Three phase circuits in holiday let

[Whitsend]

Need to extend wiring in holiday let that is adjacent to workshop with three phase supply. Currently the holiday let has a two way cosumer unit fitted with RCD and 32A mcb for power and 10A mcb for lighting. This is fed from a 40A circuit breaker off the three phase board.
The three phase board is fed via what looks like a 10mm SWA cable protected by a 63A switch fuse from the house.
The workshop has a couple of three phase machines but is only used on a hobby basis and load is well down and also won't be in use while the holiday let occupied (noise). There are a couple of spare ways in the board that could be used for the extended wiring (cooker and hob) and possibly another ring main. However this would mean that there would then be 415 volts in the holiday let. It may be possible to split the circuits between floors to minimise this. Obvious concern is that with max 63A have to split the load across phases for this to work.
Is 415v OK for domestic premises? What additional requirements are there, if any?

 

[Steve]

The only requirement is to seperate the phases for the socket outlets, so you dont have 400 volts within a certain distance (or else you'd need warning labels). Not sure if the same applies to lighting and switches.

Also, I would keep the single point of isolation for the holiday let. Dont go adding other circuits from another board.

Fit a bigger single phase CU and a bigger cable, and a bigger MCB on the 3PH board. Less hassle than trying to work around 3ph in the holiday let.

 

[plugwash]

Fit a bigger single phase CU and a bigger cable, and a bigger MCB on the 3PH board. Less hassle than trying to work around 3ph in the holiday let.

Normally I would agree with you but the trouble is this three phase board is only on a 63A feed. If you put a flat containing a cooker plus all the normal domestic loads on one phase you won't leave much for the three phase equipment in the workshop. Also volt drop calculations for the three phase submain were probablly done assuming balanced load.

I would want to have an isolator in the holiday let that could isolate everything in the holday let as I belive doing otherwise. but a full three phase board sounds like overkill. Personally I would probablly fit a 40A three phase breaker in the workshop and terminate the holiday let end using a custom arrangement of modules in a CU enclosure (since it is only a 40A feed I would just connect stuff together with 10mm singles inside the CU to get the arrangement I wanted).

IIRC if stuff on two different phases is closer than a certain distance you need danger 415V warning stickers.

edit: missed the end of a sentace, addition in bold.

 

[JohnD]

...The workshop has a couple of three phase machines but is only used on a hobby basis and load is well down and also won't be in use while the holiday let occupied (noise)...

so no contention forseen with cooker.

 

[plugwash]

still the potential issue of volt drop on the 10mm feed to consider (remember volt drop is much worse when one phase is fully loaded and the others aren't than when all phases are fully loaded)

 

[Whitsend]

Fit a bigger single phase CU and a bigger cable, and a bigger MCB on the 3PH board. Less hassle than trying to work around 3ph in the holiday let.

Normally I would agree with you but the trouble is this three phase board is only on a 63A feed. If you put a flat containing a cooker plus all the normal domestic loads on one phase you won't leave much for the three phase equipment in the workshop. Also volt drop calculations for the three phase submain were probablly done assuming balanced load.

I would want to have an isolator in the holiday let that could isolate everything in the holday let as I belive doing otherwise. but a full three phase board sounds like overkill. Personally I would probablly fit a 40A three phase breaker in the workshop and terminate the holiday let end using a custom arrangement of modules in a CU enclosure (since it is only a 40A feed I would just connect stuff together with 10mm singles inside the CU to get the arrangement I wanted).

IIRC if stuff on two different phases is closer than a certain distance you need danger 415V warning stickers.

edit: missed the end of a sentace, addition in bold.

Thanks. The holiday let is now being extended into part of the workshop where the supply is. The advice above makes sense assuming that you fit a three phase isolator to the CU and split all circuits across this balancing appropriately. This would allow reuse of the existing RCD and breakers from current CU. Would have to mark the CU as 415V present. With a bit of careful planning should be able to restrict the amount of 415V circuits "accessible" and minimise labelling requirements. Might worry the holiday makers a bit!
Also would want to restrict the amount of spare ways on the CU as this is going to max out the supply cable. I'm going to get the pencil and calculator out on this one.