Calculation of voltage drop in steady load conditions

From Electrical Installation Guide

Contents 1 Use of formulae 2 Simplified table 3 Examples

Use of formulae

Figure G27 below gives formulae commonly used to calculate voltage drop in a given circuit per kilometre of length.
If:

• IB: The full load current in amps
• L: Length of the cable in kilometres
• R: Resistance of the cable conductor in Ω/km

 for copper

 for aluminium
Note: R is negligible above a c.s.a. of 500 mm²

• X: inductive reactance of a conductor in Ω/km

Note: X is negligible for conductors of c.s.a. less than 50 mm². In the absence of any other information, take X as being equal to 0.08 Ω/km.

• ϕ: phase angle between voltage and current in the circuit considered, generally:

- Incandescent lighting: cosφ = 1
- Motor power:
   - At start-up: cosφ = 0.35
   - In normal service: cosφ = 0.8

• Un: phase-to-phase voltage
• Vn: phase-to-neutral voltage

For prefabricated pre-wired ducts and bustrunking, resistance and inductive reactance values are given by the manufacturer.

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Circuit	Voltage drop (ΔU)
	in volts	in %
Single phase: phase/phase
Single phase: phase/neutral
Balanced 3-phase: 3 phases (with or without neutral)

Fig. G27: Voltage-drop formulae

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Simplified table

Calculations may be avoided by using Figure G28, which gives, with an adequate approximation, the phase-to-phase voltage drop per km of cable per ampere, in terms of:

• Kinds of circuit use: motor circuits with cosφclose to 0.8, or lighting with a cosφclose to 1.
• Type of cable; single-phase or 3-phase

Voltage drop in a cable is then given by:
K x IB x L
K is given by the table,
IB is the full-load current in amps,
L is the length of cable in km.
The column motor power “cosφ = 0.35” of Figure G28 may be used to compute the voltage drop occurring during the start-up period of a motor (see example no. 1 after the Figure G28).

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c.s.a. in mm2		Single-phase circuit			Balanced three-phase circuit
		Motor power		Lighting	Motor power		Lighting
		Normal service	Start-up		Normal service	Start-up
Cu	AI	cos φ = 0.8	cos φ = 0.35	cos φ = 1	cos φ = 0.8	cos φ = 0.35	cos φ = 1
1.5		24	10.6	30	20	9.4	25
2.5		14.4	6.4	18	12	5.7	15
4		9.1	4.1	11.2	8	3.6	9.5
6	10	6.1	2.9	7.5	5.3	2.5	6.2
10	16	3.7	1.7	4.5	3.2	1.5	3.6
16	25	2.36	1.15	2.8	2.05	1	2.4
25	35	1.5	0.75	1.8	1.3	0.65	1.5
35	50	1.15	0.6	1.29	1	0.52	1.1
50	70	0.86	0.47	0.95	0.75	0.41	0.77
70	120	0.64	0.37	0.64	0.56	0.32	0.55
95	150	0.48	0.30	0.47	0.42	0.26	0.4
120	185	0.39	0.26	0.37	0.34	0.23	0.31
150	240	0.33	0.24	0.30	0.29	0.21	0.27
185	300	0.29	0.22	0.24	0.25	0.19	0.2
240	400	0.24	0.2	0.19	0.21	0.17	0.16
300	500	0.21	0.19	0.15	0.18	0.16	0.13

Fig. G28: Phase-to-phase voltage drop ΔU for a circuit, in volts per ampere per km

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Examples

Example 1 (see Fig. G29)
A three-phase 35 mm² copper cable 50 metres long supplies a 400 V motor taking:

• 100 A at a cos φ = 0.8 on normal permanent load
• 500 A (5 In) at a cos φ = 0.35 during start-up

The voltage drop at the origin of the motor cable in normal circumstances (i.e. with the distribution board of Figure G29 distributing a total of 1,000 A) is 10 V phase-to-phase.
What is the voltage drop at the motor terminals:

• In normal service?
• During start-up?

Solution:

• Voltage drop in normal service conditions: 

Table G28 shows 1 V/A/km so that:
ΔU for the cable = 1 x 100 x 0.05 = 5 V
ΔU total = 10 + 5 = 15 V = i.e. 

This value is less than that authorized (8%) and is satisfactory.

• Voltage drop during motor start-up:

Δ Ucable = 0.52 x 500 x 0.05 = 13 V
Owing to the additional current taken by the motor when starting, the voltage drop at the distribution board will exceed 10 Volts.
Supposing that the infeed to the distribution board during motor starting is 900 + 500 = 1,400 A then the voltage drop at the distribution board will increase approximately pro rata, i.e. 

ΔU distribution board = 14 V
ΔU for the motor cable = 13 V
ΔU total = 13 + 14 = 27 V i.e. 

a value which is satisfactory during motor starting.

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Fig. G29: Example 1

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Example 2 (see Fig. G30)
A 3-phase 4-wire copper line of 70 mm² c.s.a. and a length of 50 m passes a current of 150 A. The line supplies, among other loads, 3 single-phase lighting circuits, each of 2.5 mm² c.s.a. copper 20 m long, and each passing 20 A.
It is assumed that the currents in the 70 mm² line are balanced and that the three lighting circuits are all connected to it at the same point.
What is the voltage drop at the end of the lighting circuits?
Solution:

• Voltage drop in the 4-wire line:  

Figure G28 shows 0.55 V/A/km
ΔU line = 0.55 x 150 x 0.05 = 4.125 V phase-to-phase
which gives: phase to neutral.

• Voltage drop in any one of the lighting single-phase circuits:

ΔU for a single-phase circuit = 18 x 20 x 0.02 = 7.2 V
The total voltage drop is therefore
7.2 + 2.38 = 9.6 V

This value is satisfactory, being less than the maximum permitted voltage drop of 6%.

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Fig. G30: Example 2