Putting bathroom heater on shower circuit?

propertydev83 · 20 Jan 2017

My dad wants to install a Winterwarm WWDF20N bathroom heater. As the heater needs to be fused at 13A, he was going to add this to the shower's circuit which has a 40A MCB protected by RCD, and is a 10mm radial circuit... Basically spur off 10mm, double pole FCU 13A, then 2.5mm to heater.

I would not have thought it would overload the circuit as the shower is the only thing connected to it, but I just wanted to check if it would be ok?

Simon35 · 20 Jan 2017

What's the kW rating of the shower and the heater?

propertydev83 · 20 Jan 2017

Heater is 2kW. I've just asked if he can find out the shower's model number.

I'm waiting to hear back, but I'm already thinking it is not an option now, unless the shower is quite a bit less than 7.6kW (maximum load for 40A being around 9.6kW)?

propertydev83 · 20 Jan 2017

Shower is Triton Aquatronic 4 Ultra... Appears to be 9.5kW?

Sorry the cable sizes and MCB are not as previously stated. I am trying to look into this...

What would the cable size and MCB need to be for the 2kW heater and 9.5kW shower on a radial circuit?

stem · 20 Jan 2017

In that case. It's already overloaded. Looks like someone has fitted at 9.5 kW shower to a circuit originally designed for a 7 kW shower.

I would be expecting to see a 45A MCB and 10mm cable for the existing shower. Although the method of installation may vary this.

OwainDIYer · 20 Jan 2017

propertydev83 said:
Shower is Triton Aquatronic 4 Ultra... Appears to be 9.5kW?

Sorry the cable sizes and MCB are not as previously stated. I am trying to look into this...

What would the cable size and MCB need to be for the 2kW heater and 9.5kW shower on a radial circuit?

11 kW is 47 amp at 230 volt. Maximum 45A MCB in most consumer units. You will need to have the heater on a separate circuit, or spurred off a socket circuit.

propertydev83 · 20 Jan 2017

Sorry, I'd edited previous post.

After a bit of detective work via phone, it is 40A MCB in the consumer unit, 10mm cable to an external 80A RCD, 10mm cable to a pull cord switch, and 10mm cable to shower.

My calculation for shower and heater is 11500kW / 240V = 48A. Would it be possible to upgrade the MCB to 45A or 50A and run on the 10mm cable, dependent on how the cable is fitted in place?

If not, then spurring off a socket circuit as suggested may be possible if that is compliant? There is a socket in the loft but unfortunately it is already on a spur. (Though if he's prepared to lose the socket altogether then that spurred cable could be used...)

Simon35 · 20 Jan 2017

propertydev83 said:
Sorry, I'd edited previous post.

After a bit of detective work via phone, it is 40A MCB in the consumer unit, 10mm cable to an external 80A RCD, 10mm cable to a pull cord switch, and 10mm cable to shower.

My calculation for shower and heater is 11500kW / 240V = 48A. Would it be possible to upgrade the MCB to 45A or 50A and run on the 10mm cable, dependent on how the cable is fitted in place?

If not, then spurring off a socket circuit as suggested may be possible if that is compliant? There is a socket in the loft but unfortunately it is already on a spur. (Though if he's prepared to lose the socket altogether then that spurred cable could be used...)

I would leave the shower circuit alone, and look to either re-purpose that loft spur, or find where the loft spur is taken from and fuse at that point, with an FCU. You can then fit another socket or connection point on the loft spur radial.

EFLImpudence · 20 Jan 2017

IF it really is 10mm² and installed with no derating factors applicable then you could theoretically have a 63A device.

After all a lot of flats only have 10mm² as their supply cable.

I suggest you get someone who knows what they are doing to investigate.

propertydev83 · 20 Jan 2017

Yes, 63A is what I thought, but I'm not sure how the cable is installed without further investigation. Otherwise the rating for 10mm is 43A isn't it? So the shower and heater would be slightly too much?

If putting the heater on the sockets circuit, I don't believe there is RCD protection. Would that matter?

JohnW2 · 20 Jan 2017

Can one not apply a degree of 'total circuit diversity over time' in a case like this? The shower is only going to be operational for relatively short periods of time, so that the average load over any appreciable period of time (perhaps 30 mins, certainly 60 mins) is always going to be far less than the sum of the the two (shower+heater) loads?

Kind Regards, John

stem · 20 Jan 2017

I used to live in an electrically heated house, and the only time I used the bathroom heater was when I had a shower, so I wouldn't have thought so.

EFLImpudence · 20 Jan 2017

propertydev83 said:
Otherwise the rating for 10mm is 43A isn't it? So the shower and heater would be slightly too much?

Well, it varies. The worst would be 32A.

However, laid above a ceiling and covered with no more than 100mm. of thermal insulation it is - 45A.
Enclosed in conduit in an insulated wall is - 44A.

If putting the heater on the sockets circuit, I don't believe there is RCD protection. Would that matter?

Yes. You'd better have it RCD protected.

JohnW2 · 20 Jan 2017

stem said:
I used to live in an electrically heated house, and the only time I used the bathroom heater was when I had a shower, so I wouldn't have thought so.

That wouldn't alter what I was talking about - in fact, it would make the calculations simpler!

If one had, say, a 20-minute shower (longer than most), with the heater on just for those 20 minutes, if one assumes that the total current of both was 48A, then the the average current over any 30-minute period would be (at most) 32A, and the average current over any 60-minute period would be (at most) 16A.

How reasonable/acceptable it would be to invoke such a diversity concept in this situation, I don't know (it was really just a question - for discussion), but it is not all that different from other situations in which the application of diversity is accepted.

Kind Regards, John

stem · 20 Jan 2017

Ah OK. You are referring to diversity relating to the incoming supply, I was referring to just the circuit supplying the shower.

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Putting bathroom heater on shower circuit?

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