3kW heater - 2.5mm T+E - 32A MCB?

[Timotei]

Good Morning All.

Emergency call to local charity shop yesterday due to "no heating".

3 x 3kW "curtain heaters" on site, 4 of these on individual 16a MCB with 2.5mm T+E to 20A control switch...happy days, replace damaged control switch for 2 of these which had been damaged by being knocked (plastic replaced with metal clad).

The remaining 2 heaters are on 1 circuit, each run in 2.5mm T+E from a 32A MCB. The first (operating normally) runs to a 20A control switch, the second (not working) runs to a 13A fused switch. I was going to change the FS for a 20A control switch as overheating of the fuse has caused minor damage to the switch mechanism.....but then found that both are on the 32A MCB....at this point Friday Afternoon Fever kicked in.....so, is this all good because the heaters are a fixed resistive load and Reg 433.3.1(ii) applies.....or does basic circuit design require that the MCB provides both overload and short circuit protection to the 2 x 2.5mm cables, so 20A or 16A MCB?

At the time my instinct was that the heaters should be split to individual circuits with 16A MCBs (there are no spare ways in the DB), and as all 4 other heaters are now working I recommended that this circuit be left isolated and the heaters not used....however, after looking at the quoted reg last night, I now think I may have over-reacted, and that as the conductor "is not likely to carry overload current", the existing setup (minus the FS) is good to continue......although i would want to verify that there are no spurs from the circuit between DB and heaters (shop open while I was there and not possible to access all sockets...the shop manager did say that the sockets seem to have no logical pattern of which circuit they are on).

EICR due in April, and I have said that as well as carrying out the EICR I will take time to fully label each socket outlet / switch etc with DB and circuit numbers.

I would appreciate your thoughts.

Timotei

 

[binky]

For fixed loads, if it can't overload the cable, it's ok. There's a very long technical discussion about how that complies with regs, which I'm not going to go into, but a common example is supermarket lighting, the circuits are often wired in 2.5 cable ( or larger), on a 16 or 20 amp MCB, but drops to the lights are usually 1mm flex. As the flex is linked to a single load, ie the light, it can't overload the flex. Likewise a fused spur can be next to a fixed load, as in end of a spur rather than the start of the sour, and it will still prevent overloading of the cable supplying it. I'm not sure, in this case, a 20Amp controller provides overload protection, unless it has a fuse in it?

 

[Timotei]

I'm not sure, in this case, a 20Amp controller provides overload protection, unless it has a fuse in it?

Thanks binky....the 13A fused switch had definite signs of overheating (running at close to 13A for close to 10 hours a day) but had not melted as I've often seen when an immersion heater has been wired through a FS, so I would definitely want to swap out for a 20A switch. If we are saying that the 3kW heater can not overload, then surely no need for the 13A fuse anyway?

How is Sunny Plymouth today? I'm a South Hams boy (and long suffering (until last season) Argyle fan) stranded up here in Shropshire.

 

[Fleeting]

They aren't fixed resistive loads, the heater incorporates a motor.

 

[Timotei]

They aren't fixed resistive loads, the heater incorporates a motor.

Good Morning Fleeting, I did think this, hence my original thought to isolate the circuit for now, but when doubting my Friday afternoon clarity of thought a discussion with some other wiser (?) colleagues lead to advice to treat the heater as a fixed resistive load....but as you say, if there is a motor on board then it has the potential to overload and we are back to dividing into 2 x 16A circuits.

Appreciate your concise answer.

 

[Andy™]

32a ring with a 2.5mm spur to one point taken from MCB. all good. even says in appendix 15 that you can.

take away the ring and have just a 2.5 on a 32a and many think its dangerous and non compliant. but electrically, what's the difference?

 

[Fleeting]

It isn't a ring final circuit.

 

[binky]

Good Morning Fleeting, I did think this, hence my original thought to isolate the circuit for now, but when doubting my Friday afternoon clarity of thought a discussion with some other wiser (?) colleagues lead to advice to treat the heater as a fixed resistive load....but as you say, if there is a motor on board then it has the potential to overload and we are back to dividing into 2 x 16A circuits.

Appreciate your concise answer.

fan motor is only going to be around 60W, not worth thinking about.

Sunny Plymouth - suffering liquid sunshine in large doses . Argyle have got a new manager, who seems to be doing well, he was the England youth squad manager and is bringing in lots of new faces on loan from bigger clubs.

 

[binky]

They aren't fixed resistive loads, the heater incorporates a motor.

the appliance will still be a fixed load even with a motor rated at a total of 16A or whatever it's rated at. Its not like a multiple socket circuit where the loading can changed on a regular basis.

 

[mort2376]

Can you modify the heaters so its not running at 3kw ? Some have switches for the load others have internal switches. If you set them both to 2Kw you could put them on a 20/25a and its compliant.

If you can get a 25a MCB/RCBO for the board it will probably be ok running at 3kw. Depending on the voltage.

 

[binky]

Good Morning Fleeting, I did think this, hence my original thought to isolate the circuit for now, but when doubting my Friday afternoon clarity of thought a discussion with some other wiser (?) colleagues lead to advice to treat the heater as a fixed resistive load....but as you say, if there is a motor on board then it has the potential to overload and we are back to dividing into 2 x 16A circuits.

Appreciate your concise answer.

Personally I think it would be safer to divide into 2 circuits, just in case some ***** decides to tap ito it for another appliance, extra socket or the like. The trouble with charities is DiY types who think they know what they are doing. This could be achieved via a mini board adjacent to the main board fed from the 32A MCB with appropiate size cable.

 

[Fleeting]

the appliance will still be a fixed load even with a motor rated at a total of 16A or whatever it's rated at. Its not like a multiple socket circuit where the loading can changed on a regular basis.

Many commercial fan heaters will have it's rating in KW along with a separate rating for the motor. As they are not solely resistive they are not fixed loads.

 

[binky]

Many commercial fan heaters will have it's rating in KW along with a separate rating for the motor. As they are not solely resistive they are not fixed loads.

But it's not like you can plug an extra load in to the same point and overload what's there. As long as the max loading of the heating elements and fans at full speed don't overload the max ampage of the cable, then the cable isn't going to melt. Which is the important bit.. I take your point though.

 

[Barnet Al]

Can you modify the heaters so its not running at 3kw ? Some have switches for the load others have internal switches. If you set them both to 2Kw you could put them on a 20/25a and its compliant.

If you can get a 25a MCB/RCBO for the board it will probably be ok running at 3kw. Depending on the voltage.

I agree with mort2376, I would swap over the MCB to 25A which would probably function ok at 3kw. Alternatively, run 2.5mm T&E between the two outlets and turn it into a ring or finally, split into 2 circuits at 16A each. But I wouldn't leave it as it is.