Maximum Demand Calculation

[Bob Nicholson]

Gent's,

Appreciate your guidance on how to correctly complete the Maximum Demand request on the Electrical Installation Certificate.

I understand page 97 of the On-Site Guide but do not understand how you can work out the figure if

a. a property is empty.

b. if you install a new cct are you expected to work out every single appliance/luminaire usage in order to come up with a figure?

I am currently on my C&G 2392 and need to know the painful answer.

Anyone who just adds up the individual MCB's or takes the suppliers value, doesn't know what they are doing, IMHO!

Thanks

Bob

 

[ianmacd]

Gent's,Appreciate your guidance on how to correctly complete the Maximum Demand request on the Electrical Installation Certificate.

I understand page 97 of the On-Site Guide but do not understand how you can work out the figure if

a. a property is empty.

b. if you install a new cct are you expected to work out every single appliance/luminaire usage in order to come up with a figure?

I am currently on my C&G 2392 and need to know the painful answer.

Anyone who just adds up the individual MCB's or takes the suppliers value, doesn't know what they are doing, IMHO!

Thanks

Bob

You are saying you do not fully understand max demand BUT you state that anyone that just adds up the MCB's doesn't know what they are doing. Isn't that a contradiciton?

 

[ian932]

You are saying you do not fully understand max demand BUT you state that anyone that just adds up the MCB's doesn't know what they are doing. Isn't that a contradiciton?

Helpful !!!!

 

[Bob Nicholson]

I did not say that I didn't understand Maximum Demand & Diversity allowances, my questions are still the same, no contradiction at all!

 

[Doc Hudson]

I did not say that I didn't understand Maximum Demand & Diversity allowances, my questions are still the same, no contradiction at all!

You are quite valid in your questions Bob, there are few threads discussing this topic. Here is one I have found, (there are others).

http://www.talk.electricianforum.co.uk/showthread.php?t=3749&highlight=max+demand

I suggest have a quick read through these see if your question is answered, if not just ask again and someone else may be able to give another angle on it.

Doc H.

 

[Doc Hudson]

You are saying you do not fully understand max demand BUT you state that anyone that just adds up the MCB's doesn't know what they are doing. Isn't that a contradiciton?

Actually I believe Bob is pointing out max demand is a complex calculation not a simple 6+16+32=54, as some people may assume. There is no contradiction in his question. From my reading of the post, the question is "What is the best way for calculating maximum demand?"

Doc H.

 

[ian932]

Gent's,Appreciate your guidance on how to correctly complete the Maximum Demand request on the Electrical Installation Certificate.

I understand page 97 of the On-Site Guide but do not understand how you can work out the figure if

a. a property is empty.

b. if you install a new cct are you expected to work out every single appliance/luminaire usage in order to come up with a figure?

I am currently on my C&G 2392 and need to know the painful answer.

Anyone who just adds up the individual MCB's or takes the suppliers value, doesn't know what they are doing, IMHO!

Thanks

Bob

a. property is empty.

Table 1B

1. Assume each point as a 100w bulb add points up and calculate the load

2. Most domestic items will be covered by items 3 to 8

3. Get the load for the ovens and hobs and calculate if none fitted use the trip rating and calculate ie 40A mcb = 10A + 30% of 30A = total of 19A + 5A if you have a socket built into the control unit

4. N/A for domestic.

5. Get the load for the showers etc and calculate 100% largest then 100% second largest and 25% of the remainder

6. Get the Kw rating for the water tank elements and use 100% of the load.

7. Get the Kw rating for the UFH heating and use 100% of the load.

8. Get the Kw rating for the storage heater and use 100% of the load.

9. This is very much a case of looking at the size of circuit and taking a guess as to which of the several socket circuits will have the high loads, on a ring up ring down take the downstairs ring and apply 100% of the mcb as the load then other circuits as 40%. If you have a large kitchen with its own ring/radial that would probably be the 100% load.

10. Would probably be covered in 9.

If there is no data on elements etc. use ohms law on things like 6 to 8 to get the expected load.

b. no just apply the logic above, as a rule its only the fixed appliances and fixed lighting that you will have the load characteristics of the rings and radial sockets will be based on a % of the trip used.

 

[NozSpark]

Maximum demand is an ass....

Lets say you have a house

I had a 32A cooker circuit, a 32A RF and a 6A lighting circuit (lets say 10 fittings)

That gives you a maximum demand of 51.5A (IIRC)

Now you go in and rewire it..

so it now has a 32A cooker circuit, 32A RF (kitchen), 20A radial (D/S), 20A radial (U/S), 6A lighting (5 fittings D/S), 6A lighting (5 fittings U/S), 6A smoke alarm circuit (2 smokes)

Which gives you a new maximum demand of (not interested in working this out)

So why has your maximum demand now gone up????

It's the same house, same number of sockets & appliances, same number of occupants!!!!!!!!

 

[madeinengland]

this is the exact problem i'm having. we're supposed to fill out all paperwork for the 2392 as if we've just done a new install. the circuits we have are

1)1x16 amp radial circuit with 1 socket.

2)1x6 amp lighting circuit with 2 way 1 lamp.

then rcd protected circuits of

3)1x40 amp cooker point with 1 socket

4)1x32 amp ring circuit with 4 sockets and 1 spur.

what's the maximum demand for this?

 

[ianmacd]

this is the exact problem i'm having. we're supposed to fill out all paperwork for the 2392 as if we've just done a new install. the circuits we have are1)1x16 amp radial circuit with 1 socket. 40% = 6.4A

2)1x6 amp lighting circuit with 2 way 1 lamp. 6*0.66~4A

then rcd protected circuits of

3)1x40 amp cooker point with 1 socket 10+(0.3*40) +5 = 27A

4)1x32 amp ring circuit with 4 sockets and 1 spur. 32A

what's the maximum demand for this? 6.4+4+27+32 = 69.4A

In red

 

[madeinengland]

thanks for that ianmcd. but:

3)1x40 amp cooker point with 1 socket 10+(0.3*40) +5 = 27A

do you take 30% of entire amps or the remainder after you've taken 10 amps? so wouldn't it be 10=(0.3x30) +5 = 24A.

or is where i keep going wrong?

 

[ianmacd]

THis is something thats just come up in another thread as Table 1A and Table 1B give conflicting advice wrt cooker circuits.

 

[JUD]

this is the exact problem i'm having. we're supposed to fill out all paperwork for the 2392 as if we've just done a new install. the circuits we have are1)1x16 amp radial circuit with 1 socket. 16 x 0.4 = 6.4A

2)1x6 amp lighting circuit with 2 way 1 lamp. 100 / 230 x 0.66 = 0.29A (Only 1 lamp = 100W)

then rcd protected circuits of

3)1x40 amp cooker point with 1 socket 10 + (0.3 x 30) + 5 = 24A

4)1x32 amp ring circuit with 4 sockets and 1 spur. 32A

Total = 62.7A

what's the maximum demand for this?

In Green

 

[ITS]

If you've got a 100A Main Switch on the CU and a 100A Main Fuse - isn't the maximum demand 100A?

 

[JUD]

Not after you've applied Diversity to all the circuits.

 

[madeinengland]

here's an example ITS.

you have 100A main fuse but the house has for example:

2x6A lighting circuits

1x13A immersion heater

2x32A rings

1x32A shower.

without diversity this adds up to 121A.this would blow a 100A main fuse.

 

[sparkytim]

THis is something thats just come up in another thread as Table 1A and Table 1B give conflicting advice wrt cooker circuits.

No I don't think so. read 1B carefully "30% IN EXCESS of 10 A +5A"

= 1A " remainder of"

 

[ianmacd]

No I don't think so. read 1B carefully "30% IN EXCESS of 10 A +5A"= 1A " remainder of"

Read the whoe sentence to keep it in context.

 

[ITS]

Never mind about diversity. The installation with a 100A fuse can draw no more than 100A so therefore 100A is the maximum demand.

 

[JUD]

Never mind about diversity. The installation with a 100A fuse can draw no more than 100A so therefore 100A is the maximum demand.

We're not talking about the maximum demand the fuse can carry. We're talking about the maximum demand of an installation in normal service which will probably never come anywhere near to 100A.