Estimate voltage at a remote DC device
A long power cable has resistance. This worksheet uses current, copper wire gauge and the outgoing and returning conductors to estimate how much source voltage is lost.
Low-voltage DC run
Estimate round-trip voltage drop for a two-conductor copper run. This is for direct DC feeds, not PoE or regulated power supplies.
Planning example & assumptions
For a 150 ft one-way run of 18 AWG copper carrying 0.5 A, the nominal round-trip drop is about 0.96 V. Compare the estimated voltage at the device with its documented minimum. Verify actual cable resistance, temperature and startup current. This calculation does not apply to PoE negotiation or AC circuits.
Common planning questions
Why is the one-way cable length doubled?
Current travels through an outgoing conductor and a returning conductor. The estimate uses both lengths to calculate round-trip resistance.
Can this calculate PoE or AC voltage drop?
This worksheet estimates direct DC power feeds using nominal copper resistance. Use equipment-specific design tools for PoE, AC circuits, regulated supplies and temperature-sensitive conditions.