Total voltage drop calculator

How this is calculated#

AS/NZS 3000 limits the voltage drop from the point of supply to any point in the installation. It is one limit for the whole run, not one per circuit. So the drop in the consumer mains, every submain and the final subcircuit must add up to no more than the limit. This tool adds them.

You enter each part of the run in order from the point of supply: the consumer mains, up to three submains and the final subcircuit. For each part you give the phases, the current, the route length and the cable. The tool works each part with the same calculation as the voltage drop tool. It reads the cable's resistance and reactance from AS/NZS 3008.1, finds the mV/A·m at the power factor, and gives the drop in volts.

Each drop is then taken as a percent of that part's own circuit voltage: 230 V for single phase and 400 V for three phase. Appendix C of AS/NZS 3000 adds the percents directly, so a three‑phase submain adds to a single‑phase final subcircuit with no conversion. The running total is the drop at the end of each part.

The limit is 5 % of the nominal voltage. Where the point of supply is the low voltage terminals of a substation on the premises, which supplies only this installation, it is 7 %. The clause sets no split between the parts. The tool shows what the mains and submains take, and what that leaves for a final subcircuit from the last board.

Worked examples#

• A house: mains, a garage submain and a socket‑outlet circuit. The total voltage drop is 4.25 %. The mains and submain take 1.98 %, which leaves 3.02 % for a final subcircuit.
• A shed 30 m from the house. The total voltage drop is 4.83 %, inside the 5 % limit but close to it. The upstream parts take 3.13 %, which leaves 1.87 % for a final subcircuit.
• A factory on its own substation. The total voltage drop is 6.27 % against the 7 % limit. The mains and submain take 4.38 %, which leaves 2.62 % for a final subcircuit.
• A farm workshop 60 m from the house. The total voltage drop is 10.82 %, so the run fails. The mains and submain take 8.55 % on their own, which leaves −3.55 % for a final subcircuit. A larger submain is the fix.

Limits#

The tool works a.c. runs at 230 V and 400 V nominal. It does not check extra-low voltage circuits, which Cl 3.6.2 leaves out, or a stand-alone system, which Exception 4 judges on the utilisation voltage. Single‑core parts use the highest reactance the table gives for the formation. Use the voltage drop tool for one run with a known formation or an unbalanced neutral.

Motor starting and other short transient drops are not part of the limit. Check those against the equipment.

See also: Voltage drop, The 5 % voltage drop limit from supply to load and Total voltage drop: mains, submain and final.