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

 

[pirate]

Arr, that takes me back, me hearties...thanks Tel!

 

[Wilko]

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

I was trying not to mention the War .

 

[Master Mech]

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.

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

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

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.

There is quite a few pieces up there as this is a full fledged home theatre so here goes.

1. Arcam AVR30 Processor / Amplifier

2. Emotiva XPA-7 Amplifier - x2

3. Behringer NX6000 Amplifier - x3

4. Behringer NX3000 Amplifier

5. Buttkicker BKA1000-N Amplifier

6. Epson EH-TW9300 Projector

7. Xbox One X

8. Virgin Media V6 Box

9. Phillips Hue

10. Netgear GS116UK Network Switch

11. Netgear R7800 Network Router

12. MiniDSP 2X4 HD - X2

13. HDFury Vertex

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.

The total length of the cable is 35 meters but as you say I would need to be using 32 amps to start running into issues with voltage drop so I now need to now workout what my total load is but as all of you say it is highly unlikely I am going to be anywhere near 32 amps.

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

Is it black plastic conduit in 25mm, it looks like this stuff.

https://www.toolstation.com/25mm-heavy-duty-pvc-round-conduit/p66251

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

Thank you for that information, it seems this is my issue rather than voltage drop at this point from all the information I have received here.

Many thanks to all of you for your time & help, it is greatly appreciated.

 

[Master Mech]

@westward10 Forgot to ask, do you know if the current carrying capacity of the cable is further de-rated due to the two other cables also running in the conduit along with it?

 

[Wilko]

Hi MM - so coool stuff there

Power wise, I just looked at item 3, the Behringer NX6000 Amplifier of which you’ve got 3. I was stunned to see the spec says ea of these has 2 by 3kW amps. Yep 18kW. Aghhh. Then I looked at the power cable, it’s a plug. And it’s labelled 820W each. So probably closer to 2.4kW rather than 12kW + some efficiency factor. But it does seem like you’re going to need some air handling plant up there.