Allowable Voltage Drop: NEC 210.19 Requirements

NEC Voltage Drop References

• NEC 210.19(A)(1) FPN No. 4: Branch circuit conductors should be sized to limit voltage drop to 3% at the farthest outlet
• NEC 215.2(A)(2) FPN No. 2: Feeder conductors should be sized so combined feeder + branch voltage drop doesn't exceed 5%
• NEC 240.4: Overcurrent protection must match wire ampacity (not voltage drop)
• NEC 310.15(B): Ampacity adjustment factors for ambient temperature and conductor count

Impact of Excessive Voltage Drop

• Motors: Draw 10–15% more current at -10% voltage, overheating and reducing lifespan by 50%
• Incandescent lights: 10% voltage drop = 30% less light output
• LED lights: May flicker or fail to start below 90V (on 120V circuit)
• Electronics: Power supplies work harder, generating more heat
• Heating elements: 10% voltage drop = 19% less heat output (P = V²/R)
• Energy waste: Voltage drop = heat in the wire = wasted electricity. A 5% drop on a 20A circuit wastes 120W continuously

Frequently Asked Questions

What does NEC say about voltage drop?

NEC 210.19(A)(1) FPN No. 4 recommends (but doesn't require) 3% max voltage drop on branch circuits. NEC 215.2(A)(2) FPN No. 2 recommends 5% total for feeder + branch combined. These are Fine Print Notes (recommendations), not enforceable code.

Is voltage drop a code violation?

Exceeding 3% voltage drop is NOT a code violation — it's a recommendation (Fine Print Note). However, NEC 110.14(B) requires conductors to be sized for the load, and excessive voltage drop can cause equipment failure, overheating, and energy waste.

What is the difference between voltage drop and NEC ampacity?

Ampacity (NEC 310.16) is the maximum current a wire can safely carry without overheating. Voltage drop is the energy lost as heat in the wire. A wire can be within ampacity limits but still have excessive voltage drop on long runs.