Radial Circuit Cable Length

[Master Mech]

Hello all,

Just had an electrician wire in a another 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 is 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?

 

[EFLImpudence]

Voltage drop is only a consideration if your equipment is vulnerable to it and obviously its effect is also dependent on your actual voltage.
It is not calculated on the MCB rating but the actual load. It will be fine.

I would say the only problem is the conduit so changing the MCB to 25A or 20A might be wise if your equipment can cause an overload.

 

[JohnW2]

He has wired the radial in 4.0mm2 twin & earth cable connected to a 32 amp breaker, .... 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 is 6.0mm2 to be below the 5% maximum voltage drop stated in the regulations.

If you put an entire 32A load right at the end of 35m of 4mm² radial circuit, the voltage drop would be about 12.32V which, at about 5.36% is, indeed, fractionally over 5%.

However, for a start, there is no regulatory requirement for voltage drop to be below 5% - that is only guidance.

In any event, it's not going to happen. It is exceeding unlikely that such a circuit would ever carry a 'full' 32A load at any time and, even if it did, it would certainly not all be at the very end of the circuit. In any likely real-world scenario, even if the total load were 32A, if it were distributed along the length of the circuit, the largest VD (at the very end of the circuit) would be well under 5%.

I personally would not be at all concerned. Even if it were 'a requirement' (which, as above, it's not), you will, in practice, never see >5% VD anywhere with such a circuit.

The current carrying capacity of the cable in conduit is a somewhat different issue, and may need MCB size to be reduced.

Kind Regards, John
Edit: too slow again!

 

[Master Mech]

Voltage drop is only a consideration if your equipment is vulnerable to it and obviously its effect is also dependent on your actual voltage.
It is not calculated on the MCB rating but the actual load. It will be fine.

I would say the only problem is the conduit so changing the MCB to 25A or 20A might be wise if your equipment can cause an overload.

If you put an entire 32A load right at the end of 35m of 4mm² radial circuit, the voltage drop would be about 12.32V which, at about 5.36% is, indeed, fractionally over 5%.

However, for a start, there is no regulatory requirement for voltage drop to be below 5% - that is only guidance.

In any event, it's not going to happen. It is exceeding unlikely that such a circuit would ever carry a 'full' 32A load at any time and, even if it did, it would certainly not all be at the very end of the circuit. In any likely real-world scenario, even if the total load were 32A, if it were distributed along the length of the circuit, the largest VD (at the very end of the circuit) would be well under 5%.

I personally would not be at all concerned. Even if it were 'a requirement' (which, as above, it's not), you will, in practice, never see >5% VD anywhere with such a circuit.

The current carrying capacity of the cable in conduit is a somewhat different issue, and may need MCB size to be reduced.

Kind Regards, John
Edit: too slow again!

Many thanks to both of you for the speedy replies & excellent detailed information.

Great to know I do not need to worry about the voltage drop so thanks for putting that to bed

In regard to the conduit I guess this is an issue due to the conduit being an partial insulator of heat?

Which makes it harder for any heat given off by the cable to be dissipated into the air surrounding it? Thus why they reduce the rating of the cable for differing installation scenarios?

Also I guess what does not help is the other two cables contained within the same run of conduit?

 

[JohnW2]

Many thanks to both of you for the speedy replies & excellent detailed information. Great to know I do not need to worry about the voltage drop so thanks for putting that to bed

You're welcome. However, you may have just changed one issue into a different one (but with similar implications)! ...

In regard to the conduit I guess this is an issue due to the conduit being an partial insulator of heat? Which makes it harder for any heat given off by the cable to be dissipated into the air surrounding it? Thus why they reduce the rating of the cable for differing installation scenarios?

Exactly and ...

Also I guess what does not help is the other two cables contained within the same run of conduit?

.. that does, indeed, make things 'worse'.

Kind Regards, John

 

[ericmark]

Once you use the correction factor I get around 31 meters for 4 mm cable on a 32A MCB installation method 100.

 

[Master Mech]

You're welcome. However, you may have just changed one issue into a different one (but with similar implications)! ...
Exactly and ...
.. that does, indeed, make things 'worse'.

Kind Regards, John

Many thanks again John, so my recommendation of installing a 20 amp MCB was not a bad idea?

When I suggested this to the electrician he told me the figures stated for the current carrying capacity of the cable in different installation scenarios was a recommendation only & because all of the run was not within conduit they were not relevant.

Personally I would have though it would make little difference if only part of the run was not in conduit as the part of the cable that is inside the conduit will still heat up at the same rate regardless of how much of the cable is not inside the conduit?

I am probably talking out my behind but I am really interested to know if that makes a difference or not.

 

[EFLImpudence]

You are absolutely correct and he is wrong.

 

[Master Mech]

You are absolutely correct and he is wrong.

I cannot thank you enough for confirming that, I will get him back to fit a 20 amp MCB like I requested him to.

I personally think 20 amps is going to be more than enough for the equipment I am using & what's the worse that could happen fitting a lower rated MCB? The MCB trips.

I did ask him what the best way is to calculate the total load of all my equipment but he offered no suggestions which I think he should have helped me work it out as he needed that information to fit the correct size cable for my use case.

I am starting to think he just guessed, the equipment was being fed by two spurs from the ring main for the upstairs sockets. This was the main reason I wanted to get a dedicated circuit fitted.

I did try to work it out myself by looking 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?

 

[JohnW2]

I cannot thank you enough for confirming that, I will get him back to fit a 20 amp MCB like I requested him to.

The conduit alone would only reduce the current-carrying capacity to 30A, but the other two cables in the conduit thoretically reduce things a bit further. However, a 25A MCB (if one is available for your CU) would probably be OK.

I did try to work it out myself by looking 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?

Indeed. If you added up all the possible loads in your house (or the ratings of all the MCBs in your CU) you would probably end up with a seemingly frightening total in relation to the alleged total 'capacity' of your supply (probably 60A or 80A) - but, as you say, things are not all 'on' at once (the technical term for this concept is 'diversity'!).

Kind Regards, John

 

[Master Mech]

The conduit alone would only reduce the current-carrying capacity to 30A, but the other two cables in the conduit thoretically reduce things a bit further. However, a 25A MCB (if one is available for your CU) would probably be OK.
Indeed. If you added up all the possible loads in your house (or the ratings of all the MCBs in your CU) you would probably end up with a seemingly frightening total in relation to the alleged total 'capacity' of your supply (probably 60A or 80A) - but, as you say, things are not all 'on' at once (the technical term for this concept is 'diversity'!).

Kind Regards, John

Great to know, would you class the current carrying capacity figures like those that are published in the link below recommendations or rules?

I am just curious as he kept stressing that these figures are theoretical & to be used as a guide only.

https://www.lightwiring.co.uk/light...-cables/twin-and-earth-cable-current-ratings/

I have a Wylex consumer unit that uses NSB MCB's which Wylex not longer produce & the current NHX range does not fit in my consumer unit as they changed the design slightly so the hole in the front cover does not line up with new the MCB's.

Looks like Wylex did do a 25 amp NSB MCB. Will see if I can track one down.

 

[JohnW2]

Great to know, would you class the current carrying capacity figures like those that are published in the link below recommendations or rules? I am just curious as he kept stressing that these figures are theoretical & to be used as a guide only. https://www.lightwiring.co.uk/light...-cables/twin-and-earth-cable-current-ratings/

Those figures are to-all-intents-and-purposes 'rules', since they appear to be just a compilation of what is in Tables 4D2A and 4D5 of BS7671 ('The Wiring Regulations'). Although they appear in an Appendix of those regs which are theoretically only 'informative' (i.e. 'guidance'), the body of BS7671 is pretty clear in 'requiring' one to abide by them - and I'm sure that no-one would consider doing otherwise!

Looks like Wylex did do a 25 amp NSB MCB. Will see if I can track one down.

I would hope that would not be too difficult.

Kind Regards, John

 

[Adam_151]

Indeed. If you added up all the possible loads in your house (or the ratings of all the MCBs in your CU) you would probably end up with a seemingly frightening total in relation to the alleged total 'capacity' of your supply (probably 60A or 80A) - but, as you say, things are not all 'on' at once (the technical term for this concept is 'diversity'!).
John

I'm reminded of a commerical building I know of, adding up the breakers in the final boards is probably about 5kA, out of interest I outlined what the buidling was and what the loads were to a colleage and asked him to estimate diversity and I think we got somewhere like 800A (so that would need a transformer on site), I said I thought that was overkill, and I told him its got a standard CT metered LV service. The DNO were brought onto site to pull the fuses and find out what fuses were fitted.... 100A, and its not been a problem in over 30 years

 

[JohnW2]

I'm reminded of a commerical building I know of, adding up the breakers in the final boards is probably about 5kA, ...

Indeed - and as I implied, it gets almost as silly as that with MCBs in a domestic CU, which will often add up to 150A or more, even though most domestic installations have a 60A or 80A DNO fuse (not to mention the "100A" main switch in the CU).

Kind Regards, John

 

[RF Lighting]

There are still thousands of properties in my area supplied from an iron clad cutout containing a 30A BS 3036 fuse.