Sub main query

[JonathanA]

Good morning!

Hoping to get some advice from someone more knowledgeable and experienced than I am.

I have a 100A PME supply to a garage.

I am providing a new sub-main to a new studio/workshop which is about 30m away from a garage CU. I have run the cable through underground ducting and already backfilled the trench. The cable is 16mm 3C SWA.

The plan was to EITHER split the meter tails and provide an independent supply via an 80A switch fuse OR fit an 80A MCCB to a spare way in the existing garage CU. The studio will have a 100A DP isolating switch with all circuits protected by RCBOs.

The SWA cable has a maximum rating of 91A and I designed the installation on this basis. I realise now that the cable won't be able to take this load in practice and I think I will have to downgrade the design load from 80A to say 63A.

Needless to say I'd rather not do this unless it is strictly necessary since it will invite all sorts of difficult questions!

Like I said, I'd be very grateful for some definitive advice as to what load the existing 16mm SWA cable can safely take.

Regards

J

 

[RF Lighting]

Basing my calculations on a 2% VD, which leaves you 1% for your final circuits, you would need 35mm² cable to keep your 80A supply.

The 16mm² you have installed will be ok for at maximum a 50A supply.

Why will this raise difficult questions?

 

[JonathanA]

Ouch! and there was I thinking I should have used 25mm!

I think it fair to say that I would be out of a job if I had to either uprate the cable or limit the load to 50A so neither are going to happen!

I did check the voltage drop and found it to be within limits for 16mm though I understand it might not be now at least for lighting.

That said, I am less concerned by voltage drop and more concerned about specifying the right overload protection.

My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

Regards,

J

 

[JohnW2]

The SWA cable has a maximum rating of 91A and I designed the installation on this basis. I realise now that the cable won't be able to take this load in practice and I think I will have to downgrade the design load from 80A to say 63A.

Quite apart from the (probably limiting) issue of voltage drop to which RF has referred, in design terms you appear to be approaching this issue somewhat 'back-to-front'. The 'design load/current' should be the actual required load (or, at least, an estimate thereof). The cable is then required to have a CCC at least as great as that design load, and, in turn, the device protecting that cable has to have an In no higher than that CCC. On top of that are the VD considerations which, as above, may well be the limiting factor (particularly if lighting is being supplied)

The first question therefore should really be "what current/load do you actually need?" - since that should be your design current. The fact that you have already buried the 16mm² SWA obviously rather confuses this, so you may have to be prepared to alter what loads you plan to apply to this circuit.

Kind Regards, John

 

[deadshort]

Basing my calculations on a 2% VD, which leaves you 1% for your final circuits, you would need 35mm² cable to keep your 80A supply.

The 16mm² you have installed will be ok for at maximum a 50A supply.

Why will this raise difficult questions?

I think RF provided you will the ansewer ?

Regards,

DS

 

[JohnW2]

That said, I am less concerned by voltage drop and more concerned about specifying the right overload protection. ... My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

You already know the answer to that - as you told us, it's 91A (at 90°C). As has been indicated, at that current you would fall foul of VD requirements.

Kind Regards, John

 

[RF Lighting]

I did check the voltage drop and found it to be within limits for 16mm though I understand it might not be now at least for lighting.

How did you check it?

That said, I am less concerned by voltage drop and more concerned about specifying the right overload protection.

The voltage drop has a bearing on overload selection, and in this instance it becomes the limiting factor.

My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

50A as I said earlier.

Whilst I accept it's not what you want to hear, you can not just ignore it and hope it'll go away.

 

[JohnW2]

That said, I am less concerned by voltage drop and more concerned about specifying the right overload protection. ... ]My question really is, what is the maximum safe load for 30m of 16mm 3C XLPE SWA ducted underground?

50A as I said earlier. Whilst I accept it's not what you want to hear, you can not just ignore it and hope it'll go away.

The OP seems to be asking what is the 'maximum safe load', regardless of considerations of VD - and, as I've pointed out, he has already told us that answer (91A). I wouldn't say that VD considerations affect the 'maximum safe load' of the cable, and they would only affect safety in a more general sense if there were very voltage-sensitive equopment at the end of the cable. Don't forget that the 3% and 5% figures are only guidelines in an Appendix of the regs. All that the regs themselves require is that the voltage supplied to equipment must be hight enough as to not impair the safe operation of the equipment.

In reality, I rather suspect that the OP probably doesn't need anything approaching 80A (91A), but only he can tell us that.

Kind Regards, John

 

[RF Lighting]

Regulation 525.3 says the requirements for voltage drop is satisfied if the values in appendix 12 are not exceeded, which I read as meaning the appendix values must be met to comply with regulations.

 

[Lectrician]

I did check the voltage drop and found it to be within limits for 16mm though I understand it might not be now at least for lighting.

How did you check it?

I bet off load!

 

[JohnW2]

Regulation 525.3 says the requirements for voltage drop is satisfied if the values in appendix 12 are not exceeded, ...

You must be looking at the BRB! In the BGB, it is 525.101, and refers to the (moved) values in Appendix 4 - but I agree it is effectively the same.

... which I read as meaning the appendix values must be met to comply with regulations.

What it actually says is that the 'above requirements' (525.1, and 525.100 - the latter of which I guess is 525.2 in your BRB) are deemed to be satisfied if the (3% and 5%) figures in the Appendix are not exceeded. However,one does not have to rely on 525.101 (your 525.3) to 'deem' that 525.100 (your 525.2) is satisfied - one can satisfy 525.100/525.2 directly and that regulation merely requires that (in the absence of a relevant 'product standard') ... the voltages at the terminals [of the equipment] shall be such as to not impair the safe functioning of that equipment".

Kind Regards, John

 

[JohnW2]

I did check the voltage drop and found it to be within limits for 16mm though I understand it might not be now at least for lighting.

How did you check it?

I bet off load!

Given that, AIUI, this circuit has yet to be installed (other than burying the SWA), I presuming the 'checking' refers to calculation, not measurement!

Kind Regards, John

 

[ericmark]

Assuming 63A is about the largest MCB one can get after that it's a moulded breaker and incoming to garage 0.35Ω then at the studio/workshop likely around the 0.43Ω mark with a 5 volt drop. Using installation method C.

So within the 6.9 volt for lighting.

At 80A 0.44Ω and 7.2 volt drop so slightly over the volt drop for lighting.

However look at the simple ring and protected with a 32 amp MCB but design current for volt drop is 26 amp so moulded breaker size and design current are not always the same.

The enclosure for a moulded breaker is quite large and once you select a 80A then no real going back. I would use a switches three phase fuse box linking all incomers then you can fit fuse for garage and fuse for studio/workshop which can be subsequently swapped in size without massive expense.

I would to start with feed both with a 63A fuse although in could mean 126A total which could blow the incoming fuse it is very unlikely that both would be loaded to limit at the same time. Also a 63A fuse will take quite some overload before blowing so either supply could be loaded to 100 amp for a short time without the fuse blowing.

You clearly can't make a silk purse out of a sows ear but given the situation this to me seems best compromise. It also means if the supply proves to be not big enough you could ask for a larger supply as nothing rated at 100A is directly connected likely the switched fuse rated at 160A so you can ask for 160A supply or a split phase supply if required.

Lets see what the others reckon of my idea.

 

[Adam_151]

Provided the design load was less than the 50A* rob has calculated as the maximum current for the volt drop to stay within spec. Then I'd not have a problem with putting it on a 60A BS1361 fuse (now bs88-3)


*So no electric shower, or loads of panel heaters, etc

 

[JohnW2]

Provided the design load was less than the 50A* rob has calculated as the maximum current for the volt drop to stay within spec.

As eric has said, at 63A one would still be within the 3% VD for lighting. If one could avoid lighting (e.g. by running ELV lighting off charged batteries), then, VD-wise (5%), one could have up to about 137A on 30m of 16mm² cable - but the CCC of the cable would obviously be inadequate for that. However, a design current/OPD of 80A would be fine if there were no lighting directly supplied by the circuit.

As you and I have both said or implied, we really need to understand what the load actually would/could be.

Kind Regards, John