Maximum demand

[kingeri]

So now what's the feeling these days on working out maximum demand for domestics, the OSG method has to go out of the window surely, with more and more division of circuits going on. I have been pondering my latest job, very nice but older installation done by an old electrical engineer, changed CUs to all RCBOs (with him looking over my shoulder the whole time). Distributor's fuse is 80A.

DB1
Shower 40A
Basement sockets 32A
Kitchen sockets 32A
Living room sockets 20A
1st floor sockets 20A
2nd floor sockets 32A
Basement lights 6A
Ground floor lights/smokes 6A
1st floor lights 6A
2nd floor lights 6A

DB2
Cooker 40A
Washing machine 16A
Boiler and controls 16A
Towel rail 16A
Fridge/freezer 16A
Socket at DB 16A

 

[LeeH]

Use the tables and your experience, what else can you do?

Looks tight on paper doesn't it but 80A for dwelling is a lot.

 

[Andy78]

Well the 0.4 factor is my go to method for domestic, but that brings your max demand for this installation to 128A.

I would be working out a total of actual connected loads (towel rad, fridge, lighting points, boiler etc) and then using a sensible figure for a likely load on the socket outlets.

 

[Midwest]

Wonder if the property use has changed before or since the CU's have been replaced. New family, teenagers, more appliances. If not, and it's been okay in the past? Not very technical. I suppose if it was Xmas day & cooking the lunch, teenager in the shower for 2 hours, all the lights left on, appliances doing their thing etc etc, then it would come close to those figures. But how likely is that to happen?

Perhaps you could visit on such a day with a clamp meter

 

[Taylortwocities]

If you look at the list, you can discount a lot of those
Boiler = 3A max
Towel rail = 3A
DB socket will be tiny unless the tumble dryer is in it
Lighting - modern low energy lights everywhere? Probably an amp per circuit
Your major loads will be the cooker and that wont be 40A, what is the load really?
Shower, how big is it really?
Plus any large appliances (WM/DW etc)

IMO 80A will be fine.

 

[kingeri]

Yep it will be fine, it has been for years, I was just pondering what to put down as the max demand on the cert. It just seems like an exercise in figure fudging just to get a number below 80A. Pointless really!

 

[hightower]

Turn everything on, give it 10 minutes or so and clamp it, writing that number down?

 

[HandySparks]

I think that my 'formula' for domestic max demand from now on will be something like 35A, plus electric shower (if fitted) less, say, 10A if hob is gas. So, in this case, if hob is electric, 35A + 40A? shower =75A.

See this post:
Does christmas Cooking cause MCBs to pop?

 

[hightower]

I think that my 'formula' for domestic max demand from now on will be something like 35A, plus electric shower (if fitted) less, say, 10A if hob is gas. So, in this case, if hob is electric, 35A + 40A? shower =75A.

See this post:
Does christmas Cooking cause MCBs to pop?

Wouldn't work up here, we have a tonne of 60A fuses in the cutouts in this neck of the woods.

 

[HandySparks]

I think that my 'formula' for domestic max demand from now on will be something like 35A, plus electric shower (if fitted) less, say, 10A if hob is gas. So, in this case, if hob is electric, 35A + 40A? shower =75A.

See this post:
Does christmas Cooking cause MCBs to pop?

Forgot to say,
unless there's electric water heating or space heating, which would obviously have to be taken into account.

 

[HandySparks]

Wouldn't work up here, we have a tonne of 60A fuses in the cutouts in this neck of the woods.

A 60A fuse will easily take a small short-term overload from a shower and, in many cases there won't be other large loads running at the same time.

Edit:
It would be interesting to see the results if a few on here hung one of the cheap load monitors on their supply for a few days/weeks and reported their findings.

 

[DPG]

The problem is how circuits are arranged. You could have 5 radials on 20A MCBs in a house, or you could 2 ring finals on 32A MCBs. This would affect the max demand figures when using that method, but in reality the homeowner hasn't got any more stuff plugged in. Daz

 

[HandySparks]

The problem is how circuits are arranged. You could have 5 radials on 20A MCBs in a house, or you could 2 ring finals on 32A MCBs. This would affect the max demand figures when using that method, but in reality the homeowner hasn't got any more stuff plugged in. Daz

Quite.
I think, for domestic, the number of circuits is almost irrelevant. It's more down to what the major loads are, and when they're actually in use.

 

[Wilko]

Many local dwellings here are 60A but with gas or oil heating. And I've not seen a genuine load blown fuse, just saying. Mine has 60A fuse and 232A worth of installed MCB and absolutely no probs.
So if no change of use and no history of fuse failures or signs of heat damage, then I figure max demand must be less than fuse rating. If the calc max demand makes no sense I have used the fuse rating for that figure. Am I a bad person?

 

[hightower]

A 60A fuse will easily take a small short-term overload from a shower and, in many cases there won't be other large loads running at the same time.

My point is as @Wilko's - I'm not arguing whether a fuse can take a short-term overload, but it's bad design to have a maximum demand higher than the main fuse. Doing so would be on par with installing a 32A mcb for a 40A shower and telling the customer to keep their showers short.

 

[Midwest]

Yep it will be fine, it has been for years, I was just pondering what to put down as the max demand on the cert. It just seems like an exercise in figure fudging just to get a number below 80A. Pointless really!

Pointless task. I put down the Cutout fuse size, not really correct, but as from the replies here you'll be chasing your tail trying to calculate a figure. In my opinion, unless it's on a commercial or industrial installation, how could you properly calculate it.

Fixed loads like showers one thing, cookers (having their own designed diversity) other appliances connected via sockets, no chance.

Unless a domestic property has 2 electric showers, induction hob and or electric central heating, it's unlikely to be a problem

 

[Taylortwocities]

You might be interested to have a look at how little the DNOs allow for each house.
See extract from the EON network design manual. 2KW for an average house!!



View attachment 35255

View attachment 35254

 

[hightower]

Yes, but surely that 2kW is because they are calculating diversity on top of diversity, if that makes sense - as in not everything in the house will be on at the same time, and not all houses will have heavy loads on at the same time.

 

[Taylortwocities]

Indeed. Just thought it was interesting and shows why we don't need 50 more power stations than we actually have.

 

[sparksburnout]

Don't take this the wrong way but why are you worrying about this? Is there any evidence to suspect the current 80A fuse is not suitable? And it's not really your problem, is it? I can't understand why you are bothered by it.