Suitable MCB for Radial Circuit?

mark meli · Jul 28, 2020

Hi all,

I am installing a Radial A2 circuit which by rule it should have an MCB of 32Amps, but when I reference the table 4D1A using reference method A or B, current are 26 and 32A respectivitly..

So the question is why we go for a 32A mcb and not 25A one?

Thanks

Andy78 · Jul 28, 2020

Proper circuit design should be applied to all circuits.

If you are looking in the OSG at standard radial circuits it is made clear in that section that the stated cable sizes are minimum sizes and that calculation is necessary to design the circuit properly.

I'd pretty much ignore the OSG if you are learning circuit design.

Strima · Jul 28, 2020

Andy78 said:
I'd pretty much ignore the OSG if you are learning circuit design.

The OSG does have a couple of handy bits such as max lengths for circuits and conduit capacity for example. Also the max Zs with 80% and Cmin applied are quite handy for a quick reference.

Andy78 · Jul 28, 2020

Strima said:
The OSG does have a couple of handy bits such as max lengths for circuits and conduit capacity for example. Also the max Zs with 80% and Cmin applied are quite handy for a quick reference.

True. It's all in the regs too though.

telectrix · Jul 28, 2020

Andy78 said:
True. It's all in the regs too though.

regs book is too expensive to take to site and get it dirty.

davesparks · Jul 28, 2020

mark meli said:
Hi all,

I am installing a Radial A2 circuit which by rule it should have an MCB of 32Amps, but when I reference the table 4D1A using reference method A or B, current are 26 and 32A respectivitly..

You need to go up a cable size if its CCC is 26A.

The cable size in the book is the minimum permitted for that standard circuit, not the only size you may use.

Also you should not be directly applying the CCC from the tables, these should be adjusted using the cable rating factors for your particular installation.
[automerge]1595961665[/automerge]