Voltage Drop in Building Wiring
Most circuits in a house do not have enough current or length to produce a high voltage drop. In the case of very long circuits, for example, connecting a home to a separate building on the same property, it may be necessary to increase the size of conductors over the minimum requirement for the circuit current rating. Heavily-loaded circuits may also require a cable size increase to meet voltage drop requirements in wiring regulations.
Wiring codes or regulations set an upper limit to the allowable voltage drop in a branch circuit. In the United States, the 2005 National Electrical Code (NEC) recommends no more than a 5% voltage drop at the outlet. The Canadian electrical code requires no more than 5% drop between service entrance and point of use. UK regulations limit voltage drop to 4% of supply voltage. Following changes to the BS7671:2008 on consumers' installation, the following has become in force since 1 July 2008:
| Type of Supply | Voltage drop lighting | Voltage drop other |
|---|---|---|
| DNO | 3% | 5% |
| Private | 6% | 8% |
Voltage drop of a branch circuit is readily calculated, or less accurately it can be measured by observing the voltage before and after applying a load to the circuit. Excessive voltage drop on a residential branch circuit may be a sign of insufficiently sized wiring or of other faults within the wiring system, such as high resistance connections.
Note: voltage drops for installations are specified from the point of common coupling (i.e. where the utility supply is connected to the premises) to the point at which the electrical load is connected
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