Ampacity vs voltage drop: which one sets the wire size

The wire must pass both checks. Ampacity stops the wire from overheating. Voltage drop stops the load from being starved. Whichever check needs the bigger wire sets the size.

Two checks, two jobs

• Ampacity is the current the conductor can carry without overheating, after derating for temperature, conduit fill and continuous loads. Your code book gives the value.
• Voltage drop is the voltage lost along the run. It protects the equipment at the far end, such as a motor that will not start or lights that dim.

Short runs are usually set by ampacity. Long runs are usually set by voltage drop. Most real cases need a check of both.

Worked example: a long run where drop governs

A 40 A load sits 150 ft one way from the panel on a 240 V single-phase circuit in copper. The target drop is 3%.

The drop check needs at least 6 AWG to meet 3%, giving 5.90 V (2.46 %). Ampacity for 40 A is a separate check against the value in your code book for 6 AWG. If that value is below 40 A, the size must go up, even though the drop already passes.

Try the same numbers

Then enter the ampacity for the size you chose in the calculator’s optional field.

When the drop passes but ampacity fails

A short run can meet the drop target with a small wire that still carries too much current. Then ampacity sets the size. The calculator shows the drop, and the ampacity field tells you whether the current is within the rating you entered.

What the tool does and does not do

• It checks voltage drop with the formula on the method page.
• It checks ampacity only against the value you enter. It does not hold ampacity tables, which are copyrighted.
• It does not apply derating for temperature or conduit fill. Apply it from your code book before you enter the value.
• It does not check code compliance. Confirm the design with the authority having jurisdiction and a licensed electrician.

Sizes the tool covers

From 14 AWG to 500 kcmil: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG, 3 AWG, 2 AWG, 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG, 250 kcmil, 300 kcmil, 350 kcmil, 400 kcmil, 500 kcmil.