Limiting current on a sub main

[bernardgreen]

A small craft shop supplied from the owner's house via a 6 mm² SWA sub main. The sub main is supplied from a 16 amp MCB in the owner's house.

The 16 amp was chosen to limit the maximum load that the shop can take as well as protecting the SWA

The shop has a CU feeding two radials to sockets each with a 16 amp MCB and lighting from a 6 amp MCB.

Problem over the weekend. Staff in the shop have a 2 kW heater on radial and a 3 kW kettle on the other radial. This trips the 16 amp MCB in the house.

No power, till cannot be opened, card machine cannot be used.

No one at home so key holder for the house has to be called to open up and reset the MCB

Solution proposed.

Change MCB in house to 32 amp to protect SWA ( or 20 amp ) and fit a 16 amp MCB in the shop between SWA and the shop's consumer unit.

Question. How to fit the 16 amp MCB load limiter in the shop. There are 2 spare modules in the shop's CU but would it be acceptable to fit the MCB in there or would a separate housing be needed.

 

[ban-all-sheds]

The 16 amp was chosen to limit the maximum load that the shop can take as well as protecting the SWA

https://books.google.co.uk/books?id...IqpT_x5vgyAIVS70UCh0LmQGs#v=onepage&q&f=false

Question. How to fit the 16 amp MCB load limiter in the shop.

It is never acceptable to use a fuse or circuit-breaker as a load limiting device.

 

[bernardgreen]

It is never acceptable to use a fuse or circuit-breaker as a load limiting device.

Then what would you suggest as a way to limit the load ? The design was for a predicted maximum load of less than 16 amps.

What do camp and caravan sites use when they provide un-metered supplies of electical power to their customers. ?

 

[ban-all-sheds]

Then what would you suggest as a way to limit the load ?

Load(s) with characteristics such that they do not create overload(s).

The design was for a predicted maximum load of less than 16 amps.

Clearly the design was wrong.

What do camp and caravan sites use when they provide un-metered supplies of electical power to their customers. ?

IHNI, but I have seen, here, considerable evidence of an almost systemic disregard for safety, good practice and regulations within carqvan site operators.

 

[oldbutnotdead]

How about a bit of staff training- don't use the kettle and the electric heater at the same time (I do this in the caravan out of habit now). Or put a gas heater in (one of those wheeled things), v efficient. Or buy them fleeces and gloves

If you did want to do what you're describing then the load limiting device would have to be external to the CU in its own enclosure. 16A RCD would be my choice (add a bit more discrimination to the job), of course you'll probably need to reterminate the SWA.

And if the load limiting is on cost grounds then stick a meter in the line and bill the shop operator. Meters are not that expensive (£20 upwards)

 

[JohnW2]

Then what would you suggest as a way to limit the load ?

Why is there a desire to "limit the load" - is this a cost issue, or what? The circuit should be designed on the basis of the expected (or possible) load, not vice versa.

The design was for a predicted maximum load of less than 16 amps.

From what you say, seemingly an incorrect assessment of the design load.

Kind Regards, John

 

[bernardgreen]

I believe the reason for the laod limit was to prevent the house supply being overloaded.

Staff training ( kettle and heater not to be used at the same time ) might work after another lesson but winter is coming and staff seem a bit reluctant to toe the cold line.

 

[ericmark]

Can you change the Main Switch or RCD in the shop for a RCBO double pole and so supply all shop MCB's through the shop master RCBO? So in house 32A MCB to protect cable, In shop 20A RCBO as current limiting with that feeding the MCB's.

Only suggesting RCBO as saves having to alter buzz bars. I know with caravans I have removed buzz bar and fed the 6A lighting MCB from the 10 or 16 amp sockets MCB so total limit is 16 or 10 amp.

Other options is 20A in house and remove one 16A MCB in shop and combine both radials.

Also what about providing a caravan low wattage kettle?

 

[JohnW2]

Staff in the shop have a 2 kW heater on radial and a 3 kW kettle on the other radial. This trips the 16 amp MCB in the house.

Whilst I would not advocate a situation in which Ib>In, are you sure that the 16A MCB in the house is not a bit 'trigger happy'? 5kW is around 20A-22A and the 'typical' curve for a B16 indicates that such a current would take over half an hour to trip at such a current - and a kettle is not going to be on for that long. Indeed, the curves seem to suggest that at 20A, it might well 'never' trip. A B16 is required not to ever trip at a current of ~18A.

Kind Regards, John

 

[JohnW2]

I believe the reason for the laod limit was to prevent the house supply being overloaded.

If the house MCB were increased to 20A, that would make no material difference to the risk of 'the house supply being overloaded', but that would almost certainly never trip with a 2kW + 3kW kettle (although, as I've said, I can't really advocate any degree of Ib/In!).

Kind Regards, John

 

[Iggifer]

Staff in the shop have a 2 kW heater on radial and a 3 kW kettle on the other radial. This trips the 16 amp MCB in the house.

Whilst I would not advocate a situation in which Ib>In, are you sure that the 16A MCB in the house is not a bit 'trigger happy'? 5kW is around 20A-22A and the 'typical' curve for a B16 indicates that such a current would take over half an hour to trip at such a current - and a kettle is not going to be on for that long. Indeed, the curves seem to suggest that at 20A, it might well 'never' trip. A B16 is required not to ever trip at a current of ~18A.

Kind Regards, John

Don't forget tills, lights, phone chargers, all the other gubbins that gets plugged in a shop etc. Only small loads on their own, but they all add up.

 

[Taylortwocities]

If you did want to do what you're describing then the load limiting device would have to be external to the CU in its own enclosure. 16A RCD would be my choice (add a bit more discrimination to the job), of course you'll probably need to reterminate the SWA.

Really??? a 16A RCD will limit the load will it??? Would you care to explain how that would work?

 

[JohnW2]

Don't forget tills, lights, phone chargers, all the other gubbins that gets plugged in a shop etc. Only small loads on their own, but they all add up.

... unless there are major items about which we have been told, such loads are very unlikley to add up to anything particularly significant in relation to the 5kW 'known' load we're talking about. Don't forget that 3kW of that 'known' load will only be present occasionally, and only for a few minutes at most.

Kind Regards, John

 

[Adam_151]

Cant you simply make it one 16A radial socket circuit in the shop by putting both outgoing cables on one breaker

Or swap the incommer in the shop board to a double pole MCB

 

[stillp]

Don't forget that 3kW of that 'known' load will only be present occasionally, and only for a few minutes at most.

Do we know that? We've been told it's a "craft shop". For all we know the kettle might be used continuously for steaming something.