Voltage drop: a worked example with NEC Chapter 9, Table 8

Voltage drop is the voltage lost in the wire itself between the panel and the load. On short runs it is too small to matter. On long runs it dims lights, slows motors and makes equipment run hot. This guide works one example by hand so you can check any run, then shows when to size up.

How much drop is acceptable

For most circuits the NEC does not set a hard limit. An informational note to NEC 210.19(A) suggests sizing branch circuits for no more than 3 percent drop and no more than 5 percent combined across feeder and branch circuit. Informational notes are advice, not rules, but most engineers design to them. A few specific installations do carry enforceable limits, such as sensitive electronic equipment (647.4(D)) and fire pumps (695.7).

The formula

For a single-phase circuit (and DC), current flows out and back, so the wire length counts twice:

VD = 2 x I x R x L ÷ 1,000

• VD: voltage drop in volts
• I: load current in amps
• R: conductor resistance in ohms per 1,000 ft, from NEC Chapter 9, Table 8
• L: one-way length of the run in feet

For three-phase circuits, replace the 2 with 1.732 (the square root of 3).

Table 8 gives DC resistance at 75 °C. Some resistance values for stranded, uncoated conductors:

Worked example

A 120 V circuit feeds a 16 A load in a detached shop 150 ft from the panel. The 20 A breaker would normally call for 12 AWG copper. Check it:

1. VD = 2 x 16 x 1.98 x 150 ÷ 1,000
2. VD = 9.50 V
3. Percent drop = 9.50 ÷ 120 = 7.9 percent

That is well past the 3 percent guideline. The load will see about 110.5 V.

Try the next sizes up with the same numbers:

8 AWG lands right at the line and 6 AWG is comfortably under it. Many electricians would choose 8 AWG here and accept 3.1 percent; if the feeder to the panel already loses 1 to 2 percent, 6 AWG keeps the total under 5 percent.

When you upsize:

• The breaker stays at 20 A. A bigger conductor on the same breaker is always allowed.
• The equipment grounding conductor has to grow with them. NEC 250.122(B) says that when the hot conductors are upsized for voltage drop, the ground goes up in proportion to their circular mil area. Going from 12 AWG to 6 AWG hots means a 6 AWG ground too, not the 12 AWG that Table 250.122 gives for a 20 A breaker.
• Larger conductors may not fit the device terminals. A 20 A receptacle will not take 6 AWG, so you land the large conductor in a box and splice to a short 12 AWG tail with a connector listed for the combination.

Do the same with 240 V

Moving a load to 240 V halves the current for the same power, and the drop in percent falls by three quarters. The same 1,920 W load at 240 V draws 8 A: on 12 AWG over 150 ft that is 4.75 V, or 2.0 percent. If the equipment can run on 240 V, that is often cheaper than larger wire.

What the formula leaves out

Table 8 is DC resistance. On AC circuits with large conductors (about 1/0 and up), especially in steel conduit, reactance adds to the drop. For those, NEC Chapter 9, Table 9 gives an effective impedance to use instead. For branch circuits in the sizes above, the DC figure is close enough.

Check your own run

Our voltage drop calculator uses the same Table 8 values for copper and aluminum, single and three phase. Enter the load, the length and the size, and it shows the drop and the next size up.

Shop the sizes in this example

• 12 AWG THHN/THWN-2, 10 AWG, 8 AWG and 6 AWG
• For a direct-buried run to the shop, 10/2 UF-B or 8/2 UF-B

Verify with your local code. Voltage drop is usually a design choice, but some jurisdictions and energy codes make it a requirement. This example is general information, not a design for your installation.