BS 7671 Amendment 4: Cable Sizing for Battery Circuits

Battery circuits present unique cable sizing challenges that are now formally addressed in Chapter 57 of Amendment 4. Unlike conventional AC load circuits where current flows in one direction from source to load, battery circuits carry bidirectional current — charging from the grid and discharging to the installation. Chapter 57 requires that cable sizing accounts for this bidirectional operation.

The design current for a battery circuit is the higher of the maximum charge current and the maximum discharge current. For most lithium-ion systems, these are equal (the battery charges and discharges at the same maximum rate), but some systems — particularly those with rapid charge capability — may have asymmetric ratings. Chapter 57 requires the cable to be sized for the larger of the two currents at the continuous rating, with no diversity applied.

DC cable selection for battery circuits requires specific attention. Battery systems typically operate at voltages between 48V (small domestic) and 800V (commercial/industrial). The cable insulation rating must exceed the maximum battery voltage under all conditions, including the equalising charge voltage for lead-acid systems (approximately 15% above nominal) and the maximum string voltage for lithium-ion systems at full state of charge.

DC cables for battery circuits must be selected from those rated for DC operation. Not all cables rated for AC use are suitable for DC, because DC arcs are harder to extinguish than AC arcs (no zero-crossing). Chapter 57 specifies that cables on DC battery circuits must have an insulation rating certified for DC operation at the system voltage, and that DC isolators and switchgear must be rated for the DC prospective fault current.

Parallel cable matching receives specific requirements in Chapter 57. For high-current battery circuits where parallel cables are used, the cables must be matched in length (within 2% tolerance), cross-section (identical), and routing (same containment path). Chapter 57 specifies a maximum impedance mismatch of 5% between parallel paths, verified by measurement during commissioning. Unmatched parallel cables can lead to unequal current sharing, with one cable carrying significantly more than its rated current.

Short-circuit withstand for battery cables must account for the battery's fault current contribution. Lithium-ion batteries can deliver very high prospective fault currents — a 100 kWh commercial BESS at 400V might deliver 25 kA or more. The cable's I²t withstand must exceed the let-through energy of the protective device at the battery's prospective fault current.

ECalPro's Cable Size Calculator handles Chapter 57 battery circuit sizing including bidirectional current assessment, DC voltage rating verification, parallel cable matching, and short-circuit withstand calculations.