Sizing a Circuit Breaker

[Gary25]

I have 254V feeding a 50W anti-condensation heater via a 3c x 2.5mm cable. The load is approximately 0.196A (196mA).
Can anyone please advise what size of MCB I should use for this?
I feel that a 250mA or 300mA circuit breaker would probably be on the small side, and I'm thinking of something more like 2A or even 5A, given the size of the cable, but could that lead to damaging the heater?
The cable run is approx 130metres long.

Any help would be appreciated.

Thanks

 

[Apache]

The load on the circuit is irrelevant here - the MCB is sized to protect the cable itself.

Is this a DIY effort? Is this a new supply? Will the 130m run be used for anything else? Is there RCD protection? What is the earthing arrangement? Is this going directly into a consumer unit? How is the cable run and what specific type of cable?

 

[Gary25]

Hi.
Thanks for your response.

This is not a DIY effort, it will be installed by qualified electricians, I'm merely double checking the circuit protection requirements for my own benefit.​

This is a new supply.​

The cable will not be used for anything else, it will be fed from a panel that has very limited access. The anti-con heater will only come on upon loss of power to the enclosure it will be installed in, so not very often, if at all.​

There is no RCD protection.​

The cable will be run along cable tray.​

The specific type of cable is: TAC/EPR/SHF2/GSWB/SHF2-MUD-BLACK 0.6/1.0(1.2)Kv TO IEC 60092-353/NEK606 - Flame Retardant to IEC 60332-3-22 - Mud Resistant​

 

[poni]

The anti-con heater will only come on upon loss of power to the enclosure it will be installed in,

how that going to work then,?

 

[binky]

Indoors / outdoors?

The heater will draw what it needs, any MCB or fuse is to protect the cable supplying the heater, not the heater itself.

 

[Murdoch]

I would leave the design, installation and certification down to the people doing the install. Just make sure they give you and invoice and certificate

 

[SPECIAL LOCATION]

I have 254V feeding a 50W anti-condensation heater via a 3c x 2.5mm cable. The load is approximately 0.196A (196mA).
Can anyone please advise what size of MCB I should use for this?
I feel that a 250mA or 300mA circuit breaker would probably be on the small side, and I'm thinking of something more like 2A or even 5A, given the size of the cable, but could that lead to damaging the heater?
The cable run is approx 130metres long.

Any help would be appreciated.

Thanks

How do you think that the heater would be damaged?
MCB's are not used to protect appliances/loads.

Any appliance or load would typically have its own internal overload protection and/or a plug-top/fused spur overload protective fuse..

No different to our 5A boiler supply connected to a 32A 2.5mm T&E ring circuit..
the 32A MCB or the supply cable will not cause damage to the boiler.

The 130m 2.5mm cable could be protected by any MCB up-to 10A type B...
but if going to 16A and above, you start exceeding the max permitted earth loop impedance 80% rule of thumb.

 

[runningpiglet]

Going by max voltage drop of 5% (11V at 230 supply) then at the far end of 130m of 2.5mmsq then I reckon 4.7amps at most. So a 5 or 6 amp mcb would be appropriate?

 

[SPECIAL LOCATION]

Going by max voltage drop of 5% (11V at 230 supply) then at the far end of 130m of 2.5mmsq then I reckon 4.7amps at most. So a 5 or 6 amp mcb would be appropriate?

The volt drop down the cable will be approx 0.51V. which is ref the connected load e.g. the 50W anti condensation heater.
(18mv/A/m x 0.22A x 130m) / 1000

And as the op says this is the only thing to be connected to the cable.. you would never be drawing 4.7amps?
Plus the MCB is there to protect the cable.. Not limit the volt drop.

If volt drop is an issue, you don't decrease the MCB rating, you increase the cable CSA.

 

[poni]

250mA or 300mA circuit breaker

I have not seen a MCB that small, the smallest I have ever fitted in a consumer unit was 1A , our Regs start at 6A , I see that RS sell a 0.5A MCB (that is small)

The specific type of cable is: TAC/EPR/SHF2/GSWB/SHF2-MUD-BLACK 0.6/1.0(1.2)Kv TO IEC 60092-353/NEK606 - Flame Retardant to IEC 60332-3-22 - Mud Resistant

this cable is for use onboard ships and has a Galvanized Steel Wire Braid , you need different type of glands for this cable, also it tends not to be as good at protecting the cable from damage when exposed on the surface ,I have not heard of Mud resistant cable ( did not know this was a thing)

 

[runningpiglet]

The volt drop down the cable will be approx 0.51V. which is ref the connected load e.g. the 50W anti condensation heater.
(18mv/A/m x 0.22A x 130m) / 1000

And as the op says this is the only thing to be connected to the cable.. you would never be drawing 4.7amps?
Plus the MCB is there to protect the cable.. Not limit the volt drop.

If volt drop is an issue, you don't decrease the MCB rating, you increase the cable CSA.

Yes, I was thinking more of a future case if that 50W load were ever changed later then 4.7A would be most that cable length and size could support with a tolerable drop at the far end so probably no point fitting an MCB greater than 5A.

 

[apprentice87]

The load on the circuit is irrelevant here - the MCB is sized to protect the cable itself.

Is this a DIY effort? Is this a new supply? Will the 130m run be used for anything else? Is there RCD protection? What is the earthing arrangement? Is this going directly into a consumer unit? How is the cable run and what specific type of cable?

Is not the MCB sized to suit the load and the cable sized to suit the MCB??

john..

 

[Andy™]

if you have a circuit with a 6a MCB then it would be a reasonable assumption that the full 6a is available. not everyone is a qualified spark