Voltage drop calculator

The voltage drop of a cable run by the method of IEC 60364-5-52 Annex G, for a 0.6/1 kV cable of the metal and section chosen, against the limits of its Table G.52.1. Free; the cable designer does the same for any design, with reverse sizing.

Phases Voltage (V) Current (A) Length (m) Conductor Section mm² Power factor Conductor temperature (°C)

ΔU = 16.67 V · 4.17 %

conductor d.c. resistance R = 1.3759 Ω/km (70 °C; IEC 60228:2023 Table 4, p. 13; IEC 60228:2023 Annex B, p. 18, read in the standard text); reactance, the IEC 60364-5-52 Annex G default X = 0.08 Ω/km (IEC 60364-5-52:2009 (ed. 3.0) Annex G (informative), pp. 74 to 75)

Supply	Use	Drop limit	This run
LV supply from the public network	lighting	3.1 % (3 % limit, plus 0.1 % for the length beyond 100 m)	over the limit
LV supply from the public network	other uses	5.1 % (5 % limit, plus 0.1 % for the length beyond 100 m)	within the limit
private MV/LV substation	lighting	6.1 % (6 % limit, plus 0.1 % for the length beyond 100 m)	within the limit
private MV/LV substation	other uses	8.1 % (8 % limit, plus 0.1 % for the length beyond 100 m)	within the limit

The limits are those of the standard's informative annex; for wiring longer than 100 m they rise by 0.005 % per metre beyond 100 m, by at most 0.5 %: IEC 60364-5-52:2009 (ed. 3.0) Annex G (informative), Table G.52.1, p. 74 (read in the standard text)

Voltage: line to neutral for single phase, line to line for three phase.

The method

Formula ΔU = b I L (R cos φ + X sin φ), b = 2 single phase, √3 three phase (line to line); the percent is of the voltage entered. R is the IEC 60228 maximum R20 taken to the conductor temperature by the Annex B law; it is the d.c. resistance, the skin effect is left out. Formula source: IEC 60364-5-52:2009 (ed. 3.0) Annex G (informative), pp. 74 to 75 (read in the standard text). IEC 60228:2023 Annex B, p. 18.

British armoured cables (BS 5467, BS 6724) are read in the designer from the BS 7671 voltage drop tables in mV/A/m (Tables 4D3B, 4D4B, 4E3B and 4E4B); North American wires from NEC Chapter 9 Table 9. Table 9 is for three single conductors in conduit, 60 Hz, 75 °C; for a cable it is an approximation. The 3 and 5 percent are informational notes, not rules.

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Questions and answers

How is voltage drop calculated?

What is the maximum voltage drop allowed?

IEC 60364-5-52 Table G.52.1 (an informative annex) gives 3 % for lighting and 5 % for other uses on a low voltage supply from the public network, 6 % and 8 % from a private substation; beyond 100 m of wiring the limit rises by 0.005 % per metre, by at most 0.5 %. National rules may set their own.

What is the voltage drop of 16 mm² copper at 63 A over 120 m?

About 16.7 V, that is 4.17 % of 400 V, three phase, power factor 0.9, conductor at 70 °C. At 120 m the Table G.52.1 limits rise to 5.1 % for other uses, which it meets, and 3.1 % for lighting, which it exceeds.

Is the NEC 3 % voltage drop a rule?

Table 9 is for three single conductors in conduit, 60 Hz, 75 °C; for a cable it is an approximation. The 3 and 5 percent are informational notes, not rules.