What Voltage Drop Means
Voltage drop occurs because conductors have resistance. The longer the run, the higher the current, and the smaller the conductor, the more voltage is lost before power reaches the load.
The National Electrical Code includes informational voltage-drop recommendations for reasonable efficiency, commonly discussed as 3 percent for a branch circuit and 5 percent total for feeder plus branch circuit. Informational notes are guidance, not the same as enforceable code text unless adopted by a specification, local rule, or specific application requirement.
- Electricians use it for long branch circuits.
- Engineers use it for feeders, motors, and site distribution.
- Installers use it to compare copper and aluminum conductor sizes.
How to Calculate Voltage Drop
For single-phase circuits, voltage drop = 2 x K x I x D / CM. For three-phase circuits, voltage drop = 1.732 x K x I x D / CM. K is conductor resistivity, I is current, D is one-way distance in feet, and CM is conductor area in circular mils. Percent drop = voltage drop / system voltage x 100.
For example, a 24 amp single-phase load at 240 V is 100 feet from the source using copper conductors of 16,510 circular mils and K = 12.9. Voltage drop = 2 x 12.9 x 24 x 100 / 16,510 = 3.75 V. Percent drop is 3.75 / 240 x 100 = 1.56 percent.
Design Inputs That Matter
Use the load current that represents the design condition, not simply breaker size unless the breaker size is the appropriate design basis. Motor starting, continuous loads, power factor, conductor temperature, and raceway conditions may require deeper engineering than a simple DC-resistance approximation.
Conductor material matters because aluminum has higher resistance than copper for the same size. Distance should be the one-way circuit length in the formula shown, because the single-phase multiplier already accounts for the outgoing and return path.
- Check ampacity, terminals, and overcurrent protection separately.
- Use actual conductor size, not trade size of conduit.
- Consider both normal operation and starting or inrush where relevant.
Common Voltage Drop Mistakes
A common mistake is treating voltage drop guidance as a substitute for ampacity and code compliance. A larger conductor may reduce voltage drop but still must be installed with correct insulation, temperature rating, raceway fill, grounding, and termination rules.
Another error is double-counting distance. The single-phase formula with the factor 2 uses one-way distance. If you enter round-trip distance and keep the factor 2, the result will be twice as high.
- Do not ignore local amendments or project specifications.
- Do not use 120 V percent drop for a 240 V load.
- Do not forget transformers, feeders, and branch circuits in total drop.
Frequently asked questions
Is the NEC 3 percent voltage drop rule mandatory?
The familiar 3 percent and 5 percent values are commonly from informational guidance for efficiency, not general mandatory requirements. A project specification or local rule can still make a limit enforceable.
Should I use breaker amps or load amps?
Use the design load current appropriate to the calculation. Breaker size can overstate voltage drop if the actual load is much lower, but some designs intentionally use worst-case current.
Does voltage drop affect safety?
It can affect equipment operation and efficiency, but it is separate from ampacity and overcurrent protection. Both must be addressed.
Why is the three-phase formula different?
Balanced three-phase voltage drop uses the square root of 3 relationship between line and phase quantities, so the multiplier is 1.732 instead of 2.