Small church wants a 20-amp circuit for a bank of LED sign lights that stays on all night, load is right at 18 amps measured.

Yes, a 20A breaker with #12 AWG copper wire is acceptable for this installation, but with important caveats.

Code Compliance:

• NEC 210.20(A) requires overcurrent protection to not exceed the ampacity of conductors. #12 copper is rated 20A (90°C column, NEC Table 310.16), matching the breaker.
• NEC 210.19(A)(1) requires conductors to have ampacity not less than the load. Your 18A load is within the 20A capacity.
• NEC 210.23(A) allows 20A loads on 20A branch circuits.

Critical Considerations:

1. Continuous Load: If these lights operate for 3+ hours continuously (likely for "all night"), NEC 210.20(A) requires sizing the breaker at 125% of the load. 18A × 1.25 = 22.5A, which exceeds a 20A breaker. You'd need a 25A breaker with #10 AWG wire, or verify the load is actually under 16A continuous (16A × 1.25 = 20A).

1. Voltage Drop: For runs over 50 feet, check voltage drop. NEC 210.19(A)(1) Informational Note recommends ≤3% drop. At 18A, #12 may cause noticeable dimming on long runs.

Recommendation:

• If the load is truly 18A non-continuous, your setup works.
• If continuous, either reduce load below 16A or upgrade to 25A breaker with #10 AWG wire.
• Consider having an electrician verify the actual continuous load and wire length.

Safety Note: Churches often have insurance requirements exceeding minimum code. Check with your insurer and local AHJ, as some jurisdictions require permits for new circuits.

No. A 20A breaker with #12 THHN is not Code-compliant for this load.

NEC 210.19(A)(1)(a) requires branch-circuit conductors for continuous loads to be sized at 125% of the load.

• 18 A × 1.25 = 22.5 A minimum conductor ampacity.

NEC 240.4(D)(5) limits #12 CU THHN to 20 A (60°C column per 110.14(C)(1)(a)).

• 22.5 A > 20 A → #12 is too small.

NEC 210.20(A) requires the OCPD to be rated ≥ 125% of continuous load.

• 22.5 A → Requires a 30 A breaker (next standard size 240.6(A)).

Required Fix:

1. Conductor: #10 CU THHN (30 A @ 60°C / 35 A @ 75°C).
2. Breaker: 30 A (verify sign fixture listing allows 30 A branch circuit; if max fuse/breaker marked 20 A, you must reduce load or split circuits).
3. Terminations: Verify panel lugs and sign terminals accept #10.

Note: "All night" (≥3 hours) makes this a continuous load per NEC 100. Measured 18 A leaves zero margin for voltage drop or future creep.