Voltage Drop Calculator

Start with the circuit basics. Choose the system voltage that matches the equipment nameplate or panel voltage, then choose single-phase or three-phase. Single-phase branch circuits use the full round-trip path in the voltage drop formula, while three-phase circuits use the 1.732 multiplier. Next, enter the load current in amps. Use the actual expected running load when checking voltage drop, not only the breaker size, unless you are doing an early planning estimate.

The Find Minimum Wire Size toggle is useful when you are designing a new run and want the smallest AWG that keeps branch circuit drop at or below 3%. Leave it off when you are checking an existing conductor size. The NEC panel then separates three checks: branch circuit voltage drop, combined 5% guidance, and conductor ampacity. That separation matters because a wire can pass voltage drop at a short distance while still being too small for the amperage.

Voltage drop is the voltage lost as current flows through the resistance of the conductor. Longer runs, higher current, smaller wire, and aluminum conductors all increase the drop. SpecMath uses the standard circular mil formula with K equal to 12.9 for copper and 21.2 for aluminum. The length entered is one-way from the source to the load because the single-phase multiplier already accounts for the return path.

That example shows why long 120V circuits can need a larger conductor even when the breaker ampacity looks acceptable. A 20 amp circuit on 12 AWG copper may be legal by ampacity, but the load at the far end can still receive too little voltage for good performance.

American Wire Gauge sizes get larger as the number gets smaller. Circular mil area is what the voltage drop formula uses, because conductor area directly affects resistance. The ampacity values below are copper values from the 75 degree C column style used for many equipment terminations; aluminum is estimated at 80% of the equivalent copper value in the calculator.

Wire upsizing is not only about passing inspection. It protects equipment, reduces wasted energy, and keeps motors, compressors, lighting, and electronics operating closer to their intended voltage. A circuit that is technically protected by the breaker can still perform poorly if the run is long enough.

• Upsize when the run exceeds 50 feet on a 15A or 20A branch circuit, especially at 120 volts.
• Upsize when the run exceeds 100 feet on any circuit, regardless of amperage, before finalizing the design.
• Upsize motor loads because starting current can run 6 to 8 times higher than running current.
• Upsize continuous loads that run 3 or more hours because NEC load sizing often uses 125% of continuous load.
• Treat aluminum differently from copper and verify every termination is rated for the conductor material.

For detached garages, sheds, outbuildings, pools, pumps, and long landscape lighting runs, it may be cheaper and cleaner to install a larger feeder to a small subpanel rather than running several long branch circuits from the main panel.

Voltage drop mistakes usually come from treating breaker size as the whole design. Breaker size protects the conductor from overcurrent, but it does not guarantee that the load receives enough voltage at the end of a long run. The farther the equipment is from the panel, the more the calculation matters.

• Using 12 AWG for a 100-foot garage circuit without checking voltage drop first.
• Forgetting that feeder drop and branch circuit drop add together along the same circuit path.
• Sizing wire only to the breaker rating and ignoring conductor length, phase, and material.
• Skipping temperature and conduit derating when many current-carrying conductors share one raceway.
• Entering round-trip length even though the formula already accounts for the return conductor on single-phase circuits.

Long runs reward planning. If several loads are located far from the main panel, a dedicated feeder and subpanel can reduce material complexity and leave room for future circuits. Aluminum SER or feeder cable may be cost-effective for larger subpanel runs, while copper is often preferred for smaller branch circuits and tighter terminations.

Always plan the full installation, not just the math. Conduit bends, pull boxes, conductor fill, grounding, disconnect requirements, burial depth, wet-location ratings, and equipment nameplates can all change the final material list. The calculator gives the voltage and AWG starting point; the finished job still needs code-compliant installation details.