Conductor Derating and Voltage Drop: Size Wire That Passes Both Checks
Learn how NEC ampacity derating, IEC grouping factors, ambient temperature, and voltage-drop limits work together before you choose the final conductor size.
A wire size can pass the breaker-size habit and still be wrong. Ampacity derating asks whether the conductor can carry the load without overheating. Voltage drop asks whether the equipment still receives usable voltage at the far end. A professional design checks both, in that order.
For NEC work, mark NEC 310.16, 310.15(B), 310.15(C)(1), 110.14(C), and the voltage-drop informational notes in 210.19(A)(1) and 215.2(A)(1). For IEC-style projects, IEC 60364-5-52 is the practical reference for installation method, grouping, ambient correction, and voltage-drop targets. NEC IEC
Why Derating and Voltage Drop Are Different Checks
Derating is a thermal safety check. More loaded conductors, hotter ambient air, rooftop raceways, and insulation temperature limits reduce usable ampacity.
Voltage drop is a performance check. Long runs, high current, low system voltage, aluminum conductors, and motor starting current reduce voltage available at the load.
Upsizing for derating often improves voltage drop, but not always enough. A thermally legal 10 AWG branch circuit can still be unacceptable on a long 120V run.
Terminal temperature rules matter. Even when correction starts from a 90C THHN ampacity, NEC 110.14(C) can still force the final conductor onto a 60C or 75C terminal limit.
Code and Standard References to Put on the Worksheet
- NEC 310.16: start with the correct conductor ampacity table for material, insulation temperature, and terminal rating.
- NEC 310.15(B) and 310.15(C)(1): apply ambient-temperature correction and adjustment for more than three current-carrying conductors in a raceway or cable.
- NEC 210.19(A)(1) and 215.2(A)(1) informational notes: many designers target about 3% branch drop and 5% total feeder plus branch drop.
- IEC 60364-5-52: select the installation method, grouping factor, ambient correction, and voltage-drop limit before final cable size is accepted.
Field Examples Where One Check Is Not Enough
The numbers below are planning values, not a substitute for final code tables. They show why the calculator input should reflect the final conductor size after derating, not the first minimum size that fits a breaker.
| Circuit | Derating condition | Thermal result | Voltage-drop result |
|---|---|---|---|
| 20A branch circuit, 150 ft / 46 m | 9 current-carrying conductors, 40C | 10 AWG Cu passes ampacity after derating | 8 AWG Cu keeps 120V drop near 3.8% instead of 6.0% |
| 60A, 208V three-phase feeder, 220 ft / 67 m | 6 current-carrying conductors, 50C | 4 AWG Cu remains thermally legal | 2 AWG Cu cuts drop from about 3.4% to 2.1% |
| 32A, 400V IEC circuit, 50 m | 10 loaded conductors on tray, 45C | 10 mm2 Cu clears grouping and ambient factors | 10 mm2 stays near 1.2% voltage drop |
| 120V workshop receptacles, 100 ft / 30 m | 12 conductors in one raceway | 12 AWG can fail the adjustment check | 8 AWG may be needed when both heat and drop matter |
Worked Examples with Specific Numbers
20A receptacle run, 150 ft, 120V, 9 loaded conductors
With THHN copper at 40C and 9 current-carrying conductors, the 70% adjustment and about 0.91 ambient correction make 12 AWG too tight. 10 AWG can clear ampacity, but at 20A and 150 ft it drops roughly 7.2V, about 6.0%. 8 AWG lowers the drop to about 4.6V, or 3.8%.
60A, 208V three-phase feeder, 220 ft, warm mechanical room
Six current-carrying conductors require an 80% adjustment. At 50C, 4 AWG copper THHN can still be thermally legal for 60A, but voltage drop is about 7.0V line-to-line, or 3.4%. Moving to 2 AWG cuts the drop to about 4.4V, or 2.1%.
32A, 400V IEC circuit, 50 m tray route
A grouped tray route with 10 loaded conductors and 45C ambient can make 6 mm2 copper fail the corrected current rating. 10 mm2 copper clears the grouping and ambient check and keeps three-phase voltage drop near 4.9V, about 1.2% at 400V.
Design Checklist Before You Trust the Calculator Result
- Start with load current, continuous-load rules, conductor material, insulation rating, and terminal temperature limits.
- Apply ambient correction and conductor-count adjustment before using the selected size for voltage-drop math.
- Use one-way length in the calculator and choose the correct single-phase, three-phase, AC, or DC method.
- Recheck the total feeder plus branch drop when the panel is already far from the service or transformer.
- If the result is near a 3% or 5% target, step up one size before motors, LED drivers, or sensitive electronics complain.
Use the Final Derated Size in the Voltage Drop Calculator
First make the conductor thermally legal. Then enter that conductor size, actual current, voltage, material, phase, and one-way distance to confirm performance.