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For the people doing the work

Installer Toolset.

The checks a designer runs twenty times a day, in the browser: a NEC 220.83(B) load calc, the 120% busbar rule with derate suggestions, and the permitting database. Nothing you type leaves this page. Homeowner? The quote checker is the tool for you.

Busbar tie-in checks NEC 705.12(B)(3)(2)-(3) · 2017: 705.12(B)(2)(3)(b)-(c)

Two compliant paths onto the same busbar. The 120% rule: busbar × 1.2 − main = allowed backfeed, PV breaker at the opposite end. The sum-of-breakers rule: every load and supply breaker on the bus (excluding the main) summed must stay at or under the busbar rating, with no placement restriction at all. You only need one to pass. Works the same on a main panel or a subpanel.

Enter the ratings and run the check. Add the load-breaker sum to test both methods at once.

The order of operations pros actually run

  1. Load calc first. Know the home's real calculated load before arguing methods. It justifies derates and right-sizes everything downstream.
  2. Read the panel label, not the assumption. Confirm the busbar rating from the label, and check whether the panel accepts a listed lug kit. Feed-through or subfeed lugs are a legitimate landing option that skips the breaker debate entirely.
  3. The subpanel relocation play. Move load breakers to a properly sized subpanel. The main bus then carries only the feeder and the PV breaker, so sum-of-breakers passes at the main almost by construction.
  4. Mix the rules deliberately. Each busbar is judged on its own: 120% at the subpanel, sum-of-breakers at the main, a fully compliant combination when either alone falls short.

Residential load calculator NEC 220.83(B)

The existing-dwelling optional method, as used on plansets: 100% of the first 8 kVA plus 40% of the remainder, HVAC at 100%. Work from the panel photo; unaccounted single-pole breakers count as 1,500 VA dedicated circuits, which keeps the result conservative.

1 · The house & the panel

Dedicated 1P loads on the panel

2 · Two-pole (240V) loads, non-HVAC

3 · HVAC & EV

A/C vs electric heat: the larger of the two counts, per 220.60 they don't run together. EV chargers land in the Service Demand section at 125% of continuous rating (625.42), the treatment plan checkers expect.

Fill in the panel and hit calculate. The math shows its work, planset-style.

Requirements database

9,000+ AHJs and 1,300+ utilities, compiled from public records: codes, SolarAPP+ status, stamps, setbacks, interconnection and tap policies, offsets. Search or browse.

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