Radial Circuit Cable Length

[Master Mech]

Hello all,

Just had an electrician wire in a radial circuit in my house to my loft as I use the space for my home theatre equipment & wanted a dedicated circuit for the equipment as its consists of quite a few power hungry amps & devices.

He has wired the radial in 4.0mm2 twin & earth cable connected to a 32 amp breaker, normally I gather this would not be an issue & is common place in installations.

The problem I can see is the total length of the cable from the consumer unit to the equipment in the loft is 35 meters & after doing a little research it seems that voltage drop is going to be an issue with a cable of this length.

After using a calculator online for voltage drop the conductor size required for a 6.0mm2 to be below the 5% maximum voltage drop stated in the regulations.

That calculation is for a cable that has been surface clipped which is correct for the length of the run in the loft which is 22 meters long.

The length of the run going along & up the side of the house to reach the loft is 13 meters long & is inside 25mm conduit along with a 2.5mm2 & 1.0mm2 cable for other circuits in the house.

So am I correct in saying that using a 32 amp breaker in this situation is not appropriate & a 20 amp breaker would be the correct choice?

Alternatively the cable needs to be changed to a 6.0mm2 cable to be able use a 32 amp breaker to protect the circuit?

 

[Taylortwocities]

A radial circuit run in 4mm T&E cable and protected by a 32amp MCB is a standards circuit arrangement.

What is the total load of your “power hungry” devices? I can’t imagine that they come anywhere near 32amp.

 

[Master Mech]

A radial circuit run in 4mm T&E cable and protected by a 32amp MCB is a standards circuit arrangement.

What is the total load of your “power hungry” devices? I can’t imagine that they come anywhere near 32amp.

I will be totally honest I do not know what the total load is, I looked up all the manufacturer specs for all my equipment & I got some crazy figure like 80 amps when I added them all together so I presume these published figures are theoretical maximums but are not much use in the real world?

What is the best way to work out what my total load is?

If the voltage drop is a non issue is there any issues with the current carrying capacity of the cable due to it being in conduit & also with other cables?

 

[James]

Volt drop will not reduce by putting in a 20A breaker.

Have you used a part p registered electrician?

If so you will get or have already got a certificate that confirms compliance with the regs.

 

[Wilko]

Hi MM - You are correct that if a bigger cable was used then less voltage would be dropped for any given load in accord with Ohm’s Law, but it’s unlikely to be something that affects the use you propose. If you post up an equipment list I’m sure load estimates will be forthcoming.

 

[Burtones]

If you post up an equipment list I’m sure load estimates will be forthcoming.

That will be interesting. I would be surprised if the total load on the circuit (and therefore assumed design current) was anywhere near high enough to exceed max volt drop requirements.

Also how sure are you on the length of circuit? A quick calc shows that even if the design current was 32A you could run approx. 30m in 4mm before exceeding volt drop.

 

[Taylortwocities]

I will be totally honest I do not know what the total load is, I looked up all the manufacturer specs for all my equipment & I got some crazy figure like 80 amps

I rather think you are misunderstanding of misreading the amount of MAINS current that the devices require.
A 200 watt audio amplifier at full whack won’t pull much more current than a 60watt light bulb.
Have a look at the rating information on the back of your devices.

 

[westward10]

4mm T&E is undersized for a 32A protective device if it is contained in conduit.

 

[telectrix]

I will be totally honest I do not know what the total load is, I looked up all the manufacturer specs for all my equipment & I got some crazy figure like 80 amps when I added them all together so I presume these published figures are theoretical maximums but are not much use in the real world?

What is the best way to work out what my total load is?

If the voltage drop is a non issue is there any issues with the current carrying capacity of the cable due to it being in conduit & also with other cables?

what yous running up there, a recording studio or a pirate radio station. ?

remember it well. on my old valve radio.long before FM.

 

[Burtones]

4mm T&E is undersized for a 32A protective device if it is contained in conduit.

OP- is it conduit or plastic mini trunking? Haven't looked at the tables so not sure if it makes a difference to derating factor