MCB Selection

[johndevin]

Having a discussion at work ..

We are installing a 9.8 KW Shower.

If my calculations are correct this will draw just above 40A

The distribution board we are using does not fit a 45A breaker.

I think it would be safer to use a 40A than jump to a 50A.

I highly doubt the current would ever reach anywhere near 40A.

Whats your thoughts ?

P.S I know i could change the board but trying to avoid this ....

 

[davesparks]

Just fit the appropriate sized MCB to protect the cable and make sure its not maxed out such as the 40A as like others say its a potential fire hazard.

No,
Select the appropriate sized MCB for the load, then select the cable size to suit.

Cable size may be based on Ib if the load is not subject to overload and fault protection is provided for the cable

 

[gutterball]

can you not fit a separate board with the correct breaker?

 

[Lazlo]

This is my personal opinion and not to be taken as rude or bullying or as it is put, not helpful to the post etc etc

Carry out a cable calc using the regs book, if you can't do this or don't know how then there is no way you should be even contemplating carrying out the work...As you wouldn't really know what you are doing or why you are doing it and could end up with a fire on your hands....

 

[uksparks]

No,
Select the appropriate sized MCB for the load, then select the cable size to suit.

Cable size may be based on Ib if the load is not subject to overload and fault protection is provided for the cable

Thats a swings and roundabouts way of what I said, but yes as much as I hate to say if the way you worded it was better and more accurate.

 

[uksparks]

can you not fit a separate board with the correct breaker?

What make is the board? Will another brand not fit in? Often they fit ok if you get the right one.

 

[Lazlo]

Sorry UKSparks, I didn't mean your post I meant this thread as a whole, it is more akin to a college question....

 

[cprfenom]

What make is the board? Will another brand not fit in? Often they fit ok if you get the right one.

Don't they also advise to avoid mixing brands of MCB's?

 

[davesparks]

What make is the board? Will another brand not fit in? Often they fit ok if you get the right one.

No another brand won't be acceptable if the OP cares about doing the job properly at all. Unless the manufacturers of both the board and mcb both agree to your suggestion then the type testing etc becomes null and void.

 

[Darkwood]

I think tbh we need further input from the OP on this one or the thread is just going to go round in circles, he has had the correct advice up to press so a response from a few questions posed will be needed now.

 

[telectrix]

@ uk sparks. ...... basically, you design the circuit to accommodate the load. so e.g. the shower is 40A fixed load. it can't draw excess current as it's fixed. so assume ref. method C, you'd select a 6mm cable (47A) so that Iz>Ib. and you'd select your MCB for fault protection, it's value according to Zs tables so as the MCB would trip within the specified time. in the OP's case a 50A type B would need a Zs value of < 0.736 ( (230/5In) x 0.8)

 

[uksparks]

No another brand won't be acceptable if the OP cares about doing the job properly at all. Unless the manufacturers of both the board and mcb both agree to your suggestion then the type testing etc becomes null and void.

I meant more like how some brands are identical and fit exactly the same, yeah it's best to stick with the same but it would not hurt if you had one that fitted properly.

 

[telectrix]

I meant more like how some brands are identical and fit exactly the same, yeah it's best to stick with the same but it would not hurt if you had one that fitted properly.

yup, if it's a MK board, you must fit a MK nail.

 

[davesparks]

I meant more like how some brands are identical and fit exactly the same, yeah it's best to stick with the same but it would not hurt if you had one that fitted properly.

Some brands are just badge engineered so will be from the same manufacturer. But unless you have the paper trail to prove it you will still be up a certain creek if you mix and match

 

[Leesparkykent]

If Iz < In then the adiabatic calculation must be performed to verify fault protection of the cable is still provided.

 

[uksparks]

Some brands are just badge engineered so will be from the same manufacturer. But unless you have the paper trail to prove it you will still be up a certain creek if you mix and match

I'm not one for mixing and matching but can you explain why it's wrong to mix and match? Let's take for example a Hager MCB and a MK MCB, let's just pretend they are the same shape and fit nicely in one another's board, please explain what is so wrong about doing this other that the fact it will look different.

im bot wanting an argument just interested to hear your explanation,