Complete guide to wiring a battery backup

Scope and code basis

Battery backup installs in dwellings now fall under NEC Article 706 (Energy Storage Systems) plus 480 for stationary lead-acid and lithium banks above 100 Vdc nominal. If the battery is paired with PV, Article 690 applies to the array side and 705 governs the interconnection. Always confirm the listing: ESS equipment must be listed to UL 9540, and the cells or modules to UL 9540A for thermal runaway data.

Before pulling a single conductor, pull the manufacturer install manual and the AHJ amendments. California, New York, and most of New England have layered IRC and IFC requirements on top of NEC, particularly around indoor lithium kWh limits. A 20 kWh garage install in Massachusetts is not the same job as a 20 kWh install in rural Texas.

Sizing the system and conductors

Start with the load calc, not the battery brochure. Run a standard 220.82 dwelling calc, then identify which circuits move to the protected loads panel. Whole-home backup off a single 5 kW inverter will trip on a well pump and an AC compressor starting together. Size for continuous output at 125% per 706.31, and remember inverter output current is the nameplate continuous AC output, not peak.

DC conductors between battery and inverter are the part most installers underrate. At 48 V nominal, a 10 kW inverter pulls roughly 230 A continuous. That is 4/0 copper minimum in most configurations once you apply 706.31(B) and 310.16 ampacity tables, with overcurrent protection sized per 240.4. Lugs must be listed for fine-stranded conductors if you are using welding cable style leads.

• AC output OCPD: 125% of inverter continuous output (706.31)
• DC OCPD: within 10 ft of battery terminals, listed for DC interrupting rating at battery Voc
• Equipment grounding conductor sized per 250.122 from the AC OCPD
• DC grounding electrode conductor per 690.47 / 706.50 if applicable
• Working space per 110.26, full 36 inch depth, no exceptions for residential ESS

Disconnects, labeling, and rapid shutdown

Every ESS needs a disconnecting means within sight of the equipment per 706.15, and that disconnect must open all ungrounded conductors simultaneously. For DC coupled systems, you need both an AC disconnect at the inverter and a DC disconnect at the battery. Most listed ESS units include integrated breakers that satisfy this, but verify by drawing the one-line before ordering.

Labels are not optional and are the number one reason inspectors fail ESS jobs. Required markings include the ESS disconnect label (706.15(C)), the directory at the service equipment showing all power sources (705.10), the maximum AC and DC operating voltage and current, and the available fault current with date calculated.

Field tip: print labels on the truck with a Brother PT-E550W or similar. Handwritten Sharpie on red tape gets rejected in most jurisdictions, and a return trip for labels costs more than the printer.

Interconnection methods

Three ways to tie a battery backup into a dwelling, and the choice drives the rest of the install. Supply-side tap per 705.11 lands ahead of the main service disconnect, requires a fused disconnect, and avoids the 120% rule entirely. Load-side breaker per 705.12(B) is the most common for retrofits, where the sum of the main breaker and the inverter breaker cannot exceed 120% of the busbar rating.

The third path, and often the cleanest, is a service entrance rated transfer switch or a microgrid interconnect device (MID) that creates a separate protected loads panel. This isolates backed up circuits and dodges the 120% calculation, since the inverter is no longer parallel to the utility on the main bus. For any system over about 7.6 kW, this is usually the path of least resistance.

1. Verify main panel busbar rating stamped on the deadfront
2. Calculate 120%: bus rating times 1.2, minus main breaker, equals max backfed breaker
3. If the math fails, derate the main or move to a MID configuration
4. Locate backfed breaker at opposite end of bus from the main per 705.12(B)(3)(2)
5. Apply the permanent warning label at the backfed breaker

Location, ventilation, and commissioning

Lithium ESS in dwellings is capped at 20 kWh per unit and 40 kWh aggregate in most jurisdictions following IRC R328 or IFC 1207, with required separation from doors, windows, and means of egress. Garages are fine, finished living spaces are not, and bedroom walls are a hard no. Lead-acid still falls under 480.10 for ventilation and spill containment, which kills most retrofit ideas in finished basements.

Commissioning is where the job either passes or comes back to bite you. Torque every lug to spec with a calibrated wrench, log the values, and photograph the torque marks. Cycle the system through a utility outage simulation before the inspector arrives, and verify the protected loads actually transfer without nuisance trips.

Field tip: bring a clamp meter rated for DC and verify battery current under load matches the inverter display within 2%. A 10% discrepancy points to a shunt calibration issue or a loose lug, and you want to find it before the homeowner does.

Hand off a closeout package: one-line diagram, equipment list with serials, torque log, commissioning report, and the manufacturer warranty registration. That packet is what protects you when the call comes in two years later asking why the system did not back up the house during an ice storm.