Split Load CU

[DNS]

I was discussing the wiring of split load twin RCD consumer units with a friend last night. I wire them with the largest MCB amps next to the RCD (first feed) then decreasing down to the smallest eg. 6 amps. He disagreed and said it should be the other way, smallest first. I tried to explain that the largest demand should be first..... etc... He threw his teddy out of the window then. I have tried to find this in the books/guides, but I can't find it. Please can you settle this argument, I am right aren't I. Thanks....

 

[Midwest]

Might be wrong, but I think it is just 'Good Practice'just to provide some form of standardisation.

 

[Deleted member 9648]

It used to be good practice to place the largest load next to the supply device in order to keep load off the full legnth of the busbar....regarded by most as irrelevant now as the maximum load of the busbar should never be exceeded,and as long as that rule is complied with the distribution of load on an 10" legnth of copper is hardly important.
Did your mate give a reason for placing the lightest loads first?....I cant think of one.

 

[johnboy6083]

its strange, because whenever iwire a ph board, i keep the largest load next to the main switch, but on a 3ph board, its not really considered. 9 times out of 10,im adding circuits to existing boards, and theres no order where people have added and removed circuits ove rthe years

 

[DNS]

It used to be good practice to place the largest load next to the supply device in order to keep load off the full legnth of the busbar....regarded by most as irrelevant now as the maximum load of the busbar should never be exceeded,and as long as that rule is complied with the distribution of load on an 10" legnth of copper is hardly important.
Did your mate give a reason for placing the lightest loads first?....I cant think of one.

No he couldn't explain why, just that's the way he'd been shown. I was shown largest to the supply RCD, which make sense.... Thanks for your reply, I shall print it off and show him this morning.

 

[Engineer54]

This goes way back!! lol!! I was taught the largest load closest to the supply device on CU/DB's from when i first worked with my uncle when i was 13 and was continued to be taught the same throughout the rest of my training.

As wirepuller rightly stated, probably irrelevant these day's and is more a sign of good practice. I must admit i still like to see this practice, certainly makes far more sense than having the lightest loads next to the supply device. Even the CU/DB manufacturers, generally show this arrangement when shown in catalogues and sales blurb photo's etc!!

So historically speaking your friend and whoever taught him, ...is Wrong!! lol!!!

 

[truckster]

I was taught biggest loads next to the switch so when the shower (o whatever) kicks in it doesn't make the lights dim momentarily. Whether it's right I dunno but now its habit.

 

[Rockingit]

OK....so here's a thought:

At what point does the load on the bus bar decrease? For example, say I've got a 50A load, a 25A load and a 10A load sequentially off the bar. If I clamp over a spare couple of inches at the end of the bar nearest the 10A load, it's going to tell me it's pulling 85A, right? So where exactly do we draw a conclusion that the current is lower down the bar??

 

[Engineer54]

Not sure your clamp meter would be showing anything, as there is no current flowing in the bar past the last MCB!!

 

[Rockingit]

Not sure your clamp meter would be showing anything, as there is no current flowing in the bar past the last MCB!!

yeah, yeah, I knew I ought to have added that bit in my post! But you know what I mean.. so lets add a gap between some breakers and put the clamp over that bit........

 

[Engineer54]

OK, .... the same sort of answer really, it'll show whatever current is flowing in that part of the bus bar. It won't be showing the full current being drawn from other MCB's closer to the supply device.... Hence the reason for having the larger loads closer to the supply device.... lol!!

 

[Rockingit]

Fair enough!