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Voltage Drop Calculator
NEC-compliant. Checks the 3% branch circuit limit and 5% feeder + branch combined limit.
Voltage Drop
What is voltage drop?
Voltage drop is the reduction in voltage along a wire run caused by the wire's resistance. The longer the run or the smaller the wire, the more voltage is lost before it reaches the load.
NEC limits
- 3% — NEC recommends no more than 3% drop on a branch circuit (NEC 210.19, 215.2 informational notes)
- 5% — No more than 5% total across feeder and branch circuit combined
These are recommendations, not hard requirements in most jurisdictions — but inspectors and engineers expect them to be met.
How to reduce voltage drop
- Increase wire gauge (e.g., from 12 AWG to 10 AWG)
- Shorten the run length where possible
- Move the panel closer to the load
- Increase the supply voltage (e.g., use 240V instead of 120V)
Formula used
Single-phase: VD = (2 × I × R × L) / 1000
Three-phase: VD = (√3 × I × R × L) / 1000
Where R = resistance per 1,000 ft (copper, 75°C), L = one-way length in feet.
Use this calculator on every job — inside CircuitDesk
CircuitDesk includes this voltage drop calculator plus wire sizing, conduit fill, box fill, panel load, and more — all inside the same app where you manage jobs, quotes, and invoices.