Size Electrical Wire Calculator

What to calculate

Load current (A)The continuous plus non-continuous load the conductor must carry.

Continuous load (3 hours or more)

Conductor material

Conductor sizeUsed by the ampacity and derating modes.

Conductor insulation rating

Termination temperature ratingNEC 110.14(C): the ampacity used for sizing may not exceed the Table 310.16 value at the lowest-rated termination.

Ambient temperature (degC)

Current-carrying conductors in the raceway or cable

System voltage (V)

One-way circuit length (ft)

Circuit type

Allowable voltage drop (%)

Overcurrent device rating (A)Used by the equipment grounding conductor mode (NEC Table 250.122).

Gauge system

Gauge numberSheet steel gauge runs 3 to 38; AWG runs 4/0 (entered as -3) to 38.

Recommended conductor

8 AWG

Wire Size & Gauge Calculator

Recommended conductor area

16,510 cmil

Required ampacity

40.00 A

Table 310.16 ampacity

55 A

Ambient correction factor

1.0000

Conductor-count adjustment factor

1.000

Corrected ampacity

55.000 A

Usable ampacity after termination limits

50.000 A

Voltage drop at the recommended size

2.604 %

Size needed for the voltage-drop limit

8 AWG

Equipment grounding conductor

10 AWG

Worked out from NFPA 70, National Electrical Code — Table 310.16, Allowable Ampacities of Insulated Conductors and 4 other sources.

Reference data is transcribed from the 2023 NFPA 70 tables. Confirm the values against the edition your authority having jurisdiction has adopted.

The equipment grounding conductor shown is read from the required ampacity as a stand-in for the overcurrent device rating; size it from the device actually installed, and increase it proportionally under 250.122(B) when the phase conductors are upsized for voltage drop.

Advisory only. Conductor selection depends on the NFPA 70 edition adopted by your authority having jurisdiction, the wiring method, termination listings, conduit fill, fault duty and load classification. A licensed electrical professional must approve any installed design.

How this is worked out

5 steps

Required ampacity = 40 A.
Smallest copper conductor whose corrected ampacity reaches 40 A: 8 AWG (55 A at 90 degC x 1.0000 ambient x 1 bundling = 55.00 A, held to 50.00 A by the terminations).
Voltage drop at 8 AWG over 100 ft: 2.604 % against a 3 % allowance.
Ampacity governs; the voltage-drop allowance is already satisfied at this size.
Equipment grounding conductor for a 40 A device (NEC Table 250.122): 10 AWG.

Results are general information, not financial, tax, legal, medical or engineering advice.