What size wire for this load?
Enter the load, voltage and run length. Get the smallest conductor that meets NEC 2023 ampacity at your termination rating, after ambient and conductor-count factors, then checked for voltage drop.
Most equipment terminals are rated 75°C, and 110.14(C)(1) limits circuits of 100 A or less (or 14 to 1 AWG) to the 60°C column unless the equipment is listed and marked for 75°C. With 90°C wire you may apply the correction and adjustment factors to the 90°C ampacity, but the result can't exceed the ampacity at the termination rating (310.15(A)). Three-phase: enter the line-to-line voltage.
| Size | Table 310.16 | Usable | Drop |
|---|
Table 310.16 is the ampacity at the insulation rating for not more than three current-carrying conductors at 30°C (86°F). Usable = that value × the ambient and conductor-count factors, capped at the termination column and, for 14, 12 and 10 AWG, at the 240.4(D) overcurrent limit. Highlighted: the smallest size that meets both ampacity and your voltage drop target. An estimate from the NEC 2023 tables: local amendments and the inspector (AHJ) govern.
Pick the smallest conductor whose NEC Table 310.16 ampacity, at the terminals' temperature rating, covers the load (125% of a continuous load). At 75°C, 8 AWG copper carries 50 A and 6 AWG 65 A. Then apply ambient and bundling factors, keep 14, 12 and 10 AWG copper on 15, 20 and 30 A breakers, and upsize if voltage drop runs over 3%.
How to use the wire size calculator
Describe the load
Copper or aluminum, single- or three-phase, amps, voltage, one-way length, and whether the load is continuous.
Set the conditions
Termination and insulation ratings, ambient temperature and how many current-carrying conductors share the raceway.
Read the size
The smallest size for ampacity, the smallest for voltage drop, and the larger of the two highlighted.
What the wire size calculator does
NEC 2023 Table 310.16
60, 75 and 90°C ampacities for copper and aluminum, 14 AWG to 1000 kcmil.
Termination limit
Applies factors to the insulation's ampacity but never above the termination column (110.14(C), 310.15(A)).
Derating built in
Ambient correction from Table 310.15(B)(1)(1) and the 80% to 35% adjustment of Table 310.15(C)(1).
Small-conductor rule
Caps 14, 12 and 10 AWG copper at 15, 20 and 30 A, and 12 and 10 AWG aluminum at 15 and 25 A (240.4(D)).
Voltage drop check
Uses the Chapter 9 Table 8 resistances and upsizes when the drop is over your target.
- Ampacity table
- NFPA 70 (NEC 2023) Table 310.16, 30°C ambient, up to 3 current-carrying conductors
- Factors
- Table 310.15(B)(1)(1) ambient correction (21 to 85°C) and Table 310.15(C)(1) adjustment
- Continuous loads
- Conductor sized for 125% at the termination rating (210.19(A), 215.2(A))
- Voltage drop
- 2 (or 1.732) × length × Table 8 resistance × amps ÷ 1,000; 3% default target
- Not included
- Rooftop temperature adder, cable-specific limits (NM-B at 60°C), motors, Table 310.17 free air
Wire Size Calculator FAQ
What size wire do I need for 50 amps?
At the 75°C column of NEC Table 310.16, 8 AWG copper is rated 50 A and 6 AWG copper 65 A. With 60°C terminals or NM-B cable, 8 AWG copper is only 40 A, so 6 AWG is needed. Continuous loads, heat, bundling and long runs can push the size up, which is what this calculator checks.
Which temperature column should I use?
The lowest rating of anything the conductor connects to. Under 110.14(C)(1), circuits of 100 A or less (or 14 to 1 AWG) use 60°C unless the equipment is listed and marked for 75°C; larger circuits use 75°C. The 90°C column of THHN is a starting point for derating, not the final ampacity.
Why does 12 AWG copper show 25 A but go on a 20 A breaker?
Table 310.16 gives 12 AWG copper 25 A at 75°C, but 240.4(D) limits the overcurrent protection to 20 A (14 AWG to 15 A, 10 AWG to 30 A) unless another rule specifically allows more, such as motor circuits. The calculator applies that limit.
When do I need to derate?
When the ambient temperature is above 30°C (86°F) or more than three current-carrying conductors share a raceway or cable for more than 24 in. Multiply the ampacity by the factors in Tables 310.15(B)(1)(1) and 310.15(C)(1); neutrals that carry only unbalanced current and grounding conductors don't count.
Does this cover aluminum feeders?
Yes, from 12 AWG to 1000 kcmil using the aluminum columns. For dwelling services and feeders, 310.12 allows a smaller size than Table 310.16 in some cases; this calculator doesn't apply that, so it may pick a size larger than 310.12 requires.
Which code edition is this?
The 2023 NEC (NFPA 70). Your state or city may enforce an earlier or later edition with amendments, and the inspector (AHJ) has the final word, so treat the result as an estimate to check.
Where the numbers come from
- NFPA 70, National Electrical Code (2023): Table 310.16, Tables 310.15(B)(1)(1) and 310.15(C)(1), 110.14(C), 210.19(A), 240.4(D), Chapter 9 Table 8
- NFPA: NEC-P06 First Draft report, 2026 cycle (prints the 2023 Table 310.16 and correction tables with the proposed changes)
- Conduit.site: Table 310.16, 310.15(B)(1) and 310.15(C)(1) reproductions (2020 NEC)
- Michigan State University, Electrical Tech Note 212: Conductor properties (Table 8 resistances)
Link to this tool
Found it useful? Paste this into your website, newsletter or resource page.
More free tools
All free toolsPrice it right, then book the job
CrewRobin is the AI front office for home-service shops: an AI receptionist that answers every call, qualifies the job and books it, plus missed-call text-back, quotes, invoices and review requests.