100 Amp Wire Size Calculator
NEC Table 310.16 ampacity sizing: enter your load and get the minimum copper or aluminum wire size, derated ampacity, and breaker recommendation.
How it works
The calculator looks up your material and temperature column in NEC Table 310.16, applies the Table 310.15(C)(1) adjustment factor for the number of current-carrying conductors, picks the smallest size whose derated ampacity covers your load (125% for continuous loads), and recommends the smallest standard breaker (NEC 240.6) at or above the load — where the wire ampacity falls between standard sizes, NEC 240.4(B) permits rounding up to the next standard breaker (800 A and below). Small conductors (14/12/10 AWG) are capped by NEC 240.4(D) at 15/20/30 A breakers.
Full ampacity reference
Values: NEC Table 310.16, not more than 3 current-carrying conductors in raceway/cable, 30°C ambient. Asterisk-free summary; see the NEC for footnotes including 240.4(D).
Related tools (different jobs)
- 12V Wire Size Calculator — automotive/RV low-voltage sizing by voltage drop, not NEC ampacity.
- DC Wire Size Calculator — general direct-current wire sizing.
- AWG to mm Converter — gauge ↔ metric conversion only.
- Wire Resistance Calculator — R = ρL/A physics, temperature correction, skin effect.
- Electrical Unit Converter — volts/amps/ohms/watts plus a quick ampacity reference table.
This tool is the dedicated NEC Table 310.16 ampacity calculator: copper/aluminum, 60/75/90°C columns, conduit derating, and breaker picks.
Worked examples
100 A residential subpanel (copper)
100 A load, 75°C terminations, 3 conductors: 3 AWG copper (100 A base, 100 A derated) with a 100 A breaker. This is the default this page opens with.
100 A feeder in a crowded conduit
100 A load with 9 current-carrying conductors (70% factor), copper, 75°C: the default 3 AWG (100 A × 70% = 70 A) fails, so the tool steps up to 1/0 AWG copper (150 A × 70% = 105 A derated). With aluminum and 10 conductors (50% factor), the minimum is 250 kcmil aluminum (205 A × 50% = 102.5 A derated).
200 A service entrance
200 A, 75°C: 3/0 AWG copper (200 A) or 250 kcmil aluminum (205 A), 200 A breaker.
FAQ
What size wire do I need for 100 amps?
For a 100 amp load using the 75°C column of NEC Table 310.16: 3 AWG copper (rated 100 A) or 1 AWG aluminum (rated 100 A). Pair it with a 100 amp breaker. If your terminations are only rated 60°C, you must use the 60°C column instead: 1 AWG copper (110 A) or 1/0 AWG aluminum (100 A).
Can I use the 90°C column for THHN wire?
Only if every termination is also rated 90°C. Per NEC 110.14(C), the column you use is limited by the lowest temperature rating of any termination, conductor, or device. Most breakers and panels are rated 75°C, so even with 90°C-rated THHN you normally size from the 75°C column (3 AWG copper for 100 A).
Does the number of wires in a conduit change the wire size?
Yes. With more than 3 current-carrying conductors you must apply NEC Table 310.15(C)(1) adjustment factors: 4–6 wires × 80%, 7–9 × 70%, 10–20 × 50%. Example: a 100 A load in a conduit with 9 conductors (70% factor) needs 1/0 AWG copper at 75°C, because 150 A × 0.70 = 105 A derated — the plain 3 AWG (100 A × 0.70 = 70 A) is no longer enough.
What size wire for a 200 amp service?
Using the 75°C column: 3/0 AWG copper (rated 200 A) or 250 kcmil aluminum (rated 205 A), with a 200 amp breaker. These are the standard residential service-entrance sizes in the US.
What size breaker goes with 100 amp wire?
A 100 amp breaker. This calculator recommends the smallest standard breaker size (NEC 240.6) that is equal to or larger than your load — for a 100 A load that is a 100 A breaker, matching the 3 AWG copper / 1 AWG aluminum conductor.
What wire size for a 60 amp subpanel?
6 AWG copper (rated 65 A in the 75°C column) with a 60 amp breaker. If you use aluminum, 4 AWG aluminum (rated 65 A at 75°C) is the equivalent choice.
How do continuous loads (3+ hours) affect wire sizing?
Continuous loads must be sized at 125% of the load per NEC 210.19/215.2. An 80 A continuous load is sized as 100 A, so it needs 3 AWG copper (100 A) at 75°C instead of the 4 AWG copper (85 A) that would suffice for a non-continuous 80 A load. Tick the continuous-load box in the calculator above.
Why does aluminum need a bigger wire than copper?
Aluminum has higher resistance, so NEC Table 310.16 gives it lower ampacity per size. For 100 A at 75°C you need 1 AWG aluminum versus only 3 AWG copper. Aluminum is cheaper and lighter, but terminations must be rated for aluminum (look for CO/ALR or AL-marked lugs) and installed with proper torque and anti-oxidant compound.
Why is my 12 AWG wire limited to a 20 amp breaker when the table says 25 amps?
NEC 240.4(D) caps overcurrent protection for small conductors regardless of Table 310.16: 14 AWG → 15 A max, 12 AWG → 20 A max, 10 AWG → 30 A max. So 12 AWG copper (25 A at 75°C) still gets at most a 20 A breaker. This calculator enforces that rule automatically.
What if my load is bigger than anything in the table?
This calculator covers up to 1000 kcmil (545 A copper / 445 A aluminum at 75°C with 3 conductors). Beyond that, electricians run conductors in parallel (NEC 310.10(H)) or use busbar — that is engineered work, not a DIY calculation. The tool will tell you honestly when you are past its range.
How is this different from a 12V automotive wire size calculator?
A 12V automotive calculator sizes wire mainly by voltage drop over distance at low voltage (see our 12V wire size calculator). This tool sizes wire by NEC Table 310.16 ampacity — the maximum safe current for building wiring at 120/240V AC — which is a completely different sizing basis.
How is this different from a DC wire size calculator?
Our DC wire size calculator covers general direct-current sizing. This tool is specifically the NEC Table 310.16 ampacity calculator for AC building/mains wiring: copper/aluminum, 60/75/90°C columns, conduit derating, and breaker recommendations per NEC 240.6.
Do I need to correct for high ambient temperature, like a hot attic?
Yes, and this calculator does not do it: NEC Table 310.16 assumes 30°C (86°F) ambient. Hotter environments require temperature correction factors from NEC Table 310.15(B)(1), which reduce ampacity further. If your run passes through a hot attic or rooftop conduit, have a licensed electrician apply the correction.
Can this calculator replace an electrician or my local electrical code?
No. This is a general reference based on NEC Table 310.16. Your local Authority Having Jurisdiction (AHJ) may amend the NEC, and real installations involve grounding, conduit fill, voltage drop, and termination details this tool does not cover. Always have a licensed electrician verify the final design.
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