Which code rules apply
NEC Article 706 covers energy storage systems; Article 690 covers PV; Article 705/705-series covers interconnection. Confirm the locally adopted edition with the AHJ.
Breakers and wire
Size breakers at 125% of continuous load per NEC 210.19/215.2; check ampacity per 310.16 with 240.4(D) small-conductor limits; keep voltage drop near 3%.
Raceway and working space
Check conduit fill per NEC Chapter 9 and box fill per 314.16; maintain clear working space and listed equipment.
Formulas used
Continuous breaker
breaker >= load amps x 1.25
Voltage drop limit
keep under ~3% branch
Worked examples
40A continuous battery circuit
40 x 1.25 = 50A breaker minimum.
Common questions
Do I need a permit for a home battery?
Almost always: electrical permit plus utility interconnection approval.
Can I DIY a battery install?
Low-voltage portable units yes; permitted, interconnected storage normally needs a licensed electrician.
Sources
- NFPA 70 National Electrical Code National Fire Protection Association
- Energy Saver: Battery Storage U.S. Department of Energy