Maximum voltage drop limit
From Electrical Installation Guide
Maximum allowable voltage-drop vary from one country to another. Typical values for LV installations are given below in Figure G25.
|Type of installations||Lighting circuits||Other uses (heating and power)|
|A low-voltage service connection from a LV public power distribution network||3%||5%|
|Consumers MV/LV substation supplied from a public distribution MV system||6%||8%|
Fig. G25: Maximum voltage-drop between the service-connection point and the point of utilization
These voltage-drop limits refer to normal steady-state operating conditions and do not apply at times of motor starting, simultaneous switching (by chance) of several loads, etc. as mentioned in Chapter A (factor of simultaneity, etc.).
When voltage drops exceed the values shown in Figure G25, larger cables (wires) must be used to correct the condition.
The value of 8%, while permitted, can lead to problems for motor loads; for example:
- In general, satisfactory motor performance requires a voltage within ± 5% of its rated nominal value in steady-state operation,
- Starting current of a motor can be 5 to 7 times its full-load value (or even higher). If an 8% voltage drop occurs at full-load current, then a drop of 40% or more will occur during start-up. In such conditions the motor will either:
- Stall (i.e. remain stationary due to insufficient torque to overcome the load torque) with consequent over-heating and eventual trip-out
- Or accelerate very slowly, so that the heavy current loading (with possibly undesirable low-voltage effects on other equipment) will continue beyond the normal start-up period
- Finally an 8% voltage drop represents a continuous power loss, which, for continuous loads will be a significant waste of (metered) energy. For these reasons it is recommended that the maximum value of 8% in steady operating conditions should not be reached on circuits which are sensitive to under-voltage problems (see Fig. G26).
Fig. G26: Maximum voltage drop