Am i wrongly calculating my maximum demand?

[Engineer54]

Your customer will rue the day they went all electric, ...unless of course they have plenty of money in the old tommy tank!! lol!! ...And totally buggered, if and when they have power cuts for any length of time!!

 

[D Skelton]

Hi guys i calculating my expected maximum demand for an installation to see if the 60A cut out will handle it.

This is how I would work it out:

1X 32A ring, - 32A
1x 32A ring - 32A x 0.4 = 12.8A
1X 6A lighting, - 6A x 0.66 = 3.96A
1X10A lighting, - 10A x 0.66 = 6.6A
1X32A shower, - 32A
1X45A cooker, - 10A + (35A x 0.3) = 20.5A (It's 30% of what is left over after the 10A (plus socket if applicable), not 40%)
1X32A underfloor heating, - 32A

Then leave out the largest load (in this case one of the 32A circuits where diversity was not allowed), while you work out 40% of what is left, then add together to get a figure: (This is the part you were missing out)

(32A + 32A + 20.5A + 12.8A + 6.6A + 3.96A x 0.4) + 32A = 75.144A

Then add up all circuits pre-diversity and work out 40% of that total:

45A + 32A + 32A + 32A + 32A + 10A + 6A x 0.4 = 75.6

Providing there is no difference greater than 5A between those two numbers, you can take whichever is the lowest of those numbers to be your maximum demand, otherwise, take the highest. In this case, your maximum demand is 75.144A

This is how I was taught in college anyway.

Seems to me you'll need to shove an 80A in to replace the existing 60A. But don't worry about upgrading the 16mm tails to 25mm ones as 16mm tails are rated at 87A anyway, so you'll be fine there.

Hope that helps

 

[Octopus]

If you're doing a re-wire why don't you fit fewer circuits and then the "old" diversity calculation will probably work.

But seriously as more and more homes are getting 8 or 10 way boards the approach to calculating loads needs to be revised.

Personally I add up all the breakers and multiply by 0.4!

 

[Farmelectrics]

dont you mean divide by 0.4

 

[Farmelectrics]

stand corrected multiply it is just done calcs

 

[Octopus]

dont you mean divide by 0.4

100 divided by 0.4 = 250

100 multiplied by 0.4 = 40

oops - sorry just seen your next post!

 

[Farmelectrics]

murdoch can you apply this 0.4 calculation to 3 phase boards aswell

 

[Rockingit]

Agree here totally with Murdochs thoughts on how the traditional methods (whichever one you use) are increasingly misleading with more circuits on wider boards. A method I'm starting to use more and more is the principal of starting with your max incoming capacity (20kW, say) and then 'spending it' around the install. You almost always have lots of spare to save.

 

[telectrix]

also bear in mind that a 60A BS1361 fuse will run all day up to 100A without blowing.

 

[Farmelectrics]

is that because all fuses breakers rcbos etc take up to one half times its load

 

[Farmelectrics]

not ideal though as the cables them selves are taking more current than they should

 

[telectrix]

agreed, but such currents would only be short duration.