Confused by Scaddan example in Electric Wiring for Domestic Installers

[jack_sprat]

Hi,
I'm a self-build amateur - I'll be running cables and getting system sign-off/inspection by certified competent person, and am just working through circuit design basics.

I'm referring to Scaddan's 'Electric Wiring for Domestic Installers' (16th ed for 18th ed IET), which includes example for addition of kiln to existing house (pp88-93).

• Existing total current 118A, with diversity is 100A, on an 80A fuse, 80A meter, 80A consumer unit.
• New load (+kiln), with diversity is 137A.

I'm okay with all of the calculations presented (sizing tails, sizing MCB, cable, and shock risk &c.) but I don't understand how he concludes in his summary that "main fuse needs uprated to 100A"

He notes that existing tails are inadequate but if taking account of diversity on existing system results in a design load that's 20A over the fuse, meter, and CU ratings, surely that was all already needing upgraded, and the increased design load will need more than 100A capacity.

Is anybody able to explain? Is it in a table ? (I'm waiting on the 'Brown book' on-site guide)

thanks - JS

 

[mainline]

Hi,
I'm a self-build amateur - I'll be running cables and getting system sign-off/inspection by certified competent person, and am just working through circuit design basics.

I'm referring to Scaddan's 'Electric Wiring for Domestic Installers' (16th ed for 18th ed IET), which includes example for addition of kiln to existing house (pp88-93).

• Existing total current 118A, with diversity is 100A, on an 80A fuse, 80A meter, 80A consumer unit.
• New load (+kiln), with diversity is 137A.

I'm okay with all of the calculations presented (sizing tails, sizing MCB, cable, and shock risk &c.) but I don't understand how he concludes in his summary that "main fuse needs uprated to 100A"

He notes that existing tails are inadequate but if taking account of diversity on existing system results in a design load that's 20A over the fuse, meter, and CU ratings, surely that was all already needing upgraded, and the increased design load will need more than 100A capacity.

Is anybody able to explain? Is it in a table ? (I'm waiting on the 'Brown book' on-site guide)

thanks - JS

Take your figures, apply a diversity factor of about 0.4 and then decide

 

[davesparks]

I suspect the suggestion of up-rating to 100A is based on the fact that is the maximum size you can normally get on a domestic single phase supply and not actually based on the calculation.


This nicely shows the difference between theory and reality.

In theory they need a bigger supply, which would almost certain be 3 phase as you almost never get single phase supplies over 100A.

In reality a 63A supply would be adequate in most cases.

Electricians need to learn both the theory and reality as both are important parts of the job.

 

[davesparks]

Hi,
I'm a self-build amateur - I'll be running cables and getting system sign-off/inspection by certified competent person, and am just working through circuit design basics.

Have you made arrangements for getting it signed off already? This is not normally permitted/possible and generally very few decent electricians would be willing to take this on.

Certifying electrical work involves signing for 3 seperate stages of the job, design, installation, inspection and testing.
If you are designing and installing then you will have to sign-off those elements of the job and the electrician you employ will have to sign for the inspection and testing that they do.

 

[Markyd]

Have you made arrangements for getting it signed off already? This is not normally permitted/possible and generally very few decent electricians would be willing to take this on.

Certifying electrical work involves signing for 3 seperate stages of the job, design, installation, inspection and testing.
If you are designing and installing then you will have to sign-off those elements of the job and the electrician you employ will have to sign for the inspection and testing that

Should be a good answer...if at all

 

[jack_sprat]

Have you made arrangements for getting it signed off already? This is not normally permitted/possible and generally very few decent electricians would be willing to take this on.

Certifying electrical work involves signing for 3 seperate stages of the job, design, installation, inspection and testing.
If you are designing and installing then you will have to sign-off those elements of the job and the electrician you employ will have to sign for the inspection and testing that they do.

... and therein lies the issue: engaging a 'decent electrician' who not only turns up but even returns calls and is a competent enough joiner to route and rout cables and backboxes in CLT.
TBH, I don't want to be doing this either, but a combination of failed main contractor, Brexit, and COVID have meant that I've had to take on much more of this build than I'd ever anticipated. So I'm making the best I can of the situation - I can't secure a reliable sparky for design, or first or second fix, so I'm going with the one who's said he'll inspect and sign off my installation (and who I've worked with as 'mate' in the past) - it's far from ideal but, as is often mentioned in other contexts, this is where theory has banged up against reality.

 

[mainline]

... and therein lies the issue: engaging a 'decent electrician' who not only turns up but even returns calls and is a competent enough joiner to route and rout cables and backboxes in CLT.
TBH, I don't want to be doing this either, but a combination of failed main contractor, Brexit, and COVID have meant that I've had to take on much more of this build than I'd ever anticipated. So I'm making the best I can of the situation - I can't secure a reliable sparky for design, or first or second fix, so I'm going with the one who's said he'll inspect and sign off my installation (and who I've worked with as 'mate' in the past) - it's far from ideal but, as is often mentioned in other contexts, this is where theory has banged up against reality.

So why doesn't your matey sparky do the design, after all he will be the one to sign his name to it.

If your supply really isn't enough then you can always lock out for instance (the kiln) when the cooker is in use.