UK reg to say 2 different size cables cant be used on one circuit if rcbo rated to lower cable?

Alexking1 · Jun 13, 2023

Came across a cooker circuit with 10mm and 40A rcbo. Last leg was 6mm. Isolator to outlet. I downgraded to rcbo to 32A to accommodate the 6mm. Does the cable have to be the same throughout? If so what is the reg for this please.

timhoward · Jun 13, 2023

Alexking1 said:
Came across a cooker circuit with 10mm and 40A rcbo. Last leg was 6mm. Isolator to outlet. I downgraded to rcbo to 32A to accommodate the 6mm. Does the cable have to be the same throughout? If so what is the reg for this please.

No there isn't a reg requiring consistent cable throughout a circuit. (In fact 433.3 mentions the scenario of a reduction in CCC.)
If the 6 sq mm leg was clipped direct (when it can carry 47 amps before any further de-rating) it might have complied.

Basically 433.1.1 needs to be met for the whole circuit unless over-current protection can be legitimately omitted (see sections 433.2, 433.3 )

bennie · Jun 13, 2023

Not sure if theres a reg on it but it's fine to do that on a radial circuit as long as the protective device is the correct rating for the smaller cable, as you've done. What's the wattage of the oven? The cable might need to be changed if it needs to be on a 40 amp.

Alexking1 · Jun 13, 2023

bennie said:
Not sure if theres a reg on it but it's fine to do that on a radial circuit as long as the protective device is the correct rating for the smaller cable, as you've done. What's the wattage of the oven? The cable might need to be changed if it needs to be on a 40 amp.

Total wattage 8215w. No socket at isolation switch. So with diversity 32A works.

Alexking1 · Jun 13, 2023

timhoward said:
No there isn't a reg requiring consistent cable throughout a circuit. (In fact 433.3 mentions the scenario of a reduction in CCC.)
If the 6 sq mm leg was clipped direct (when it can carry 47 amps before any further de-rating) it might have complied.

Basically 433.1.1 needs to be met for the whole circuit unless over-current protection can be legitimately omitted (see sections 433.2, 433.3 )

Thanks Tim.

davesparks · Jun 13, 2023

Alexking1 said:
Came across a cooker circuit with 10mm and 40A rcbo. Last leg was 6mm. Isolator to outlet. I downgraded to rcbo to 32A to accommodate the 6mm. Does the cable have to be the same throughout? If so what is the reg for this please.

Why did you change the RCBO to 32A?

No there is no requirement for the same size cable to be used throughout as long as all cables used have adequate fault protection and, if required, overcurrent protection.

davesparks · Jun 13, 2023

bennie said:
The cable might need to be changed if it needs to be on a 40 amp.

The OCPD would need to be changed back to 40A but the cable is unlikely to need changing.

oscar21 · Jun 13, 2023

All seems a bit over the top to me and even if it were a technical regs breach you have to look at common sense. One of the first scenarios when I was in college doing the 2391 20 odd years ago was to calculate the cable size for an electric shower. following the regs to the letter it needed to be 25mm cable, the tutor said "obviously thats a load of -------s because it wont fit into the terminals so always us your common sense." Which has stuck with me ever since.

Only yesterday I wired a point for an oven that wasn't on site yet and some single ovens nowadays come with a plug so in this instance we wire a 6mm to an outlet plate and wire a single socket next to it in a dual box to give the appliance installer options. I only use a short length of 2.5 for the socket because, well, its just a single socket.