Large number of 13A sockets in a steel building: Design considerations.

[Mark42]

This weekend I’m installing 20 2-gang 13A sockets around the inside walls of my new 20m x 8m steel building.

These are only for small car battery chargers, so total continuous load will be trivial. Plus one fused connection unit (directly in the ring) for the electric shutter at the far end of the building.

Cabling is surface 3-core 2.5mm SY girder-clipped to the horizontal steel members. It’s expensive, so cable cost is a limiting factor. I have one 100m reel only.

I have designed the install as below. It is not in accordance with ‘old-skool’ floor area guidance, but I believe it’s OK. Does anyone disagree: ie have I missed anything?

Basically it’s one huge ring around the walls of the building, with sockets equally spaced. Total ring cable length 60m. Considering volt drop at this length I will feed the ring from a 20A MCB. 32A would probably be OK, but I don’t need it, and 20A is a concession to ‘make up’ for exceeding the normal floor area suggestion.

Of course it could be split into two 20A radials down each side of the building, but I see no electrical reason for this.

I’ll lightly solder the stranded cables together in each box before connecting to the socket outlets, to reduce both ring resistance and the chances of the ring going open circuit for any reason in the future. Earthing will be excellent: in addition to the CPC, every metalclad box is bolted directly to the frame, boxes are of course strapped to the sockets internally, and the whole structure is bonded.

There is one small corner of the garage which will be used as a working area, with power tools etc. This has a separate 32A conventional ring, and a separate 16A radial to an industrial 16A socket for the arc welder. Both on a different phase to the ring above.

The main electric shutter is on a separate circuit – close to the CU so cable usage is insignificant. Should a battery charger fault pop the main RCD on the big ring circuit phase, it would not affect this shutter, so I could still get into the building without faffing around with manual overrides.

I’m a great believer in imaginative solutions, and in not always following guidance like an unthinking sheep. But I’m not so arrogant (yet!) not to ask for advice from those more experienced than I am. So, have I been daft and missed anything here? It’s my own place so I want to do it right, once, then not touch the thing for 20 years.

Cheers, Mark.

 

[malcolmsanford]

Personally Mark I would have installed a sub board for this as you never know what the future will hold. Then you would have no issues with disconnection times, length of runs etc etc.

It is a large ring final mate and your design though works by the sounds of things just seems to be a case of trying to do something a little off the wall, just to prove it can work, personally I like simple.

Not sure why your going to the trouble of soldering cables together, to be honest what advantage if any will you achieve, you may by doing this reduce the impedance slightly, but not sufficient to affect disconnection times on a 20amp MCB unless your thinking of a "c" type for the chargers, which you may have to consider with the start up on those babies. also if you need to fault find and split the ring there will be a few curses.

But with the RFC being RCD protected your ZS can be 1667, which negates the soldering really

Also by having such a large RFC on a single RCD your chargers are going to have Protective Conductor Currents and you may start getting leakage tripping on it, I'm assuming it's a 30mA additional protection one, so do you need the RCD protection?

Not sure about what you mean if you have a 20amp MCB for the RFC and then state the main RCD, is the shutter on a split board without RCD protection ? That sounds to me if you have an upfront RCD protecting the entire installation and if that is the case would not RCBOs be a better option, if you want RCD protection.

 

[micknew]

The best advice would be to get a qualified electrician in who would carry out the works and give you certification for insurance purposes.

 

[malcolmsanford]

The best advice would be to get a qualified electrician in who would carry out the works and give you certification for insurance purposes.

I suppose it would depend if this steel building is in the UK or Malaysia

 

[Mark42]

Personally Mark I would have installed a sub board for this as you never know what the future will hold. Then you would have no issues with disconnection times, length of runs etc etc.

Sorry, Malcolm I didn’t make it clear. Of course it’s on a sub-main: the building is 100m from the main house.

Supplied by a 4-core 10mm u/g SWA fed from a 32A 2-phase MCB in a main house board. Cores are L1, L2, N and PE, plus armour terminated with Piranah nuts (I don’t like banjos on plastic boxes) and bonded.

It’s a split-load 17[SUP]th[/SUP] Edn CU reconfigured as a split-phase DB. The single phase main switch removed, two 25A 2P RCDs installed, fed from the cable above via a separate 3P&N isolator acting as the local main switch. Non-standard I know, but it’s much cheaper than installing a 3P board for only a few ways. This is an unusual split-phase 240/480V ‘farm’ supply

Perhaps a photo is better, I’ll get one later.

It is a large ring final mate and your design though works by the sounds of things just seems to be a case of trying to do something a little off the wall, just to prove it can work, personally I like simple.

Yes, but I like saving expensive (and beautiful) SY too. I think putting in two rings is not more simple than my proposal, and just consumes wire and DB ways for no electrical reason. Unless I’m wrong, and that’s why I’m here asking questions.

Not sure why your going to the trouble of soldering cables together, to be honest what advantage if any will you achieve, you may by doing this reduce the impedance slightly, but not sufficient to affect disconnection times on a 20amp MCB unless your thinking of a "c" type for the chargers, which you may have to consider with the start up on those babies. also if you need to fault find and split the ring there will be a few curses.

OK I agree. I don’t like finely-stranded cable in socket plates that’s all. The terminal screws tend to displace the strands, the strands can annoyingly bunch up on insertion, especially when there’s 5 sq mm of them loosely twisted together, there’s always one bloody tiny live strand sticking out so you have to do it again, in poor light, and so on. I agree it’s overkill. As to faults? That’s what side cutters are for! But yes, point taken.

It will be a B-curve breaker: they are only very small ‘Accumate’ chargers for battery maintenance. I don’t know if they have excessive earth leakage from filter circuits or whatever. Time will tell, but yes, nuisance tripping would require attention, then I agree RCBOs on each circuit might be better.

The wiring may as well include the high-integrity earthing scheme as standard. Costs nothing.

But with the RFC being RCD protected your ZS can be 1667, which negates the soldering really

Yes I know, but why design down to worst-case when at no cost you can design up? My children will play with stuff in there, hoses, wet cars, no shoes on a concrete floor, wet clothes, who knows? If I don’t see Zs in the tens of ohms I’m not happy. Yes, overkill again!

Also by having such a large RFC on a single RCD your chargers are going to have Protective Conductor Currents and you may start getting leakage tripping on it, I'm assuming it's a 30mA additional protection one, so do you need the RCD protection?

Not sure about what you mean if you have a 20amp MCB for the RFC and then state the main RCD, is the shutter on a split board without RCD protection ? That sounds to me if you have an upfront RCD protecting the entire installation and if that is the case would not RCBOs be a better option, if you want RCD protection.

As above, you may be right.

I used two 2P RCDs, which I had in stock, as main switches on each phase. Therefore there are no unprotected ways. It looks fine to me so far IF there’s no nuisance tripping.

The 'important' shutter will be on the phase which will have no connected loads when the building is unattended, so nuisance tripping is unlikely (so long as my wiring's waterproof!)

I did use RCBOs in my main house for everything so will consider changing the 2P RCDs for 2P switches and then installing RCBOs where necessary.

Thanks for the advice. Hope this all makes sense. It's a bit hard in words!

The best advice would be to get a qualified electrician in who would carry out the works …

What, like you, you mean? OK, but tell me first that what I am proposing is electrically dangerous or in some way electrically incorrect. Then I’ll consider it.

 

[Engineer54]

So what sort of earthing arrangement are you on at this farm, TT system or PME??

 

[needasparks]

Did I read that correct, your having a spare un-used phase introduced (2 phase & N).

 

[Silly Sausage]

Instead of soldering use bootlace crimps.
If you're going for permanent connections (>20 years you say), use 2 in 1s....
Twin Cord End, 10.0mm - Red - Twin Cord Ends - Ferrules & Cord Ends - Crimping Tools | Crimp Terminals | Electrical Connectors

Leave plenty of slack cores in in case you ever need to cut them off.

I think there's a reg about stranded conductors, can someone point it out?

Have fun making off 40+ SY glands!

 

[lamb]

You may need to consider the flammable gases that can be produced when batteries are in charging mode. Depends on the battery type and how many.

 

[Engineer54]

Did I read that correct, your having a spare un-used phase introduced (2 phase & N).

No, .... he means that he has a 3 phase supply, but he's intentionally loaded up the other two phases so he can have his shutter door on on the remaining unloaded phase!! Not to worry he seems to know best!! lol!!!

 

[micknew]

Perfect example of a little knowledge being dangerous.

 

[Engineer54]

Perfect example of a little knowledge being dangerous.

Just look at the length of the sub-main from the house and then the 60m ring circuit!! Then he's talking about his kids playing with water hoses in there with bare feet!!! Very sensible with batteries being charged.

Still no mention about LABC or what type of earthing arrangement he has at this farm....

 

[Mark42]

Did I read that correct, your having a spare un-used phase introduced (2 phase & N).

No, .... he means that he has a 3 phase supply, but he's intentionally loaded up the other two phases so he can have his shutter door on on the remaining unloaded phase!! Not to worry he seems to know best!! lol!!!

LOL back, gentlemen. I’m not that bloody stupid. Maybe I was not clear. There are ONLY TWO phases on the site. It’s (antique) ‘spilt phase’ installed after the war to save copper. 240V x 2 at 180 degrees. Welcome to rural England, things move slowly here

The two phases will both be used and will have a reasonably balanced load when the building is IN USE. But when unoccupied (ie most of the time) only the maintenance chargers will be in operation, all on a single phase. Maybe 2A tops, intermittent, even the most absurd stickler can’t tell me to balance that across phases! (Can you?) Hence the ‘clean’ second phase, unlikely to trip out when nothing is in use.

Instead of soldering use bootlace crimps.

… I think there's a reg about stranded conductors, can someone point it out?
Have fun making off 40+ SY glands!

Brilliant idea: thanks! Pack of 6mm twin ferrules ordered already.

Is the stranded reg (which I also can’t find) about actually needing to use stranded where movement is expected?

Hmm… SY glands. I hesitate to admit it but I again use a non-standard method: unwind the braid, twist it, fit g/y sleeving, sleeve the two blacks with brown & blue, then add an inch or so of 12mm shrink-fit over the lot where the outer sheath ends. It looks neat, costs little, and can then be used with a standard cable gland. The sleeved braids can then be terminated in the metalclad box earthing lug. Flame away, gentlemen.

SY glands are a rip-off price, and I think only really indicated when EM protection is required.

You may need to consider the flammable gases that can be produced when batteries are in charging mode. Depends on the battery type and how many.

Good point, but it’s a big open, ventilated, area and only a few lead-acid car batteries. It’s only little maintenance chargers for rarely-used vehicles, to stop the batteries from self-discharging. No chance of hydrogen gassing off - I believe that only happens when really cooking batteries with a big charger, or with industrial batteries like in electric pallet trucks.

So what sort of earthing arrangement are you on at this farm, TT system or PME??

An excellent question, but one I hesitate to ask for advice about for fear of unleashing a torrent of conflicting opinions! (And urban myths.)

It’s TN-C-S and true PME. Two-wire HV supplying a pole trannie, split phase 480V from there to three houses in total. Overhead LV to the other two, with the poles spiked, and new u/g LV to my place with a spiked u/g joint in my yard. (It was a happy day when I could finally saw down those ugly poles in my garden.)

So what to do with a sub-main in a separate building 100m+ from the main house?

I assume standard practice would be to ‘run’ the PME on the SWA armour to protect the cable, terminate downstream in a plastic enclosure, sheath the nut, then install a TT system for the separate building. But I don’t understand why in my case.

I’m using 4-core 10mm: two phases, one neutral and a spare as a PE. Does this not count as a bonding wire? It’s (just) big enough (the DNO’s incomer is 35mm 100A x 2). So using this 10mm ‘bond’ should bring my steel building into the PME equipotential zone? Yes or no? Were I using ONLY the SWA armour, I agree it may not be enough since steel armour can get scuffed and rot over the years.

I plan to do some experiments: install a local earth spike as for a TT system, then measure the earth loop impedance at the sub-main with the PME only, the earth spike only, and with both in parallel. I suspect the PME, even down that long wire, will be better. What do others think?

My initial plan was to install two spikes in the wet earth on opposite corners of the building, bonded with 25mm to the structure, as if I was going for TT, but use the PME as well. Is this a naughty thing to do? Is there likely to be a PD between the DNO’s value for earth and my local true earth? And if so, would that cause a current to flow in the earth cables?

And the busted neutral saga? Would not my earth spikes take care of that? Or at least reduce any potential on the extraneous metalwork to a non-lethal level while waiting for a breaker to trip? But as I said my supply neutral is now underground, unlike my neighbours’ which looks like bits of bellwire on sticks, so I believe my local neutral is secure.

But it’s not my trade so I may have something completely wrong here!

 

[Engineer54]

Who built the steel framed building, you or was this building existing. I'm asking as i want to know if the concrete floor has any reinforcement ??

 

[Mark42]

Just look at the length of the sub-main from the house …

Yes, I know, it’s ridiculous. I really should move the house!