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NEC 705 Interconnection Current Limit

NEC 705.12 · 120% Busbar Rule

When to use: Verifying load-side (busbar) interconnection of a PV/inverter source to an existing service panel. Per NEC 705.12(B)(3)(2), the sum of the main supply breaker plus all backfeed (source) breakers must not exceed 120% of the busbar rating. The proposed inverter backfeed breaker is sized at inverter output × 1.25 (continuous), rounded up to the next standard size, and the backfeed breaker is located at the opposite end of the busbar from the main.

Panel & Source Parameters
A
A
0 if none
A
Continuous
A
Calculation Steps
Busbar × 1.20 = 225 × 1.20 = 270.0 A
− Main (200.0) − existing (0.0) = 70.0 A headroom
headroom ÷ 1.25 = 56.0 A max inverter
Required new backfeed = 40 × 1.25 = 50.00 → 50 A (std)
✓ PASS — Within 120% Rule
56.0A
Max Inverter Output
Results
Busbar × 120%270.0 A
Main Breaker200.0 A
Existing Backfeed0.0 A
Required New Backfeed50 A
Allowable Backfeed Headroom70.0 A
Max Inverter Output56.0 A
NEC References
NEC 705.12(B)(3)(2) — 120% rule
Sum of supply OCPD ≤ 120% busbar
Backfeed breaker at opposite end of bus
Continuous source × 1.25