Outbuilding Electric Heater

[hansPulesh]

Hi all, I've had an outbuilding/office built at the end of my garden and am thinking of installing some electric heaters out there.

The outbuilding has it's own RCD protected consumer unit and having calculated it, a 2kw heater, 12 meters away from the consumer unit requires a 1mm cable?

Is this correct or am I way off? It just seems strange that a power hungry device such as an electric heater draws so little power?

 

[EFLImpudence]

2kW @ 240V is 8.33A - end of.

More to the point, I think what you have realised is that 1mm² cable can handle a fair bit of current (16A in ideal conditions).

[Perhaps this will help those who insist on using 1.5mm² (20A) cable for a few amps of lighting.]

However, there is a strange regulation that states the minimum allowed for 'power circuits' is 1.5mm².
So, 1.5mm² it will have to be - unless you are just referring to the heater flex.

Is the supply suitable for some heaters?

 

[JohnW2]

The outbuilding has it's own RCD protected consumer unit and having calculated it, a 2kw heater, 12 meters away from the consumer unit requires a 1mm cable?

The current-carrying capacity of cables is higher than one might expect. However, the first problem you are up against is that the Wiring Regulations do not permit cable smaller than 1.5mm² to be used for anything other than lighting circuits.

This 12 metres you mention - is it from the CU in the outbuilding or the house CU? - and are you talking about 'fixed wiring' or the flex of the heater? How many of these heaters are you contemplating, and what is the outbuilding's CU supplied from?

Kind Regards, John

 

[hansPulesh]

Sorry, will make it clearer. The outbuilding is at the end of the garden. The CU in the outbuilding is approx 35 meters away from the CU in the house. The cable supplying the outbuilding CU is a 4mm SWA cable.

The 12 meters that I mentioned was the distance of the electric heater from the CU in the outbuilding.

EFLImpudence - when you ask if the supply is suitable for some heaters, how do you mean? - The heater is one of those wall hung fire places form B&Q.

 

[JohnW2]

Sorry, will make it clearer. The outbuilding is at the end of the garden. The CU in the outbuilding is approx 35 meters away from the CU in the house. The cable supplying the outbuilding CU is a 4mm SWA cable. The 12 meters that I mentioned was the distance of the electric heater from the CU in the outbuilding.

Thanks for clarifying - it's obvioulsy a very large outbuilding!

EFLImpudence - when you ask if the supply is suitable for some heaters, how do you mean? - The heater is one of those wall hung fire places form B&Q.

I think EFLI was questioning whether the available electricity supply (and the current-carrying-capacity and voltage drop in the supplying SWA etc.) was adequate for 'some' (i.e. by implication 'several') such heaters. How many do you propose? What is feeding the SWA - an MCB in the house CU, a 'switch fuse' or what - and what is its rating?

Kind Regards, John

 

[hansPulesh]

At the moment, the SWA is not being fed by anything. There are however 2 options open on it's feed.

1. Can be fed from the house's CU via a 30Amp MCB.
2. The mains supply inside the house is connected to a black box (I can't remember what they are called but it works almost as a junction box), from where the tails carry the power into the meter. The 4mm SWA could be connected to a spare set of terminals in this box.

The max load that the 4mm SWA can carry at that distance is 33Amps.

If there are any other better ways of supplying the SWA, please do let me know.

I was thinking of using 2 of these 2kW heaters.

 

[JohnW2]

At the moment, the SWA is not being fed by anything. There are however 2 options open on it's feed.
1. Can be fed from the house's CU via a 30Amp MCB.
2. The mains supply inside the house is connected to a black box (I can't remember what they are called but it works almost as a junction box), from where the tails carry the power into the meter. The 4mm SWA could be connected to a spare set of terminals in this box.

As for (2) that 'black box' is presumably a 'Henley Block' (still generally called after the name of the historically original manufacturer!). You could not connect the SWA directly to that - you would have to go via a fuse or MCB - usually done with a 'switch fuse' unit. If you don't need more than 32A, either (1) or (2) are certainly options. In the case of (1), would the MCB in the house CU be RCD protected? If it would be, then one downside of this approach would be that a fault in the outbuilding could take out all the circuits in the house were protected by the same RCD - with (2), that problem does not arise.

I was thinking of using 2 of these 2kW heaters.

If you're going to only have two heaters, and if there will be no other major loads, then that should be fine.

Kind Regards, John

 

[hansPulesh]

Henley block - that's the one.

I'm also considering putting in an in line water heater which draws 3.7Amps. As this won't be in constant use however, I'm hoping it won't trip any MCB's? I know this will be the max that the 4mm SWA can handle.

I also forgot to mention that the SWA is laying on the floor under a decking frame. Being in constant contact with the cold floor during winter times where the ambient temp will be near enough 0 C, it's max load capaciy should also increase. These will be the only times when both heaters would be in use as well as the in line water heater (although this will only be for short periods of time).

During summer months, there would be no need for the heaters and so the cable should not carry more load than it is designed to do so.

 

[JohnW2]

Henley block - that's the one. I'm also considering putting in an in line water heater which draws 3.7Amps. As this won't be in constant use however, I'm hoping it won't trip any MCB's?

Even when that were on, together with a couple of 2kW heaters the total would only be fractionally over 20A, so seemingly no problem.

Kind Regards, John

 

[hansPulesh]

At the moment, the SWA is not being fed by anything. There are however 2 options open on it's feed.
1. Can be fed from the house's CU via a 30Amp MCB.
2. The mains supply inside the house is connected to a black box (I can't remember what they are called but it works almost as a junction box), from where the tails carry the power into the meter. The 4mm SWA could be connected to a spare set of terminals in this box.

As for (2) that 'black box' is presumably a 'Henley Block' (still generally called after the name of the historically original manufacturer!). You could not connect the SWA directly to that - you would have to go via a fuse or MCB - usually done with a 'switch fuse' unit. If you don't need more than 32A, either (1) or (2) are certainly options. In the case of (1), would the MCB in the house CU be RCD protected? If it would be, then one downside of this approach would be that a fault in the outbuilding could take out all the circuits in the house were protected by the same RCD - with (2), that problem does not arise.

I was thinking of using 2 of these 2kW heaters.

If you're going to only have two heaters, and if there will be no other major loads, then that should be fine.

Kind Regards, John

How best can the SWA be fed without causing problems like the one that you described (the RCD in the house taking down all of the circuits connected to it?)

 

[JohnW2]

How best can the SWA be fed without causing problems like the one that you described (the RCD in the house taking down all of the circuits connected to it?)

As I said, if you went with your option (2) and fed the outbuilding SWA via a switch-fuse from a pre-CU Henley block, then the outbuilding circuits would not affect any RCD in the CU.

Kind Regards, John

 

[hansPulesh]

Many thanks for all of your help.

 

[hansPulesh]

At the moment, the SWA is not being fed by anything. There are however 2 options open on it's feed.
1. Can be fed from the house's CU via a 30Amp MCB.
2. The mains supply inside the house is connected to a black box (I can't remember what they are called but it works almost as a junction box), from where the tails carry the power into the meter. The 4mm SWA could be connected to a spare set of terminals in this box.

As for (2) that 'black box' is presumably a 'Henley Block' (still generally called after the name of the historically original manufacturer!). You could not connect the SWA directly to that - you would have to go via a fuse or MCB - usually done with a 'switch fuse' unit. If you don't need more than 32A, either (1) or (2) are certainly options. In the case of (1), would the MCB in the house CU be RCD protected? If it would be, then one downside of this approach would be that a fault in the outbuilding could take out all the circuits in the house were protected by the same RCD - with (2), that problem does not arise.

I was thinking of using 2 of these 2kW heaters.

If you're going to only have two heaters, and if there will be no other major loads, then that should be fine.

Kind Regards, John

Apologies for bringing this post up again after such a long time but have only just got around to looking into this.


Under option (2) above;

Can anyone post a link to show what a "'switch fuse' unit" is?

Could I use something like this?

http://www.ebay.co.uk/itm/SHOWER-WE...pplies_Electrical_ET&var=&hash=item4ae2d73f9a