Electrical supply to shower pump

[Penners]

I need some advice on powering a shower pump, please.

The pump is rated at 330W and a 5A fuse is specified. Would it be acceptable practice to power it from the lighting circuit?

Thanks for any advice.

 

[Ushy M]

Mr M here Penners - how much load is already on this supply? If this is upstairs on a lighting ring, there should be enough capacity on there to power a shower, but it's not ideal. You'll need a fused spur in any case, and it would be best to make this a temporary measure only and rewire at some point in the future. It's best to keep lighting and power and separate -it would be better if this could be provided as a fused spur off a ring main.

 

[Penners]

Many thanks for your reply, Mr M.

Yes, it's the upstairs lighting circuit. The load at present is very little - about 500W if everything's on. The lighting circuit is cabled in 1.5 mm^2. Full load amperage of the pump is 1.9A.

I note your comment about fusing the spur. But given the small lighting load at the moment, do you still think I should only make this a temporary arrangement?

 

[Flyfisher]

The 5A lighting circuit is good for 1200W, so the 500W of lighting and 450W of shower pump should be OK. The only thing I'd be wary of is the start-up current surge of the pump motor. I have a 2.2kW air compressor, which is comfortably within the capacity of a fused 13A plug. However, it would regularly blow the plug fuse. I was advised to connect it to a 16A 'industrial' outlet with a 16A 'slow-trip' MCB in the fusebox. The electrician who did this work for me said that fuses can be weakened by regular surge currents and will eventually blow at some point. Whatever the reasons, my compressor has never tripped the MCB.

As for accepted practice, I've lost track these days with all the regulation changes. The main thing, of course, is safety and I can't see any safety problems with running a properly fused spur from the lighting circuit.

Having said all that, my instinct agrees with Mr M and I'd connect a shower pump into a power circuit using a 5A switched spur (so that a pump problem wouldn't fuse all your lights) - never forgetting the certification requirements of course, or was it wired into the lighting circuit by the previous occupant? :wink:

 

[Penners]

never forgetting the certification requirements of course

Perish the thought...

Thanks for the advice, FF. I note your comments about the startup amperage. The 1.9A spec. is described as "full load" - do you think that includes the startup surge, or not?

What's the formula for amps - I can't remember. Is it A = W/V or the other way round?

 

[Me!]

Yes, current is watts / volts.

My personal experience of some electric motors is that thier initial start-up current surge can be much higher than even their full-load running current. However, even though the heat generated in a cable = the current squared x the resistance, remember that a startup surge is only for a very short period, so I wouldn't worry too much about that.

Has your lighting circuit got an earth though? Many old houses don't, and you may need one for a pump.

However, this thread has reminded me that my mother used to have a bayonet plug on her iron, so that she could plug it in to a light socket. I even remember double bayonet adaptors that allowed you to plug two such objects in to the same unfortunate light socket!

 

[Flyfisher]

I agree with Me! (er, that reads a bit odd, but you know what I mean :wink: )

I think the "full load" spec will be the current drawn when the motor is working at full load. This will be the maximum constant current, which can be used for load calculations. The start-up peak current will be much higher but very brief.

Another thought: if you have fast-acting MCBs instead of fuses in your fusebox then you might get 'nuisance tripping'. I have a chop-off saw that is probably only about 750W but it will sometimes cause the 13A MCB in my workshop to trip. Each time it trips I make a mental note to buy a 'slow-acting' MCB (the different versions are actually described by 'type x', but I can't remember the x***) but my mental notes don't seem to last very long these days.

Good point about lighting circuit earths though. Does the pump have an earth wire? An RCD in the circuit might be a good idea; it might even be a requirement of the latest regs for all I know.


*** I checked and found this explanation: http://www.tlc-direct.co.uk/Technical/Distribution/Mcb.htm

 

[Penners]

Has your lighting circuit got an earth though?

Yes, the lighting circuit - indeed the whole house wiring - is pretty up-to-date. All circuits are properly earthed via TW&E cable, and the consumer unit has a master RCD plus MCBs on all circuits. The RCD is not slow-acting, though. A light bulb blowing can trip it, which can be a bit of a pain (especially if the computer's on!) but it's a small enough inconvenience.

I've thought over the advice that I've had from you all (for which many thanks) and on balance I think I ought to undertake the modest extra work involved in running the pump on a power circuit. I can run a 2.5mm radial supply from one of the 32A MCBs in the consumer unit to the pump without too much hassle, and incorporate a 5A DP fused switch. So (unless you want to tell me otherwise) that seems a more advisable solution.

Thanks again for your helpful thoughts on this topic. Much appreciated.

By the way, despite my tongue-in-cheek comment above, I do intend to get this installation inspected and certificated by a proper sparky. While we were buying this house, I kicked up a fuss because the recently installed kitchen didn't have a Part P certificate, forcing the vendor to obtain one. So I don't think my conscience would allow me to neglect this!

 

[Flyfisher]

I don't think it's necessary to provide a supply all the way back to the consumer unit (although there's no harm in doing so).

I believe it's still acceptable to take a spur from an existing socket, using 2.5mm T&E. Might make routing the cable a bit easier, depending on your layout.