Which MCB for this cable?

[officeguy]

Hi Everyone,

Note: I live in a part of the UK where there is no Part P.

We have a room that has electric heating (almost) installed.

We have 2 X 1000W electric heaters. A single 1.5mm T&E cable is being run from the CU to the first heater fused switch (fused to 5A). Then, another 1.5mm T&E cable is run from this fused switch to the next fused switch (also 5A fuse) for the second heater. I believe this to be a "Radial" circuit.

It is my understanding that in "free space", the 1.5mm cable used can take up to 20A. In principal, this should be fine for both the heaters (about 8.3A in total for both). However, about 6m of the T&E is under insulation (resting on the ceiling). I believe this to reduce the rating of the cable to 10A. So still ok I believe for the 2 heaters.

However, what MCB should we use in the CU that will protect the cable? Is a 10A MCB still ok, given that it allows overload for a period of time?

Any other tips are appreciated.

Thanks

 

[flameport]

10A is fine, provided the circuit is not excessively long.

 

[deadshort]

Hi, 16a would be fine.

Regards,

DS

 

[officeguy]

Thanks for the quick reply folks!

No, not long at all. The CU is almost touching the ceiling. The cable goes across the ceiling (about 6m), then down the opposite wall for a remaining run of about 7.5m.

It would be appreciated if someone could explain why the 10A is still ok, given the long time for an MCB to trip. I'm not saying that you're wrong, I'm just lacking the knowledge and I'm just interested

Also, why does the length matter regarding the MCB size?

Thanks

 

[EFLImpudence]

It would be appreciated if someone could explain why the 10A is still ok,

Not sure I understand why you say 'still ok'.
The load is 8.3A, the conductor under (the worst case) thermal insulation has a capacity of 13A so 10A protective device complies.

given the long time for an MCB to trip.

Not sure why you say 'long time'.
The heaters cannot overload the cable so a short circuit or fault to earth will disconnect in 0.1 seconds or less.

Also, why does the length matter regarding the MCB size?

The longer the cable, the higher the resistance (impedance) so the lower the short circuit/fault current will be.
The higher the rating of the protective device, the higher fault current needed to disconnect in time.

 

[officeguy]

It would be appreciated if someone could explain why the 10A is still ok,

Not sure I understand why you say 'still ok'.
The load is 8.3A, the conductor under (the worst case) thermal insulation has a capacity of 13A so 10A protective device complies.

given the long time for an MCB to trip.

Not sure why you say 'long time'.
The heaters cannot overload the cable so a short circuit or fault to earth will disconnect in 0.1 seconds or less.

Also, why does the length matter regarding the MCB size?

The longer the cable, the higher the resistance (impedance) so the lower the short circuit/fault current will be.
The higher the rating of the protective device, the higher fault current needed to disconnect in time.

Got it, thanks!

Where did you get 13A from? I thought covered in insulation reduced the cable rating by 50%, so 10A?

Also, when I say long time, even though the heaters wont overload the cable, I guess I'm just trying to protect the cable in-case someone in the future plugs in higher power heaters. What if both heaters ran at 5A each (so 10A total)?

 

[JohnW2]

10A is fine, provided the circuit is not excessively long.

In terms of overload protection, a 10A OPD is OK for protecting a cable of ≥10A CCC of any length (and some would say that - in adding your qualification, are you thinking of fault protection (i.e. Zs)? Of course, short of installing different cable, the OP has no real choice, since a 6A MCB would be inadequate for a 2kW load.

Kind Regards, John

 

[JohnW2]

The heaters cannot overload the cable....

I nearly made that comment in my recent post, but I hesitated when I realised that we do not know the nature of the heaters. If they contain fans, then overload is presumably not impossible.

Kind Regards, John

 

[EFLImpudence]

That is true.

 

[EFLImpudence]

Where did you get 13A from? I thought covered in insulation reduced the cable rating by 50%, so 10A?

10A would be for running through the insulation (for more than 50cm.).

Above the ceiling covered by insulation:
<100mm. thick - 16A
>100mm. thick - 13A.


2 x 5A = 10A so fine.

 

[JohnW2]

>100mm. thick - 16A
>100mm. thick - 13A.

Slight typographical correction needed, I believe!

Kind Regards, John

 

[JohnW2]

The longer the cable, the higher the resistance (impedance) so the lower the short circuit/fault current will be.
The higher the rating of the protective device, the higher fault current needed to disconnect in time.

All true - but, to put it in perspective, if one assumes a Ze of 0.8&#937;, and also assuming I've done my quick sums correctly, I think that 1.5mm ² T+E would have to be around 104 metres in length for the impedance to be too high to achieve the required disconnection time with a 10A MCB.

Kind Regards, John

 

[EFLImpudence]

I know but I was only answering the OP's question because length was mentioned.

 

[EFLImpudence]

Slight typographical correction needed, I believe!

Kind Regards, John

Oooh thanks. Sorry.

 

[JohnW2]

I know but I was only answering the OP's question because length was mentioned.

Indeed- and, as you will have seen, I did question flameport about the reason he introduced the issue of length.

Kind Regards, John