Recomendations...

[steviez]

Hi,

I am working on a project at a local garden center and have to put in a new 8 way 3 phase board next to an existing 8 way 3 phase board. It is wired via 16mm SWA cable. Would it be better to:

1. Bring a new feed to the board from the meter
2. Bring a new feed to the board from the existing one

I am only an apprentice and would like to get to know the best way to do this so I can learn before my boss asks me

Thanks

 

[Spark123]

Not a lot to go on as we don't know the loading on the existing board nor what the new board will feed.
I always think it better to have a nice big feed protected by fuses (to provide discrimination) but without knowing more information is impossible to say.

 

[holmslaw]

..

 

[steviez]

Thanks for your replies. The existing board is a Square D 125Amp and its fully loaded feeding a few ring mains, lights and sub mains to some green houses. The new board will only use 4 ways to start with but the customer has requested the same sort of board installed so he has future expansion. It will supply a large fan (about 15- 20 Amp) some out door bulk head fittings, a small ring and possibly a small oven.

The board will sit next to the existing one so I was looking at the idea of connecting it from that via a 3ph MCB.

Do you think it would be better to run another SWA from the mains up to this board?

 

[Adam_151]

You need to assess the loading on the existing and new boards (allowing some for future expansion of the spare ways), to decide whether you need a new submain or whether it can be added to the existing

If you do decide to add it to the existing them bear in mind what homslaw and spark123 have said.... there could possibly be issues with getting two lots of cables in the same supply terminals (though if the isolators are 125A and the cables are 16mm it might be less of an issue), and that running a submain from an MCB has the potential of introducing discrimination issues.

You might therefore want to enquire into the availability of an unfused tap off for the board in question

 

[steviez]

I think the best way would be one of these:

1. See if it is possible to bring a new feed from the mains connection (meter point)

2. Look at the possibility of connecting the sub main from the incoming side of the main isolator.

The load on the existing board changes at different times of day and while I do not believe it goes anywhere near the max load of the cable or board I would feel better doing one of the above.

Thanks

 

[steviez]

Just been to the job in question today to replace a faulty fan controller so I managed to take a look a bit more:

The building has a 3phase supply with 3x 100 amp service fuses, it is then connected to a 63amp MEM switch fuse via 16mm tails. From the switch fuse it goes up to the roof space via a 16mm 4 core armoured cable. The board is full (24 single ways) but not all are in use at once and when they all are it pulls about 40 - 60 amp per phase depending on what is in use.

There is no room to bring a new feed from the switch fuse so was thinking of running the new board from the main board via 16mm cable and 3 phase breaker, does that sound OK?

Also one question... why has the original electrician fused it down to 63amp via the switch fuse before it goes to the board? it seems a little bit on the low side for an entire coffee shop / garden centre.

Thanks

 

[Adam_151]

It might pay to do some load monitoring of the board in question before adding additional loads... does your company have equipment to do this... it can be hired in but chances are no one has priced for this!

Whats been priced for ooi, I assume *not* a new submain back from the intake?

 

[steviez]

It might pay to do some load monitoring of the board in question before adding additional loads... does your company have equipment to do this... it can be hired in but chances are no one has priced for this!

Whats been priced for ooi, I assume *not* a new submain back from the intake?

We have the equipment to do those kinds of tests and have done so. If I remember correctly the entire board does not go above 100amp (over 3 phases) so its well within its capacity.

We are lucky in that the customer has given us an open budget as he wants it done correctly first time. We are going to take a few circuits out of the old board and put them in the new one to balance it out and also fit RCBO's to a few circuits that need it as there is NO RCD protection on any circuit at the moment.