60amp cut out fuse homework

[junior-sparky]

od morning everyone, I'm new to the forums and looking forward to all the help and advice your experience can give me.

I've been asked to find out that if we found a 60amp cut out fuse in a normal domestic property will this be sufficient gor the property. Now my tutor said there's a equation counting all circuits and seeing if we need to upgrade or we're happy to stick with the 60amp. Im sure he said adriabatic equation or I coukd of been wrong.

Rcd 1
1. Upstairs lights 6a
2. Outside light 6a
3. Cooker 40a

Rcd 2
1. Whole house sockets 32a
2. Downstairs lights 6a

There's also spare ways for garden shed power and a separate circuit for a future conservatory.


Cheers

 

[Andy78]

This question concerns two terms. Maximum demand and diversity. Look them up, get your books out, and you will have it cracked in no time.
Post your workings out and answers on here and someone will no doubt help you out.

 

[Farmelectrics]

60a would be ok highly unlikely to get anywere near the 60a but as said work out your maxiunm demand and apply diversity ib = power divided by voltage

 

[junior-sparky]

Would max demand just be adding up mcb amps? Only been in college 2 minths so still learning

 

[RogertheBodger]

You may also have to look at the requirements for future circuits in the spare ways mentioned in your post. How many circuits/Breaker size, for Garden shed and Conservatory?

 

[junior-sparky]

If I could figure out how to put images on here I'd show the sheet we have to work from.

RCD 1-

Circuit 1 - upstairs lights = there's 8 lights on it and using the worst case of 100w each as there is no rating I get a load of = (rounded up) 4 amps.

Circuit 2 - outside light = again 100w = (rounded up) 1 amps.

Circuit 3 - cooker = 2 x 5500kw ovens = (rounded up) 48 amps Diversity>>>>
48 -10 = 38, 30 x 38 / 100 = 11.4 amps, + 10amp from start + 5amp for a socket === (rounded up) 27 amps.



RCD 2 -

Circuit 1 - Whole house sockets = how to you calculate this without knowing the ratings and whats on it?, any help please.

Circuit 2 - downstairs lights = 10 lights on and again I'll use max 100w = (rounded up) 5 amps.


So far = 37 amps. so all good.


There's no info on there amps just says there's future scope to have soon.

 

[Rockingit]

Appendix 1 in the On-site Guide walks you through the 'official' version.

As far as socket circuits are concerned, you are meant to allow 100% of maximum load for the first (so that would be 32A in this case), then 40% of each additional circuit. Well, if you think about it, if you have two rings side by side, each on 32A overload protection, which one are you going to call the first, and how come the other is only 40% of it if they're doing principally the same job in the same place??

In practice, it's all about looking at what is going on: A 32A ring in a kitchen/utility room has lots of white goods plugged into, kettles etc, so it's reasonable to assume that it's going to average a much higher current draw than say a domestic ring supplying the lounge and bedrooms which these days only has lots and lots of switched mode power supplies plugged in and maybe the odd hairdryer - in other words pretty close to naff all.

Diversity is the word we give to explain this principle - that the current loadings on circuits are diverse, different, and change over time. For example, in a house you might be cooking lunch and doing the washing at lunchtime, but in the evening you're just sat at your computer here on the forum with a few lights on and making the occasional coffee, so the overall load on your house hits rock bottom, and then is almost negligible overnight depending on how lazy you are about turning stuff off standby mode! Even a 2kW washing machine isn't drawing 8 amps all the time it's on - it spends most of its life either thinking or spinning around - its only when it's heating water and the elements are on that it's drawing that much juice. Likewise, a cookers elements turn on and off, and so on.

Things like showers (instantaneous water heaters) don't tend to change much as they are a fixed load for a fixed time - when a 9kw shower is on, it will keep drawing 9kw until you turn it off again (put simply. In practice it probably isn't 9kw's!!).

Double check your workings on the cooker, and I think if you go back over your maths and DONT keep rounding up, you may find you're pretty close to smack on 60A all-in (haven't checked that, btw!).

 

[Farmelectrics]

another way was add up all breakers and multiply by 0.60

 

[Rockingit]

I'll give you all hope on the subject.....

I've a domestic customer who at the main intake CU (DB1 of 5) has distribution circuits of 1 x 63A, 1 x 50A, 2 x 32A, 2 x 16A, 1 x 10A and 1 x 6A. That's 231 Amps in total. Their main intake is a standard 80A fuse fed from a 22kW pole transformer. And it's all running quite happily with not a hint of overload in sight, nowhere near it.

 

[junior-sparky]

Many thanks for the explanation it has helped alot.

Like you said, how do you decided which comes comes first with ring diversity as total could be either really low or hi dependent on which you choose.

Ive double checked my cooker duversity and came up with 26.34 amp so I'd be happy with a 32a or 40a mcb with 6mm t&e maybe 10mm to be extra great.