garage electrics

Wozzaaah · Jul 16, 2007

I need to uprate the supply cable to my garage (my Portamig's a bit hungry

)and I'm getting conflicting advice from two different sparks. One says I need a switchfuse on the supply end with an rcd protected CU at the other, the other says it HAS to be an rcd at BOTH ends. One also says the cable HAS to be buried wherever possible, the other makes no mention of it having to be buried at all as long as it's not vulnerable. Ideally I'd like to clip it to the wooden fence as its such a long run it would be a right pain to dig a trench that far. It is a straight run from house to garage and doesn't cross the lawn at all. The cable will be 16mm 3 core SWA, XLPE. Any sparks on here to give me a definitive answer (possibly backed up by documentation) please? I've googled and googled but there's no mention of it HAVING to be buried, only how deep to bury it if choosing burial. I've got a pdf copy of the IEE 16th edition but not had time to look through it yet.
Woz

rtbcomp · Jul 16, 2007

I'd put RCD and MCB as close to the consumer unit as possible, this protects the maximum amount of cable. Especially if you leave it exposed.

If the CU already has an RCD it will be 60mA, in this case fit a 30mA RCD in the garage. If this trips the main 60mA should not trip.

Don't put your lights on the RCD, if you get an emergency that causes a trip you want the lights to stay on!

irondarren · Jul 16, 2007

I often found trip switches and welders don't go together, something to do with the large draw of a welder. Perhaps the RCD at home is a bit old, but it can be a pain

Luke · Jul 16, 2007

fwiw, my portamig 211 trips out a 16A trip, when its cold. Seems better when it's warmed up.

i might just put a 20A in.

malcolm · Jul 16, 2007

irondarren said:
I often found trip switches and welders don't go together, something to do with the large draw of a welder. Perhaps the RCD at home is a bit old, but it can be a pain

I had trouble with my ancient syncrowave blowing trip switches on start up - that's in-rush current to fill up the transformers. I ended up with a type D trip switch that allows a bit more current for the first 100ms without tripping. The normal one is B type, and C type are often used for welders.

Never had RCD trouble though (apart from yesterday when the house was struck by lightening which messed up a couple of bits and pieces). The RCD compares current out vs current in and assumes any difference is current going to earth which would be a bad thing.

Rail_Head · Jul 16, 2007

Wozzaaah said:
I need to uprate the supply cable to my garage (my Portamig's a bit hungry )and I'm getting conflicting advice from two different sparks. One says I need a switchfuse on the supply end with an rcd protected CU at the other, the other says it HAS to be an rcd at BOTH ends. One also says the cable HAS to be buried wherever possible, the other makes no mention of it having to be buried at all as long as it's not vulnerable. Ideally I'd like to clip it to the wooden fence as its such a long run it would be a right pain to dig a trench that far. It is a straight run from house to garage and doesn't cross the lawn at all. The cable will be 16mm 3 core SWA, XLPE. Any sparks on here to give me a definitive answer (possibly backed up by documentation) please? I've googled and googled but there's no mention of it HAVING to be buried, only how deep to bury it if choosing burial. I've got a pdf copy of the IEE 16th edition but not had time to look through it yet.
Woz

A cable is adequately protected by an MCB. When (if?) someone slices through it with a metal implement, the inner live conductor will have a perfect short to the earthed metal sheath (assuming SWA). The fault current will be very high and trip the MCB - so an MCB is plenty adequate. One RCD is fine. 2 RCDs should never be installed in a line - they wont discriminate and in the event of a fault either could trip.

As far as I can remember (its a while since I last sat my 16th Edition!) its not allowed to clip to a fence???

16mm sq??? that will be a lot of notes!!

Wozzaaah · Jul 16, 2007

Rail_Head said:
A cable is adequately protected by an MCB. When (if?) someone slices through it with a metal implement, the inner live conductor will have a perfect short to the earthed metal sheath (assuming SWA). The fault current will be very high and trip the MCB - so an MCB is plenty adequate. One RCD is fine. 2 RCDs should never be installed in a line - they wont discriminate and in the event of a fault either could trip.

As far as I can remember (its a while since I last sat my 16th Edition!) its not allowed to clip to a fence???

16mm sq??? that will be a lot of notes!!

Thanks Rail_Head, so i assume that if a fault current is high enough to trip an MCB then it would also blow my 60a switchfuse? The sparky I've asked to come and look at the job says it has to be RCD protected on the supply side as well which contradicts what another sparky told me. The trouble is the guy who's coming to look at it is probably the one who'll do it and certify it so I suppose we'll have to do it his way. I'm just going to be doing the groundwork and getting everything ready for him to connect. Looks like I'll be digging a 50m trench after all

.

JimM · Jul 16, 2007

Hi

I'm no electrician myself but would recommend you ask this over at the uk.d-i-y newsgroup, there are plenty of sparks on there and they seem to know their stuff.

My initial thoughts however would be

1. RCD in the garage would be adequate
2. Depending on the distance you may need to have an independant earth at the garage end (or at least it may be advisable)
3. Don't think a trench isn't required, not sure about clipping to a fence though
4. Pretty sure this would fall within the scope of Part P so technically you can't do it yourself (wouldn't stop me though

HTH

Jim

Wozzaaah · Jul 16, 2007

Rail_Head said:
16mm sq??? that will be a lot of notes!!

I've been quoted £3.55+vat per metre and I need about 70m. I think I can get a bit off that as well as my mate has a mate who works at a local wholesalers. I could probably get away with 10mm but If I'm doing the job I might as well spend a bit extra and futureproof it. The problem I have is the length of the run.

rtbcomp · Jul 16, 2007

Rail_Head said:
A cable is adequately protected by an MCB. When (if?) someone slices through it with a metal implement, the inner live conductor will have a perfect short to the earthed metal sheath (assuming SWA). The fault current will be very high and trip the MCB - so an MCB is plenty adequate. One RCD is fine. 2 RCDs should never be installed in a line - they wont discriminate and in the event of a fault either could trip.

But what if the cable is worn through or the cut isn't clean? IMHO the cable should be protected by an RCD. If you have a 60mA RCD near the CU and a 30mA in the garage the one in the garage should trip first. Even more so as the 30mA RCDs trip faster than the 60mA ones.

Luke · Jul 16, 2007

You're going to have to work hard to not get an appreciable voltage drop over that sort of distance.

Wozzaaah · Jul 16, 2007

JimM said:
Hi

I'm no electrician myself but would recommend you ask this over at the uk.d-i-y newsgroup, there are plenty of sparks on there and they seem to know their stuff.

My initial thoughts however would be

1. RCD in the garage would be adequate
2. Depending on the distance you may need to have an independant earth at the garage end (or at least it may be advisable)
3. Don't think a trench isn't required, not sure about clipping to a fence though
4. Pretty sure this would fall within the scope of Part P so technically you can't do it yourself (wouldn't stop me though

HTH

Jim

Thanks Jim

1. the new cu will have a 60a 30ma main switch/rcd
2. The sparky i spoke to said the same, he'll check the earthing.
3.???? this is the one I'm concerned with as I don't do diggin if i can help it

4. He's part-p certified so that's not an issue (although it wouldn't stop me either

)

Wozzaaah · Jul 16, 2007

Luke said:
You're going to have to work hard to not get an appreciable voltage drop over that sort of distance.

Hence the need for 16mm cable! An 8kw load over 70m gives a volt drop of 6.1v, well within the 4% limit. 12.1kw over 70m gives a drop of 9.1v which is the maximum 4%. That will allow a total load of 52.17amps, which should be well within my usage.

Luke · Jul 16, 2007

cool, I admit i hadn't done the maths.

Drains · Jul 16, 2007

You could step it up to 10,000v or so, put it down some bell wire, then a step down in the shed.

Oops, sorry, just thinking out loud. Take no notice.. :laughing:

Wozzaaah · Jul 16, 2007

Luke said:
cool, I admit i hadn't done the maths.

Hehe, nor did I, I just used the voltage drop calculator on TLC's website! :laughing:

Wozzaaah · Jul 16, 2007

simondrains said:
You could step it up to 10,000v or so, put it down some bell wire, then a step down in the shed.

Oops, sorry, just thinking out loud. Take no notice..

Hmmm, maybe I'll make one of them Tesla coil thingys and work out a way to power my portamig off it :laughing:

irondarren · Jul 16, 2007

wozz if it was me, id runn the power to a distribution box
http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/Contactum_Insulated/index.html
Something like these little guys, you need fuse to populate them about 7 quid each, but this would allow power for other items. Just take power from you main box and run power to the garage strait to one of these. Then you can set up fuses for each item etc. welder using 32amp socket, 240v normal plug sockets and lights. So i guess a 4 way unit would be good enough and it wont break the bank. I also believe you can get the fuse units with rcd's in.

Wozzaaah · Jul 17, 2007

Thanks Darren. I've got the garage end sorted, an 8 way split-load metalclad cu. It's just the supply side I'm querying regarding which sparky is correct.

Davek0974 · Jul 17, 2007

Hi all,

I'm in the middle of getting this done myself for my new workshop after moving house. I'll admit it's only an 10mtr run of cable though, being done by a sparky although i would feel happy doing it myself. Heres the setup...

30A mcb at house end,
4mm SWA run,
6 way consumer unit at garage containing...
1x20A rcd for sockets only,
1x6A mcb for lights,
1x15A mcb for 3hp rotary phase converter,
1x6A mcb for outside lights.

The rotary converter then feeds a 3-phase panel which splits the supply between 2 lathes and a milling machine.

It is only used by one person hence the light load design. The cable had to be buried, so i had to take up the block pavers in the front garden and re-lay them after.

Make sure the rcd only feeds the sockets as mentioned above.

Dave

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