16A supply

[robinj66]

After much delay I need to get on with providing a 16A supply for my SIP 13/200 belt drive compressor (see http://www.mig-welding.co.uk/forum/showthread.php?t=40074).

The garage is very close to the house and has been supplied with power from the main consumer unit (about 5 metres away). One cable supplies the lighting circuit & a separate cable feeds the 13A sockets (3 sets of double sockets). This circuit also supplies the downstairs sockets including the kitchen (not cooker). This has never given me a problem despite running a freezer in the gargage (permanently on), a lathe, mig welder, a direct drive 3HP compressor as well as the usual power tools and music. The circuit is protected by a B16 mcb and the 6-way consumer unit (installed prior to 1999) has a 63A 30Ma RCCB.

Ideally the C/U should be relaced with a larger 17th edn unit so that I could provide a dedicated feed to the garage but money is tight atm. My questions are :-
- will it be OK to fit a 2-way consumer unit in the garage with 16A mcb's supplying the 13A sockets and the 16A socket(s) on separate circuits (using the existing feed to the garage.

- does the C/U in the garage actually need a master switch or can I fit one simply with the 2 mcb's?

Any and all comments much appreciated

 

[tlovatt]

You could change the main feeder to be a larger MCB (subject to checking the incoming power supply) and connect it before the RCD, then fit a small consumer unit in the garage with an RCD to feed the sockets, the small 4 way consumer units are only about £20 delivered including the MCBS (2 ways are the incoming RCD and 2 ways for outgoing MCBs)

 

[robinj66]

Thanks for the reply.

Is it safe to just increase the rating of the mcb in the main C/U bearing in mind it services the kitchen sockets?

 

[Mr shiney]

You should not increase the MCB, just be careful on the overall load you put on the circuit.

 

[robinj66]

Thanks - that was my thinking

 

[printcom]

What size is the cable running from the house to the garage ?

 

[Mr shiney]

What size is the cable running from the house to the garage ?

Good point. It will need to be 4mm minimum I should imagine depending on distance.

I ran armoured cable to my garage in a trench and that was 10mm as it was a 25m run round the house from the main box. That's rated for 73amps.

 

[tlovatt]

You should not increase the MCB, just be careful on the overall load you put on the circuit.

If you do not increase the sise of the main MCB (subject to cabling size checks) then there in no point putting in any extra MCBs, just add the 16A socket to the existing circuit as it is protected with a 16A MCB anyway.

 

[Mr shiney]

True, but..... The existing circuit will have been sized for the ring that is on it. It would be a rare occurrence that a circuit was wired way beyond the fuse capacity fitted to it. The fuse will be rated at a percentage below the failure rate of the cable to give the required protection but not a huge amount.

If running a new cable from the main box would be my method then you put a new breaker in to match the cable. As I said earlier I would go with a 10mm armour cable which is rated for 73amps and stick a 60amp breaker on it.

I would not consider increasing the breaker on an existing circuit in order to extend it. That's an easy way of burning the house down!!!

 

[printcom]

If you consider what robinj66 wants to run on this circuit 16A is woefully inadequate, I have a 3HP clarke belt drive compressor and that requires a 20amp C type breaker to be reliable. What happens when the compressor kicks in whilst using the Mig or any of the other equipment?
In my opinion the amount of power potentially required in the shed builds a strong case for doing the job properly - and that does not include sharing a house circuit.
Also bear in mind that a fire/accident caused by shoddy (unauthorized non part P) work will not be paid out by you insurers and the council may jump down your throat.
If the jobs done properly I have found most part P lecky's will test and sign off your work for a reasonable amount of beer money.

 

[robinj66]

Thanks for all the replies - I thought this would end up being more complicated than I originally thought

Best solution would be to run a separate supply to the garage but there isn't any room in the current C/U so I was exploring the idea of using what is already in place. That seems to be working fine for welder, compressor, lathe ect (but I do understand that the new compressor will drastically change the load on the circuit).

I can't categorically say what size the existing cable is - my (very limited) understanding is that 2.5mm is standard for a domestic ring main and that that would be adequate for household sockets. Wierdly the upstairs sockets are governed by a 32A mcb (?).

The outer cable "looks" bigger than other 2.5mm that I have come across so could be 4mm but obviously that needs to be checked properly. If it were 4mm I understand that this would be OK for this compressor ( the motor [plate is shown in the earlier thread).

I think I'm going to have to bite the bullet and get a sparks in - anyone operate in this neck of the woods (South Kent) ?

 

[tlovatt]

You can not tell the size o the cable very easily by the outer. 2.5mm is the cross sectional area not the diameter so the difference to 4mm is not that obvious unless you are used to working with it.

I would ask have you tried running the compressor off a 13A plug, a 2.2kw motor should be fine (takes about 10A) it may just stress the fuse over time.

You can have a ring using 2.5mm with a 32A MCB (the cable goes from the consumer unit, to each socket and then back to the consumer unit) or a single feeder (from the consumer unit out to the sockets but with no return to the consumer unit) this would then normally be fused at 16A

 

[robinj66]

The compressor blows 13A fuses (but didn't trip the 16A mcb). The plate for the motor is attached to my earlier thread and states a 14.7A requirement, hence it was suggested that I need a 16A supply.

 

[brightspark]

The compressor blows 13A fuses (but didn't trip the 16A mcb). The plate for the motor is attached to my earlier thread and states a 14.7A requirement, hence it was suggested that I need a 16A supply.

thats because the starting current is about 55amps till full speed so will pop the 13amp fuse

 

[Mr shiney]

To be technically correct.... The starting current will be about 19amps bit the point is valid.

 

[brightspark]

To be technically correct.... The starting current will be about 19amps bit the point is valid.

on a 2.5kw motor on a compressor it IS 55 amps unless 2 of my my new fluke clamp on ammeters with inrush current are wrong

 

[Robotstar5]

on a 2.5kw motor on a compressor it IS 55 amps unless 2 of my my new fluke clamp on ammeters with inrush current are wrong

Fluke? - cheap tat!

 

[brightspark]

Fluke? - cheap tat!

there all probably made in china under licence expensive cheap sh ,.,.,.te

 

[tlovatt]

I am not sure if you can get slow blow fuses for standard 13A sockets, but a 13A running current should be fine

 

[Mr shiney]

on a 2.5kw motor on a compressor it IS 55 amps unless 2 of my my new fluke clamp on ammeters with inrush current are wrong

I would challenge your fluke. On a 2.2kw which us 3hp the start load is 19amps.

A clamp meter is not a reliable measurement of starting current.