Storage systems invert the cable problem that PV presents. On a PV string, voltage is high and current is modest, so voltage drop dominates. Between a battery bank and an inverter, voltage is low and current is enormous — 100 A continuous is ordinary on a 48 V household system — so thermal capacity, heat dissipation and terminal quality dominate, and the voltage-drop budget collapses to a fraction of a volt. Cable chosen by PV instinct is almost always too small for a battery run.
This guide covers what changes, the arithmetic that decides it, and the protection details specific to lithium-based systems.
Why storage DC is a different problem from PV
| Aspect | PV string | Battery to inverter |
|---|---|---|
| Typical voltage | 400 – 1000 V DC | 48 V (or 200 – 500 V high-voltage stacks) |
| Typical continuous current | 10 – 20 A | 50 – 200 A |
| Binding constraint | Voltage drop | Thermal capacity + heat dissipation |
| Voltage-drop budget | ≤1–2 % of 600 V (6–12 V) | ≤1 % of 48 V (0.5 V) |
| Load profile | Daytime, sun-following | Continuous charge/discharge, night included |
| Fault current | Limited by module string | Very high on lithium banks |
Two consequences follow immediately. First, the same ampacity rules that leave PV strings comfortably sized will run hot on a battery trunk. Second, every extra milliohm matters twice — once as heat, once as sag under load, because on a 48 V bus a single volt of drop is already 2 %.
Sizing: the four steps in order
Step 1 — design current with the continuous factor. Inverter ratings are continuous; apply the usual 125 % factor to a continuous load, so a 100 A inverter plans for 125 A of conductor capacity before any correction.
Step 2 — thermal capacity. Use the applicable ampacity table for the insulation class (90 °C XLPE battery cable is the common construction) and then respect the terminal limitation: lugs and breaker terminals are often rated 75 °C, which caps the column you may use regardless of the insulation. Capacity also derates in hot battery rooms and in bundles — the same derating logic described for rooftops in the rooftop cable guide applies to a battery enclosure at 40 – 50 °C.
Step 3 — voltage drop on the real numbers. For a two-wire DC run: ΔU = 2 × I × L × ρ / A, with ρ ≈ 0.023 Ω·mm²/m for warm copper. Worked examples for a 48 V bank, 100 A continuous, one-way length as shown:
| One-way run | 25 mm² | 35 mm² | 50 mm² | 70 mm² |
|---|---|---|---|---|
| 2 m | 0.37 V — 0.77 % | 0.26 V — 0.55 % | 0.18 V — 0.38 % | 0.13 V — 0.27 % |
| 3 m | 0.55 V — 1.15 % | 0.39 V — 0.82 % | 0.28 V — 0.57 % | 0.20 V — 0.41 % |
At 2 m, 25 mm² squeaks under 1 %; at 3 m it fails. And drop is only half the story — the other half is heat:
| Cross-section | Conductor heat at 100 A (2 m run) |
|---|---|
| 25 mm² | 37 W |
| 35 mm² | 26 W |
| 50 mm² | 18 W |
| 70 mm² | 13 W |
37 W dissipated continuously inside a battery cabinet is a self-inflicted thermal problem — it warms the batteries, which ages them, in a loop. One size up halves the heat.
Step 4 — terminals and workmanship. At these currents the joint, not the conductor, is the weak point: compression lugs crimped with the correct die, torqued to spec, and checked after the first weeks of thermal cycling. A terminal that runs warm will eventually carbonise its insulation regardless of how generous the cross-section is.
For high-voltage stacked systems (200 – 500 V), the pressure eases: a 400 V / 25 A stack over 3 m on 10 mm² drops under 0.1 %, and sizing becomes a conventional thermal exercise. The low-voltage, high-current corner is where storage cabling bites.
Protection: the lithium fault-current problem
A lithium bank can deliver far more fault current than the same capacity in lead-acid, and every protective device in the DC path must be able to interrupt that current at DC voltage — which is a much harder duty than AC interruption, because DC does not pass through zero to extinguish the arc.
Practical implications:
- Class T fuses are the common recommendation on lithium battery mains: a high DC interrupting rating in a compact package. An underspecified breaker or fuse may clear the circuit once and fail, or fail to clear at all.
- Fuse at the battery positive, as close to the terminal as practical — conductor length between the battery and its protection is unprotected length.
- Disconnect ratings matter as much as conductor ratings: switches and breakers in the DC path need DC ratings at the system voltage, not AC ratings borrowed from a datasheet.
Cable construction completes the picture: fine-stranded, flexible copper conductors make terminal work feasible at these sizes, and tinned variants earn their premium in humid or coastal battery rooms — the same corrosion logic as in the conductor material comparison.
Regulatory context in one paragraph
Storage wiring sits under its own rules, not the PV array rules: NEC Article 706 governs energy storage systems in the US, systems are certified as complete units under UL 9540, and the battery/inverter combination typically defines the permitted conductor sizes and protective devices in its installation manual. The manual's conductor table is a floor, not a ceiling — field-routed distances are usually longer than the certified configuration assumed, and the sizing arithmetic above is how you cover the difference honestly.
Five mistakes worth avoiding
- Reusing PV sizing instincts. 100 A at 48 V is a different regime; check heat, not just drop.
- Skipping the 125 % continuous factor. Battery loads are continuous by definition; the margin is not optional.
- Ignoring the 75 °C terminal cap. Insulation rated 90 °C does not help if the lug terminates at a 75 °C rating.
- Long unprotected runs from the battery terminal. Protection belongs at the source, before the first metre of cable.
- AC-rated breakers in DC paths. DC arcing does not self-extinguish; devices must carry explicit DC interrupting ratings.
FAQ
Q1: Can I use PV wire (UL 4703) for battery-to-inverter connections? A: Its electrical construction is suitable — 90 °C, UV-rated single-core copper — but PV wire is built around string currents and UV duty, not 100 A continuous trunk runs. What matters more in practice is conductor size, fine stranding for terminal work, and following the ESS installation manual and NEC 706 requirements for the circuit.
Q2: Why is voltage drop so much harder at 48 V? A: The budget scales with voltage. One percent of a 600 V string is 6 V; one percent of a 48 V bus is 0.48 V — and the current is five to ten times higher, so the conductor must be several sizes larger just to lose the same fraction.
Q3: What size cable does a 100 A battery inverter need? A: Plan for 125 A after the continuous factor, then choose by ampacity table, terminal temperature rating and the drop table above. In metric terms this typically lands at 50 – 70 mm² for short runs; confirm against the applicable table and the inverter manual before purchase.
Q4: Why do lithium systems specify Class T fuses? A: Lithium banks can source very high fault current, and a protective device must interrupt that current at DC voltage. Class T fuses combine a high DC interrupting rating with fast clearance; ordinary AC-rated breakers or automotive fuses may not interrupt a lithium DC fault at all.
Q5: Is welding cable acceptable for battery runs? A: Its flexibility is attractive, but acceptance depends on the listing and the jurisdiction — welding cable is not listed for building wiring in the same way, and AHJs commonly reject it where a listed battery cable construction is required. Fine-stranded, listed battery cable gives the same workability without the compliance question.
Sourcing note
🔴 Disclaimer: conductor tables, protective-device ratings and ESS certification requirements vary by jurisdiction and system model; the figures above are typical worked examples and must be confirmed against the applicable code edition, the UL 9540 system certification and the battery/inverter installation manual before design sign-off. This article is engineering guidance, not a design certification.
