Critical Loads Subpanel & NEC 220 Sizer
Before paying a solar contractor $5,500 for an unnecessary 400A Main Panel Upgrade (MPU), build your backed-up Critical Loads Subpanel schedule below. Computes NEC 125% continuous load rules, 240V well pump & A/C locked-rotor surge (LRA), copper feeder AWG sizing, and 24h/48h LFP battery storage.
1. Select Critical Branch Circuits to Back Up
Check every 120V and 240V circuit moving into your backed-up Critical Loads Subpanel:
Why the NEC 125% Continuous Rule Matters
Under NEC Article 215.2 and 220, any load expected to run continuously for 3 hours or more (such as EV charging, HVAC blowers, lighting, and server/router racks) must be sized at 125% of its continuous ampere rating so thermal-magnetic breakers never nuisance-trip during summer blackouts.
Cutting Central A/C Surge by 65%
A standard 3.5-ton or 4-ton 240V air conditioner draws 75 to 95 Locked Rotor Amps (18,000W+ instantaneous surge) for 400 milliseconds at startup—tripping a 10kW inverter into overload. Installing a $360 Micro-Air EasyStart soft-starter drops LRA surge to ~26 Amps (6.2kW).
Explore All 10 NEC 220 & Service Guides
Need single-line diagrams for NEC 705.12 120% busbar rules, supply-side taps, or Span Smart Panel vs manual Square D QO subpanels?
Open NEC 220 Load Calculation Vault →