How to Size a Cable: A Step-by-Step Walkthrough

Cable sizing comes down to two checks: ampacity and voltage drop. Ampacity tells you the minimum conductor size that won't overheat. Voltage drop sometimes forces you to a larger size than ampacity alone would require. You run both checks, then use the larger result.

This walkthrough goes through a complete example so you can see how the two interact.

The setup: a 30 A circuit, 120 feet from the panel

Say you're adding a 30 A, 240 V circuit, a new HVAC disconnect, and the run from the panel to the unit is 120 feet. You're using copper conductors in conduit. The ambient temperature in the conduit area is normal (under 30°C) and there are only two current-carrying conductors in the conduit.

Step 1: Find the design current

The design current is the expected continuous load, derated to 125% if it runs continuously for 3 hours or more. An HVAC unit in most climates runs continuously in summer, so it qualifies.

Say the unit's nameplate shows 24 A running current. Apply the 125% factor:

Design current = 24 A × 1.25 = 30 A

Your breaker and conductors must be sized for 30 A minimum.

Step 2: Pick the minimum conductor size by ampacity

From NEC Table 310.12, copper conductors at 75°C in conduit:

10 AWG handles exactly 30 A at 75°C. That's your ampacity floor. Under normal conditions (no additional derating), 10 AWG passes this check.

If there were additional conductors in the conduit requiring a derating factor, you'd apply that now and might need to step up. With only two conductors, no adjustment is needed.

Step 3: Calculate voltage drop for the ampacity-selected wire

10 AWG copper has a cross-section of 10,380 circular mils. For a 240 V single-phase circuit:

V_drop = (2 × K × I × L) / A
V_drop = (2 × 12.9 × 30 × 120) / 10,380
V_drop = 92,880 / 10,380
V_drop ≈ 8.95 V

As a percentage of 240 V: 8.95 / 240 = 3.7%

That's above the 3% guideline. 10 AWG passes the ampacity check but fails the voltage drop check at this run length.

Step 4: Find the size that meets the voltage drop target

You need V_drop ≤ 3% of 240 V = 7.2 V. Rearrange the formula to solve for the required area:

A = (2 × K × I × L) / V_drop_max
A = (2 × 12.9 × 30 × 120) / 7.2
A = 92,880 / 7.2
A ≈ 12,900 circular mils

8 AWG copper has 16,510 circular mils, which exceeds the requirement. 8 AWG it is.

Check: V_drop with 8 AWG:

V_drop = (2 × 12.9 × 30 × 120) / 16,510
V_drop = 92,880 / 16,510
V_drop ≈ 5.63 V → 2.3%

Under 3%. The circuit uses 8 AWG copper, even though ampacity alone would have allowed 10 AWG.

Step 5: Choose the larger result

This is the step people skip. Ampacity said 10 AWG. Voltage drop said 8 AWG. Use 8 AWG.

The rule: ampacity and voltage drop each set a minimum. Use whichever minimum is larger (numerically lower gauge number).

A wire size calculator runs both checks simultaneously and returns the correct result directly, useful when you're working through multiple circuits or want to compare copper versus aluminum.

Where derating changes the picture

The example above had clean conditions. Real jobs often don't.

Multiple conductors in conduit. If you're pulling five current-carrying conductors in the same conduit, NEC 310.15(C)(1) requires derating to 50% of base ampacity. For 10 AWG that's only 15 A, you'd need to step up on the ampacity check before even running the voltage drop numbers.

High ambient temperature. In an attic at 45°C, a 75°C-rated conductor gets derated by a factor of about 0.71. 10 AWG base ampacity of 30 A becomes 21 A. Not enough for a 30 A circuit.

Aluminum instead of copper. If you switch to aluminum, you need two gauge sizes larger for equivalent ampacity (8 AWG aluminum handles roughly 30 A). The K constant for aluminum is 21.2 instead of 12.9, so the voltage drop will be higher for the same gauge. You'd redo the voltage drop check with 8 AWG aluminum.

A common mistake: stopping at ampacity

The ampacity table in the NEC is often treated as the complete answer to conductor sizing. It's not, it's half the answer. For short runs (under 30–40 feet for most circuits), voltage drop is usually negligible and ampacity alone is fine. For longer runs, the voltage drop check is the one that actually drives the decision.

In the example above, skipping the voltage drop check and installing 10 AWG would put the HVAC unit 3.7% below rated voltage at full load. Over a summer of continuous operation, that's worth getting right.

Summary

  1. Calculate design current (multiply by 1.25 if continuous load)
  2. Look up minimum AWG from NEC ampacity table, apply derating if needed
  3. Calculate voltage drop for that wire size
  4. If drop exceeds 3%, solve for the required area and find the next AWG up
  5. Use whichever size is larger

That's the whole procedure. The math is straightforward enough to do by hand for a single circuit; for multiple circuits or comparing materials, use a cable sizing calculator to work through the combinations faster.

Conductor sizing should be verified against the current NEC and the code adopted by your local jurisdiction. These calculations are estimates for planning purposes. A licensed electrician should review all permanent installations.