power supplies work harder and motors run hot.
For speaker lines the same resistance steals power transfer and damping. The percentage result compares power delivered to the speaker with an ideal zero-resistance cable, using the standard squared voltage-divider coefficient. An 8 ohm load tolerates long runs; a 2 ohm parallel load on thin cable can lose a fifth of its speaker power. This is the arithmetic behind running heavy speaker cable or moving the amps closer to the boxes.
Worked example: A 16 A continuous load at the end of 150 ft of 12 AWG on 120 V
- 1.12 AWG resistance: 1.98 ohms per 1000 ft = 0.00198 ohms per foot.
- 2.Vd = 2 × 16 × 0.00198 × 150 = 9.5 V.
- 3.Percent: 9.5 / 120 = 7.9%, far past the 3% guideline. 8 AWG cuts it to 3.7 V (3.1%).
12 AWG is undersized for that run; move up to 8 AWG or shorten the run.
Max one-way run for 3% drop at 120 V
| Load | 12 AWG | 10 AWG | 8 AWG |
|---|---|---|---|
| 10 A | 91 ft | 145 ft | 231 ft |
| 15 A | 61 ft | 97 ft | 154 ft |
| 20 A | 45 ft | 73 ft | 116 ft |
Field notes
- Distribute power, then convert: one 208 V run to a distro near the load beats four long 120 V stingers, in copper and in drop.
- Warm cable is the field symptom of a marginal run; a cable reel left coiled under load concentrates that heat into a fire risk. Uncoil reels in use.
Frequently asked questions
How much voltage drop is acceptable?
The common guideline is 3% on a branch circuit and 5% total from source to load (NEC informational note figures). At 120 V that is 3.6 V and 6 V respectively.
What gauge speaker wire do I need for a long run?
Keep insertion loss under about 0.2 dB. Into 8 ohms, 12 AWG reaches that limit at roughly 50 ft one way; 4 ohm loads halve the distance. Thicker cable or shorter runs beat heroic math.
Does voltage drop matter for LED fixtures and amps with switch-mode supplies?
Less than for motors and tungsten: switch-mode supplies regulate over a wide input range but draw more current as voltage falls, which increases cable heating. Deep drops still cause brownout resets at inrush.