20 Amp Radial Circuits on 20 Amp BS 3036 Fuses.

[CAWORK]

Good Afternoon All,

Although I don't recall ever coming across the above situation myself, I have heard/read lately about the above not been permitted.

I was reading an old On-Site Guide (probably about 20 years old) where in a table covering Final Circuits for BS 1363 socket outlets it states the protective device for 2.5 mm Ring Finals can be any type, for a 30/32 amp Radial Final Circuit wired in 4 mm cable a cartridge fuse or circuit breaker must be used.

However, and this is what I am asking about, for a 20 Amp Radial Final Circuit wired in 2.5 mm cable it states that the protective device can be any type (which to me would indicate a 20 amp 3036 is acceptable, although, I understand, not a common 3036 rating to come across).

I have seen the exact thing above been said in several (fairly old) DIY books.

Regarding the above, I understand 3036 fuses can sometimes take double the rating to blow, so thinking about it, 40 amps to blow a 20 amp 3036 would be a big strain on 2.5 mm cable.

Regards

 

[EFLImpudence]

Although I don't recall ever coming across the above situation myself, I have heard/read lately about the above not been permitted.

I was reading an old On-Site Guide (probably about 20 years old) where in a table covering Final Circuits for BS 1363 socket outlets it states the protective device for 2.5 mm Ring Finals can be any type,

Yes, but it states only 30A BS3036 or 32A MCB.

for a 30/32 amp Radial Final Circuit wired in 4 mm cable a cartridge fuse or circuit breaker must be used.

Yes, because of the 3036's derating factor - 0.725, you would need 6mm² (30/0.725 = 41A CCC).
It is fine with 32A MCB.

However, and this is what I am asking about, for a 20 Amp Radial Final Circuit wired in 2.5 mm cable it states that the protective device can be any type (which to me would indicate a 20 amp 3036 is acceptable, although, I understand, not a common 3036 rating to come across).

Well, you can have any device because 2.5mm² has a CCC of 27A (20/0.725 = 27.6 near enough???).

Regarding the above, I understand 3036 fuses can sometimes take double the rating to blow, so thinking about it, 40 amps to blow a 20 amp 3036 would be a big strain on 2.5 mm cable.

Yes they have a fusing factor of 2, MCBs have a FF of 1.45,this is taken as the standard, hence 1.45/2 = 0.725.

 

[EFLImpudence]

Yes, but it states only 30A BS3036 or 32A MCB.

Just to add - a 30A cartridge is acceptable as well.

 

[CAWORK]

Just to add - a 30A cartridge is acceptable as well.

That must be why it says 'any'

Yes, but it states only 30A BS3036 or 32A MCB.


Yes, because of the 3036's derating factor - 0.725, you would need 6mm² (30/0.725 = 41A CCC).
It is fine with 32A MCB.


Well, you can have any device because 2.5mm² has a CCC of 27A (20/0.725 = 27.6 near enough???).



Yes they have a fusing factor of 2, MCBs have a FF of 1.45,this is taken as the standard, hence 1.45/2 = 0.725.

That I have always been fully aware of.


The reason for me posting this topic is because I have heard lately a few times lately people saying a 2.5 20 amp radial is not permitted to be protected by a 20 amp 3036, but that's not what the on-site guide I was looking at suggested.

Regards

 

[EFLImpudence]

The reason for me posting this topic is because I have heard lately a few times lately people saying a 2.5 20 amp radial is not permitted to be protected by a 20 amp 3036, but that's not what the on-site guide I was looking at suggested.

Perhaps they are worried about the additional 0.6A.

It is not unknown for the OSG to be wrong.

It's up to you to decide which you consider correct using the available data.