How to Size Electrical Cables per NEC

Oversize a cable and you waste money on copper and larger conduit. Undersize it and you get overheating, insulation failure, or a fire. The NEC gives you a systematic process to land on the right size — but the tables and correction factors trip up even experienced engineers.

Cable sizing per NEC comes down to four steps: determine your load current, look up the base ampacity, apply derating factors, and verify voltage drop. Here's how each step works.

Step 1: Determine the Load Current

Start with the actual current the conductor will carry based on the load calculation. For motor circuits, use NEC Table 430.250 (three-phase) or 430.248 (single-phase) for full-load current rather than the motor nameplate — NEC 430.6(A)(1) requires table values for standard conductor sizing.

For continuous loads (operating for 3 hours or more), conductors must be sized for at least 125% of the continuous load per NEC 210.19(A)(1) for branch circuits and 215.2(A)(1) for feeders. A 100A continuous load requires conductors sized for 125A.

Step 2: Look Up Base Ampacity

The primary table is NEC Table 310.16 (formerly Table 310.15(B)(16) in pre-2020 editions). It lists allowable ampacity for insulated conductors rated 0-2000V in raceway, cable, or direct burial, with not more than three current-carrying conductors, at 30°C ambient.

Choose your column based on conductor material (copper or aluminum) and insulation temperature rating:

Important: even with 90°C-rated wire, you may be limited to the 75°C column if your terminals are rated 75°C (which most are). NEC 110.14(C)(1) requires conductor ampacity to be based on the lowest-rated component in the circuit — usually the termination.

Select a conductor size whose ampacity equals or exceeds your design current from Step 1.

Step 3: Apply Derating Factors

Two conditions reduce the base ampacity:

Temperature correction (NEC 310.15(B)(1))

If the ambient temperature where the cable is installed exceeds 30°C (86°F), you must reduce the ampacity. The correction factors are in Table 310.15(B)(1). For example, at 40°C ambient, the correction factor for 75°C-rated conductors is 0.88.

Adjusted ampacity = base ampacity x temperature correction factor.

This matters in hot environments: attics, boiler rooms, and outdoor installations in warm climates. Rooftop installations may require an additional temperature adder before applying correction factors — check NEC 310.15 for rooftop-specific provisions.

Bundling adjustment (NEC 310.15(C)(1))

When more than three current-carrying conductors share a raceway or cable, heat buildup reduces each conductor's capacity. The adjustment factors from Table 310.15(C)(1):

When both corrections apply, multiply them together: adjusted ampacity = base ampacity x temperature factor x bundling factor.

A useful strategy: start with the 90°C column ampacity (higher starting value), apply derating factors, then verify the result doesn't exceed the 75°C column value for termination limits. This gives you the most usable ampacity from your conductor.

Step 4: Verify Voltage Drop

NEC 310.16 sizes cables for heat. Voltage drop is a separate check. NEC recommends no more than 3% on branch circuits and 5% total from the service entrance to the final outlet.

Use: Vd = 1.732 x L x I x Z / 1000 (three-phase) or Vd = 2 x L x I x Z / 1000 (single-phase), with Z from NEC Chapter 9, Table 9.

If voltage drop exceeds the recommendation, upsize the conductor — even though it already passes the ampacity check. For a detailed walkthrough, see our voltage drop calculation guide. Or use the ekx cable sizing calculator to handle ampacity, derating, and voltage drop checks together.

Worked Example

Size a feeder for a 150A continuous load on a three-phase, 480V system. Copper THHN conductors in steel conduit. Ambient temperature is 35°C. Three current-carrying conductors. Run length is 200 feet.

Step 1: Design current = 150A x 1.25 = 187.5A (continuous load factor).

Step 2: From NEC 310.16 at 75°C column (terminal limit), 3/0 AWG copper = 200A.

Step 3: Ambient is 35°C. Temperature correction factor for 75°C conductors at 35°C = 0.94. Only three current-carrying conductors, so no bundling adjustment. Adjusted ampacity of 3/0 = 200 x 0.94 = 188A. That just barely exceeds 187.5A — technically passing but with no margin.

Using the 90°C approach: THHN at 90°C, 3/0 AWG = 225A. After derating: 225 x 0.96 (90°C factor at 35°C) = 216A. That exceeds the 200A terminal limit, so we're capped at 200A. Still passes with better margin.

For comfort, you might select 4/0 AWG (230A at 75°C) to avoid razor-thin margins.

Step 4: Voltage drop for 3/0 AWG at 200 feet, 150A (actual load, not the 125% design value): Vd = 1.732 x 200 x 150 x 0.082 / 1000 = 4.26V, or 0.89%. Well within 3%.

Final selection: 3/0 AWG copper THHN in steel conduit. 4/0 if you want margin.

Key Takeaways

• Start with the actual load current — use NEC motor tables for motors, multiply by 1.25 for continuous loads
• Base ampacity comes from NEC 310.16 — match the column to your termination temperature rating, not the wire insulation rating
• Derate for ambient temperature above 30°C and for more than three current-carrying conductors in a raceway
• Always verify voltage drop separately, especially on runs over 100 feet
• Use 90°C-rated wire with derating math for the most efficient conductor utilization