Junction boxes size

Good morning,

i'm trying to find information about how to calculate junction boxes sizing by uk regulations. Can anyone help, please?

Best regards

fab.elec said:

Good morning,

i'm trying to find information about how to calculate junction boxes sizing by uk regulations. Can anyone help, please?

Best regards

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Welcome to the forum mate.

'Junction boxes' Are you sure this has been translated correctly?
Usually junction boxes are sized as to what is going in them. Fore example, the more terminals the bigger they are.

Spoon said:

Welcome to the forum mate.

'Junction boxes' Are you sure this has been translated correctly?
Usually junction boxes are sized as to what is going in them. Fore example, the more terminals the bigger they are.

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Thanks for your reply
I'm pretty sure about the translation. In many countries the regulations contain specific requirements about boxes sizing, but i don't seem to find anything in BS7671, that's why i'm asking in this forum

fab.elec said:

Thanks for your reply
I'm pretty sure about the translation. In many countries the regulations contain specific requirements about boxes sizing, but i don't seem to find anything in BS7671, that's why i'm asking in this forum

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Very interesting. What does your regulation say about box sizes?

I also can not find anything about sizing of junction boxes in BS7671 or the On-Site Guide.

Spoon said:

Very interesting. What does your regulation say about box sizes?

I also can not find anything about sizing of junction boxes in BS7671 or the On-Site Guide.

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For example, regulations in Italy says that junction boxes require at least 20% of free space. NEC regulations in the USA gives formulas to calculate boxes size based on the dimensions of conductors and terminals that go inside it

Could you point me to the BS7671 on-site guide you're talking about?

fab.elec said:

Could you point me to the BS7671 on-site guide you're talking about?

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In the On Site Guide there is a section on 'Cable capacities of conduit and trunking', Appendix E, (page 133)

Also, a section of this which states:

Under BS7671 we do not use a standard formula to determine junction box sizes in general. They must be suitable for the purpose and installed with good workmanship. A competent electrician / designer / specifier should know what is and is not suitable under the prevailing conditions.

However, junction boxes that are inaccessible for maintenance must be constructed and installed to the 'maintenance-free' specification of BS5733, in which case the manufacturer will usually stipulate the maximum number of cables / terminals to be fitted in order to achieve MF compliance for that particular product.

Ain’t we talking about their current carrying capacity, i.e. 5A or 30A?

No, under some regs there are specific requirements for the physical dimensions of the box. Under NEC article 314 for example, there are two ways of sizing a box. If it is one of the standard sizes (there are a couple of dozen) then the maximum number of cables of any size can simply be looked up in a table. A square 4 x 4 x 1 1/2" box has a usable volume of 21 cubic inches and may be used to terminate no more than 14x #18 conductors, 12x #16 conductors etc. If the box is of any other size or you need to calculate for a selection of cable sizes you can use the volume requirement per conductor terminated, per clamp installed, per ground strap etc as tabulated, e.g. each #10 conductor requires 2.5 cu. ins. The volume is always printed on the box.

This is one of the things I would point out to people who think BS7671 is vague; it tries to avoid being prescriptive, in this case entrusting choice of suitable dimensions to the installer.