Radial Voltage Drop Calculation?... By Load or MCB Rating?

At the end of any one radial adding all the voltage drops in the cables that feed it, 4% .

In each section the current flowing and the resistance and Mr Ohm's law tell you the voltage lost.

As a quick estimator the rule of 16 is not quite right, but close for 1st cut, is that 1m length of a copper core  of 1mm2 is 16 milli-ohms.


As the core  area goes up, resistance falls pro-rata, so 4mm2 cable is 4 milliohms per metre, 16mm2 would be 1 milliohm per metre etc.

Cable resistance scales with distance, so 2m of cable drops twice as many volts as 1m etc.

If the cables will be running hot, then use 19 instead of 16. (but 16 factorises and multiplies easier so suits the mental arithmetician.)




Maximum load - for 13A general use sockets where anything could be plugged in, the MCB rating is what limits you, and you assume worst case, where the load is as near the end as is possible with the socket layout.


For fixed known loads, use the actual load figures.

regards Mike.

edit we crossed in post but same gist as post above

At the end of any one radial adding all the voltage drops in the cables that feed it, 4% .

In each section the current flowing and the resistance and Mr Ohm's law tell you the voltage lost.

As a quick estimator the rule of 16 is not quite right, but close for 1st cut, is that 1m length of a copper core  of 1mm2 is 16 milli-ohms.


As the core  area goes up, resistance falls pro-rata, so 4mm2 cable is 4 milliohms per metre, 16mm2 would be 1 milliohm per metre etc.

Cable resistance scales with distance, so 2m of cable drops twice as many volts as 1m etc.

If the cables will be running hot, then use 19 instead of 16. (but 16 factorises and multiplies easier so suits the mental arithmetician.)




Maximum load - for 13A general use sockets where anything could be plugged in, the MCB rating is what limits you, and you assume worst case, where the load is as near the end as is possible with the socket layout.


For fixed known loads, use the actual load figures.

regards Mike.

edit we crossed in post but same gist as post above