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

0.285% Voltage drop seems a remarkably low allowance.

5% is commonly allowed, though remember that this is the TOTAL voltage drop from origin to point of use. So if most of this has already been used in a sub main such a low allowance is possible.


I do however consider it possible that you have mis understood the wording "total voltage drop" 

Total means the total voltage drop from origin to point of use.

It does not mean the total over a number of circuits. 


In my view if a circuit is used for a fixed load, then voltage drop calculations may be based upon the actual load. 16 amp circuit with a 10 amp load, base the voltage drop on 10 amps.

It might be better practice to design for 16 amps in case needs change.

If a circuit supplies one or more socket outlets, then voltage drop calculations should presume full load, who knows what might be plugged into the outlet(s) in future.

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

Should this work not be done by a qualified designer?

Thanks all.  Oh right!?... I was of the understanding that the allowance is for the whole installation?...

https://www.tlc-direct.co.uk/Book/4.3.11.htm
Qoute form Example 4.8 from the above link:-  "It is important to appreciate that the allowable volt drop of 4% of the supply voltage applies to the whole of an installation. If an installation has mains, sub-mains and final circuits, for instance, the volt drop in each must be calculated and added to give the total volt drop as indicated in {Fig 4.10}."


...is that incorrect?  Cheers!

Cheers Broadgage!

"The whole installation" means all parts of the installation that are relevant to the circuit in question.

So, just to confirm!??... If I have 14 Radials/Ways leaving the Distribution Board to feed different equipment throughout the Nano Brewery (Converted Garage!).  I only need to worry about keeping each Radial/Way under the 4% Voltage Drop allowance?  And not the whole installation?  Is that correct?  I have listed the radials below for reference.  Thanks for your feedback!

Radial 1 - Control Panel (Switching x4 Immersion Elements,  x1 1.2kW Pump,  x1 Extractor Fan)
Radial 2 - Instant Water Heater
Radial 3 - x4 250W Pumps
Radial 4 - Fermenter 1 & Inkbird Temperature Control
Radial 5 - Fermenter 2 & Inkbird Temperature Control
Radial 6 - Power Sockets for HLT (Hot Liquor Tank)
Radial 7 - Fridge & Freezer
Radial 8 - Ambient Room Heater
Radial 9 - LED Lighting Main Room
Radial 10 - LED Lighting Cool Room
Radial 11 - LED Flood/Security Lights
Radial 12 - Conditioning Room Chiller
Radial 13 - x2 Electric 510W Winches
Radial 14 - Security/Alarm System

Yes, that is correct. Don't go overboard either, a 5.1% worst case drop is not a disaster! In fact, all of the loads you have are not particularly sensitive, LED lights of the "not dimmable" type don't usually flicker, and the rest are non-critical. The effect of voltage drop to motors is reduced starting torque, and possibly increased current consumption for fixed loads. It might just cost you a little more money for the electricity, or more money for cables if you reduce the drop to a low level. The choice is part of the design process.


David CEng.

Cheers David!

You need to consider the total voltage drop from the origin of the installation up the point of use.

In a very simple installation, the consumer unit may be very close to the orign which is generally accepted to be the power companies electricity meter. Strictly speaking, one should calculate the voltage drop in the cables from the meter and into the consumer unit, but it is common to ignore this as being to small to worry about for short cables.

In such a simple installation, simply calculate the voltage drop in each sub circuit and ensure that it does not exceed 5% for power and 3% for lighting. If the voltage drop is excessive, then a larger cable is the obvious answer, but alternatives exist.


A slightly larger or more complex instalation may have the relevant consumer unit located in a workshop, outbuilding, or brewery or other place that is distant from the origin of the installation. A relatively large cable is then installed from the origin to this local consumer unit. Known as a sub main.

In such a situation the voltage drop in this sub main should be calculated.

AND then the voltage drop in each sub circuit calculated. 

The total of the voltage drop in the sub main and in the sub circuit should not exceed 5% for power and 3% for lighting.


Calculate the maximum load that can reasonably be expected on the submain, this may call for judgment and common sense rather than simply adding everything up. Calculate the voltage drop in this submain. Aim for no more than about  1% to 1.5%. Consider a larger cable if needed.

An apparently over sized sub main may be prudent to allow for expansion.


Then calculate voltage drop for each sub circuit. If say 1.6% has already been lost in the submain, then remember that only another 1.4% for lighting or another 3.4% voltage drop for power circuits is available.


If the submain is long, then I might allow as much as 2.5% voltage drop therein. That only leaves 0.5% available for voltage drop on lighting circuits, which is achievable for short and lightly loaded circuits.


Other arrangements may be applicable to VERY large installations, but are unlikely in this case.

Cheers Broadgage!


It is actually the first instance...  "the consumer unit may be very close to the orign which is generally accepted to be the power companies electricity meter."  ...as Scottish Power installed a new supply into the garage.