Submain help please

[dansk]

Hi,

Ive not had to run submain often so i just wanted some advise.

Survey today - 4Kwp on outbuilding roof.

The outbuilding already has a supply from the house in 2.5 SWA, i advised a fresh supply (TBC once i calculated VD) but i suspect 8mm SWA.

The main CU in the house has a spare way but the whole board is protected by a main RCD switch, its quite an old thing and customer has already told me it trips for the slightest thing. I dont want to touch this and was going to connect into the tails prior the main CU.

Questions:

Do i have to put the SWA on a breaker before running it out to the sheds? or can i terminate it in an adaptable box and connect directly into henly?

the supply to the house is PME, so i will make the sub CU a TT in the shed.

Next question - does the inverter need a AC isolator if its located next to the sub CU?

Which end to i earth the sheething to if its PME at the house and TT in the shed?

Thanks

 

[telectrix]

Hi,

Ive not had to run submain often so i just wanted some advise.

Survey today - 4Kwp on outbuilding roof.

The outbuilding already has a supply from the house in 2.5 SWA, i advised a fresh supply (TBC once i calculated VD) but i suspect 8mm SWA.

The main CU in the house has a spare way but the whole board is protected by a main RCD switch, its quite an old thing and customer has already told me it trips for the slightest thing. I dont want to touch this and was going to connect into the tails prior the main CU.

Questions:

Do i have to put the SWA on a breaker before running it out to the sheds? or can i terminate it in an adaptable box and connect directly into henly? the SWA needs overload protection

the supply to the house is PME, so i will make the sub CU a TT in the shed. why?

Next question - does the inverter need a AC isolator if its located next to the sub CU? PV not my field- don't know

Which end to i earth the sheething to if its PME at the house and TT in the shed? see above. gland both ends.

Thanks

what's 8mm SWA by the way?

 

[dansk]

Thanks -

8mm SWA - you not heard? its the new armoured cable....kidding, i meant 10mm.

Overload protection - sure i should have realised this.

TT at sub CU - why not? exporting PME over 80 metres maybe not such a good idea, and also the array will need bonding anyway so i might as well stick a rod in and earth two things in one go.

 

[yellowvanman]

If you have PME at one end and TT at the other, in theory the SWA should be earthed at the 'source' end, which to my mind is the house, even though you are generating at the remote end. This is based on the fact that if the cable gets damaged most of the fault current will be coming from the house!

Meter does need an isolator, which you put on the inverter side of the meter and the MCB or RCD can act as the isolator on the 'other' side, although there is a debate about that as the DTI guide puts the isolator between the consumer unit and the meter!

 

[Graeme Harrold]

An MCB can not act as an isolator unles it switches both the Line and Neutral. If there is room Id fit an isolator switch as well, then there is absoulutely no doubt in how to shut off the PV Power. Just off the top of my head, 16A over 80m, burried (I presume), dont think 10mm SWA is going to be big enough for a 1%VD

 

[yellowvanman]

A single pole MCB on a PME system can act as an isolator - see 537.2.1.1 Red book. Double pole for TT.

Not saying you're wrong with the voltage drop but maybe a bit more marginal than you think. If you consider 16A flowing in a 10mm conductors its not going to raise the temperature of the copper to 70C, and the voltage drop figures are given at cable rated temperature.

 

[dansk]

Thanks for the help -

I know 10mm wont acheive 1%, but i checked up inthe DTI, and allthough 1% is preferbly they understand its not always possible, and in my instance its not possible due to cost restrainst. The Guide also says in anycase the VD must compley with the big red book, and im well within the limits for that.

It gunna look a bit naff with a big isolater either side of the meter, but safty first.

Cheers.

 

[Deleted member 38070]

View attachment 6890 you could pop the gen meter on the top of ac isolator ?? (only one isolator required near inverter) pic of TT sub (2) in garage ;-)
agree with
yellowvanman

i put isolator on source side (when switched off also turns meter off)

 

[Matt_M]

Just for people's information here are equations for working out cable sizes based on how much current they will carrying:

Minimum CSA of Cable Required to Maintain < 1% Power Loss in DC Cables

Am = 2 x Lm x Ist
0.01x Vmpp x k

Rearranging Formula for Length

Lm = Am x 0.01 x Vmpp x k
2 x Ist

Where: Am = CSA of cable
Lm = Length of Cable Run between Array and Inverter
Ist = String Current
Vmpp = System Voltage
k = 56 – factor for electrical conductivity of copper

 

[edexlab]

yellowvanman " A single pole MCB on a PME system can act as an isolator - see 537.2.1.1 Red book. Double pole for TT".

This is correct generally but with Pv the isolators must be lockable in the off position only an Mcb/ Rcd / Main switch does not comply for this reason.

Also Dansk I get the impression that you think he means to put an isolator literally either side of the meter

this is true but he meant one at the origin of the submain and one adjacent to the Inverter

also I think you would be wise to go with the 1% volt drop on a 80m run it'll be cheaper than replacing the cable for a larger size if you do have any problems with local voltage

 

[yellowvanman]

This is correct generally but with Pv the isolators must be lockable in the off position only an Mcb/ Rcd / Main switch does not comply for this reason.

Doesn't an isolation kit apply here then?

 

[PV Martin]

i had the same issue with isolators on a recent job we installed on 16 new builds and the CUs were all very visable in the Lounge's of all properties. i didnt really want to put a big red industrial isolator in someones lounge? after seeking advice from Elecsa i was informed that an MCB could NOT be used as a means of isolation..... but i could fit a DP isolator in line within the CU as long as i provided an MCB type locking device OR the CU door was lockable.

 

[dansk]

All points taken on board - i will up the cable to 25mm which should prevent any problems.

This is what im going to do:

House: 20a MCB, to JB, terminate into 25mm SWA out to shed 80m run, into small CU, then isolator, gen meter and inverter.

 

[edexlab]

Doesn't an isolation kit apply here then?

yes but you can lock these devices in the on position therefore it does'nt comply we had a landlord who insisted on this approach to prevent his tenants from switching ths system off ,but we refused to do it and pointed him to the Niceic who agreed with us

 

[woko]

I find the sunny design software is good for PV cable calcs.

 

[malcolmsanford]

yellowvanman " A single pole MCB on a PME system can act as an isolator - see 537.2.1.1 Red book. Double pole for TT".

This is correct generally but with Pv the isolators must be lockable in the off position only an Mcb/ Rcd / Main switch does not comply for this reason.

Also Dansk I get the impression that you think he means to put an isolator literally either side of the meter

this is true but he meant one at the origin of the submain and one adjacent to the Inverter

also I think you would be wise to go with the 1% volt drop on a 80m run it'll be cheaper than replacing the cable for a larger size if you do have any problems with local voltage

Just out of curiosity this must be a recommendation from a source other than the BS 7671-2008 I guess?