Free tool
Voltage drop calculator for AV and security runs
Size a cable run in seconds. Enter the voltage, current, one-way length, and conductor, in AWG or mm², and get the drop in volts and percent, with a flag when you cross the 3% and 5% limits. Free, and nothing leaves your browser.
Voltage drop
4.45 V
Drop
1.93 %
Voltage at load
225.6 V
Within the 3% rule of thumb for feeders.
Copper and aluminium DC resistance at 20 °C. Warmer conductors read slightly higher. Estimate only, not a substitute for a qualified electrician.
Long runs lose voltage: this tells you how much
A speaker line, a PoE injector feed, or a DC supply to a remote display all lose voltage over distance because copper has resistance. Too much drop and the far end browns out or drops offline. This calculator uses the conductor's real resistance to show exactly how many volts you lose and what fraction of the supply that is.
The formula
- Drop = factor × current × resistance × length, with the run counted out and back.
- Single-phase and DC use a factor of 2; three-phase uses √3.
- Rule of thumb: keep feeders under 3% and total under 5%.
Figures use copper and aluminium DC resistance at 20 °C; a hot conductor reads a little higher. Planning the whole job? Avitly pulls cable lengths straight off the floor plan, so the runs you calculate here match the ones on your quote.
Common questions
What is an acceptable voltage drop?
The common rule of thumb is 3% on a feeder and 5% total from source to load. Those are guidelines, not law: check the standard you work to, since NEC, IEC, and local wiring regulations word the limit differently. Below 3% almost nothing misbehaves; above 5% you start seeing displays browning out, PoE devices dropping off, and amplifiers losing headroom.
Do I use one-way or round-trip cable length?
Enter the one-way length, the distance from source to load. The calculator doubles it for DC and single-phase AC because current flows out and back, and applies a factor of √3 for three-phase. Measuring the round trip yourself and entering that would double the drop.
How do I fix a voltage drop that is too high?
In order of how easy they are: use a thicker conductor (each AWG step down cuts resistance about 26%), shorten the run or reroute it, raise the supply voltage so the same power draws less current, or split the load across two circuits. Thicker cable is usually cheapest because the labour is identical.
Does temperature change the result?
Yes. This calculator uses DC resistance at 20 °C. Copper resistance rises roughly 0.4% per °C, so a conductor running at 60 °C in a warm ceiling void reads about 16% higher than shown. On runs that are already near the limit, size up.
Is copper or aluminium better for long runs?
Copper has about 61% more conductivity per unit area, so for the same drop, aluminium needs roughly two AWG sizes larger. Aluminium is cheaper and lighter for long feeders, but needs rated terminations and anti-oxidant compound. For AV and IT work inside a building, copper is almost always the right call.
Does this work for speaker cable and 70V lines?
Yes, for the DC and single-phase cases. Constant-voltage 70V and 100V distributed lines are exactly why the calculator exists: they run high voltage at low current so the drop stays small over long distances. Enter the line voltage and the total current the taps draw.
Design it once. Sell it, install it, service it.
Avitly runs the whole install off one floor plan: quote, client-signed offer, bill of materials, rack layout, then device credentials and tickets after handover. Presales to maintenance, no re-entry.
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