Category
MEP calculators
MEP — mechanical, electrical and plumbing — is where the building meets its services. Today our MEP catalog focuses on electrical work: NEC-based wire sizing that finds the smallest AWG meeting both ampacity and voltage-drop limits, with copper and aluminum tables at 60, 75 and 90 °C insulation ratings. Plumbing flow and HVAC load tools are next on the roadmap.
All MEP calculators
Sizing a conductor: ampacity and voltage drop
Two checks govern wire size for any branch or feeder circuit:
- Ampacity — the conductor must safely carry the load current without exceeding its insulation temperature rating. NEC Table 310.16 lists the allowable ampacity for each AWG at 60, 75 and 90 °C, in copper and aluminum. For continuous loads (running three hours or more), the load is scaled by 1.25× per NEC 210.20(A) before this check.
-
Voltage drop — long runs lose voltage to resistance. The NEC
recommends a maximum 3 % drop for a branch and 5 % combined for feeder + branch
(informational, see 210.19). We compute drop using the standard formula
VD = (2 · K · I · D) / CMfor single-phase (and the √3 variant for three-phase), withK = 12.9for copper andK = 21.2for aluminum.
The wire-size calculator returns the smallest AWG that satisfies both checks — and shows you which check was binding, so you can decide whether to upsize conductor or shorten the run.
What these calculators don’t replace
Final electrical design must be reviewed and stamped (where required) by a licensed electrician or PE per local jurisdiction. NEC adoption varies by state and city — verify the edition and any amendments your AHJ enforces before committing to a design.