Voltage drop along a run, plus ampacity after derating. Cables are often sized by voltage drop rather than current — a cable that carries the load comfortably can still leave too little voltage at the far end.
Standard method — stated scope limits apply
Load & Route
Cable Data (from manufacturer)
Per-conductor values straight off the cable datasheet. Set X to 0 for a DC circuit.
Ampacity Derating (optional)
Base ampacity comes from the applicable cable table for your installation method — it isn't invented here.
Enter cable and load data.
About This Calculation
Sizing a power cable is really two independent checks, and the cable must pass both: current-carrying capacity (ampacity), so the conductor doesn’t overheat, and voltage drop, so equipment at the far end still sees an acceptable supply voltage. A cable sized only for ampacity can still fail on voltage drop over a long run — and vice versa on a short, heavily loaded run. Ampacity itself isn’t a single number: it’s derated for ambient temperature, grouping with other loaded cables, and installation method (buried, in conduit, in free air), all of which reduce how well the cable can shed heat.
Frequently Asked Questions
Which usually governs: ampacity or voltage drop?
Short, heavily loaded runs are usually governed by ampacity. Long runs, even at modest current, are often governed by voltage drop — the drop accumulates with distance regardless of how much thermal margin the cable has.
What voltage drop limit should I design to?
There is no single universal number, but 3% for a branch circuit and 5% total from source to the furthest load are common general guidelines (matching typical national wiring code recommendations) — always confirm against the actual governing code and any site-specific standard.
How much does grouping really derate a cable?
It can be substantial — a cable grouped tightly with several other loaded cables in the same tray or conduit can lose 30-50%+ of its free-air ampacity, because it can no longer shed heat to the surrounding air as effectively.
Does a bigger cable always fix a voltage drop problem?
Usually yes for resistive drop, since resistance falls as cross-sectional area rises — but at some point a larger conductor becomes impractical to terminate and the fix shifts toward reducing run length, raising system voltage, or splitting the load closer to source.