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# Garage supply CPC

Discuss Garage supply CPC in the Electrical Forum area at ElectriciansForums.net

#### ianphilip007

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hi something strange came up on todays elecsa assessment. my job had a garage suply fed via a spare way in the consumer unit which fed a 5 way garage consumer unit. Now the CPC was sized using the adiabatic equation correctly. He mentioned that as the supply circuit was feeding a C.U with more than 2 ways the cpc in the supply cable was classed as a main earthing conductor and may be undersized, shock horror how can this be,. i then told him how its size had been calculated and he said no more and that it was fine. didnt really understand why the cu having more than 2 ways would make any difference, as the fault current would be dependent on the zs of the supply cable, and we size cpc's and main earthing conductors either by ref to table or via calculation. cant find anything in section 54 bs7671 with reguards to what he was talking about . anyone heard of this ??

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#### Paul.M

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Ian I think it boils down to only having one cpc per and not using that cpc for any other circuit. Taking it to another db would class it as an earth and not a cpc. But if the figures work out fine and he's happy with it, happy days.

#### ianphilip007

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yes i agree but what baffeled me is the fact that a 2way db would of been ok but because it had 5ways. sizing of main earthing conductors ansd cpc's using adiabatic is the same. just nearly fell off my chair when he siad it lol but as you say all ok

and another day and ive learnt something new so all good.

#### Paul.M

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I wonder if this is a "rule of thumb" type of thing? Without using diabetic equation only 2 circuits are allowed???? Do the maths and if it works out, great?

I've just finished work and in the pub atm, I'll have a look in the books when I get home. This has got me scratching my head.

#### darkwood

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Its covered in section 543, reg 543.1.2 and its in reference to a protective conductor which is common to 2 or more circuit and not as he explained or you interpreted him but review notes (ii) which allows you to calculate it if you dont wish to follow table 54.7 which then you need to use the most onerous of the values of fault current and operating time of the various circuits when determining the correct size protective conductor..... usually quicker and easier to apply table 54.7 as your values are not normally known if your pricing the job thus not giving you the option to calculate

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#### Risteard

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Taking it to another db would class it as an earth and not a cpc.
Not sure where you're going with that but I wouldn't agree. There is only one Earthing conductor in an installation so distribution circuits will have cpcs.

#### ianphilip007

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Its covered in section 543, reg 543.1.2 and its in reference to a protective conductor which is common to 2 or more circuit and not as he explained or you interpreted him but review notes (ii) which allows you to calculate it if you dont wish to follow table 54.7
thats the only thing i could find in relation to what he was saying when i looked afterwards. but as you say that would apply to a 2way cu just as it would to a 5way. no real problem as the cpc was sized correctly just wanted to know what he was refering to thanks.

#### darkwood

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Ive corrected many a clark of work in my time although they dont like to be proven wrong they usually keep their distance after but as i edited my reply and added the fact that the values needed to use adiabatic equation are not usually known in the design or pricing stage so table 54 allows for worse case senerio and is generally the chosen method.....

So any supply cable up to 16mm that supplies multiple circuits will require a protective conductor to match the csa of the line conductor unless you have measured values to work out the adiabatic equation, but teaching, eggs, suck... comes to mind here hence you got confused regarding 2way board excemption.... id be inclined to ask him to clarify this

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#### Paul.M

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Not sure where you're going with that but I wouldn't agree. There is only one Earthing conductor in an installation so distribution circuits will have cpcs.
I'm just going by definition of of cpc and earth mate.

Cpc protects the circuit from the furthest point to the met and the earth starts at the met to the delta/star transformer.

Db1 to bd2 is a cpc and the earth if its supplying 2 or less circuits. If supplying more look at 543.1.2 as DW says. It would be nice to have an answer to this because I've fitted garage dbs with more than two circuits and I would like to know for future.

#### ianphilip007

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i agree, in this instance the design calculations were done by myself so i knew the expected fault current so was able to size using the adiabatic. it only came up as i used t+e within the building and swa 3 core outside so the reduced cpc size in the t+e was the factor. other wise the cpc would have complied with the table

#### topquark

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I think you can only classify it as an earth if it's connecting an EMT back to the main MET, otherwise it's an CPC for a distribution circuit.

#### ianphilip007

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as far as i can see as long as the cpc of the supply cable is sized for the pefc/ disconnection time at the second db then there is no problem.

#### ianphilip007

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I think you can only classify it as an earth if it's connecting an EMT back to the main MET, otherwise it's an CPC for a distribution circuit.
if im honhest thats the way i see it its a cpc for the distribution circuit to the garage, to protect that cable. i never would of classed it as a main earthing conductor.

#### darkwood

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The only thing i would highlight here is you need to know the worst value of the circuits connected to the D.B. when you are doing the calcs and not the supply D.B. supply ELI reading although its not impossible to estimate i tend to find values can differ though from estimated when actually taking the reading at the end of the job as many factors come into play that could raise the resistance value.

#### topquark

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if im honhest thats the way i see it its a cpc for the distribution circuit to the garage, to protect that cable. i never would of classed it as a main earthing conductor.
I think that's where it bets a bit "grey" as the earth terminal bar in the CU (when more than one final circuit) can become an EMT (with the possibility of adding bonding etc).

#### Paul.M

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So at what point does a cpc become a main earth for db2?

And why am I getting into this right now whilst in a beer garden?

#### ianphilip007

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are you refering to the worst values of ZS darkwood ??

#### ianphilip007

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So at what point does a cpc become a main earth for db2?

And why am I getting into this right now whilst in a beer garden?
id just drink my beer lol i really dont think it matters that much as if you size it as a main earth or as a cpc it will be the same using the adiabatic. or if you just make the cpc the same as main conductors you will have a correctly sized cpc or main earth.

#### darkwood

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as far as i can see as long as the cpc of the supply cable is sized for the pefc/ disconnection time at the second db then there is no problem.
This would be correct if it was supplying one circuit but it is supplying multiple circuits so the most onerous recorded or estimated value has to be taken into account which would usually (not always) be the smallest cpc, as garage circuits usually dont tend to be long runs anyway compliance is usually achievable without the need to upgrade the cpc in the T&E but il stress this isnt always the case.

This is why following table 54.7 from the off is usually the best method less working out and avoids occasions that may catch you out.

#### topquark

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So at what point does a cpc become a main earth for db2?

And why am I getting into this right now whilst in a beer garden?
I don't think it ever should, but is sometimes misused. If you needed this situation (to accomodate bonding etc at the supplied location) then you should install an EMT and supply a separate earthing conductor from that EMT back to the MET.

Focus on the beer mate! I cried off going coz it was pi**ing down and it's over a mile.

#### ianphilip007

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yes i agree re supplying one circuit, but the highest pefc will be at the second db. so if the cpc for the supply cable is sized according to the fault current hear i cant see a problem re the final circuits within the garage .

#### Paul.M

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id just drink my beer lol i really dont think it matters that much.....
Best thing I've heard all day Ian. Ps well done on passing your assessment. Hope the biscuits didn't cost you too much lol .

#### ianphilip007

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no plain rich tea he ignored the chocolate rocky bars hehe

#### ianphilip007

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I don't think it ever should, but is sometimes misused. If you needed this situation (to accomodate bonding etc at the supplied location) then you should install an EMT and supply a separate earthing conductor from that EMT back to the MET.

Focus on the beer mate! I cried off going coz it was pi**ing down and it's over a mile.
do you mean install a seperate main bonding conductor ?? re (tncs)

#### topquark

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do you mean install a seperate main bonding conductor ?? re (tncs)
Only if there is bonding to do (ie extraneous conductive parts in the outbuilding) AND you are exporting the earth.

For this particular case, (attached?) garage, in practice then you'd just bond back to the MET IF you had any bonding to do.

What you have done is fine.

#### darkwood

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yes i agree re supplying one circuit, but the highest pefc will be at the second db. so if the cpc for the supply cable is sized according to the fault current hear i cant see a problem re the final circuits within the garage .
In reply you are cascading mcb's which due to the reduced earth size allowed for standard circuits this then sets up a siuation where it has a knock on effect when the circuit is reduced again, in the scope of things this would give premature limits to circuit sizes especially for circuits with mcb ratings approaching that of the D.B. supply mcb.. e.g. 40amp (b) D.B. supply down to a 32amp ring main supply off sub board, tabled guides for circuit sizes would be contravened in some situations leaving a non compliant zs value hence you should apply table 54.7 unless you can prove through the adiabatic equation that the circuit your installing will comply ..... its usually too late at the end of the job to find your values are higher than expected and because you could say have a zs value of 0.9ohms covering the D.B. it would only give an allowance of 0.25ohms left for a ring main hence its so easy end up with a installation that dosn't comply.
Its was introduced i assume because it was a situation often tripping electricians up when supplying multiple circuits and finding they couldn't comply so it was implemented to ensure you would comply and if you so choose to hve a smaller earth you had to calculate it and prove its compliance.

#### Guitarist

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This is quite an interesting thread, as I'm sure a great many electricians supply other DB's with T&E.

#### topquark

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This is quite an interesting thread, as I'm sure a great many electricians supply other DB's with T&E.
Yep, mine is (and I can't "blame" anyone else).

#### darkwood

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See it all the time Guitarist but as i said it doesn't mean it doesn't comply it means you got to prove it does and i often used to find an earth upgrade to be neccessary esp' on the longer supplies with close fuse ratio's in the cascade.

P

#### pencilpusher

I had exactly the same issues a while ago but I cannot find the thread. Anyway, Lenny helped me out with the "let through figures" in relation to the amount of fault current the protective device in the main DB will actually "let through". If it's a modern MCB (in my case an RCBO) then let through figures that can be used in the adiabatic are lower than the fault current calculated. It meant that the supply to garage CPC could be smaller. The problem was the T&E CPC feeding the adaptable box where it met the SWA. I will see if I can find the figures I refer to.

I believe these figures (I2T) are manufacturer specific and need obtaining from the specific manufacturer of the PD.

EDIT: Found it; http://www.electriciansforums.net/e...ectrical-regulations/48549-cpc-size-help.html

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#### Guitarist

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See it all the time Guitarist but as i said it doesn't mean it doesn't comply it means you got to prove it does and i often used to find an earth upgrade to be neccessary esp' on the longer supplies with close fuse ratio's in the cascade.
Hope there are no plumbers reading this thread. I would hate for their preconception that electricians "just stick wires in" to be spoilt. :rolleyes2:

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