Discuss Total load on distribution board in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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F

Farraday

Hi

im looking for someone to help me understand the total load that can be pull from a dis board.

Ie: I have 3phase dis board on a 250amp breaker. The board is filled with mcbs of mixed ratings. However if I add the rating of each MCB together it far exceeds the 250amp breaker on the board.

Now ow I know each load will not pull the amount of amps required to trip the MCB. Can someone explain how to work out how much can be put in a board please.
 
I'm not sure I fully understand your question but the first thing you need to bear in mind about MCB' is their function is to protect the cable that's connected to them from being overloaded to a point where it's damaged or starts a fire.

The second thing is Google the term 'electrical diversity factor', the definition of which may partially answer your question.
 
And the third thing to note is that it's over 3 phases.
 
Can someone explain how to work out how much can be put in a board please.
Dear oh Dear what next, obviously the total load connected to the Distribution board should not exceed it's rating and incomer rating, the size of MCBs is regardless, your answer is not to exceed the Distribution board rating as listed by the manufacturer, get a grip man
 
Dear oh Dear what next, obviously the total load connected to the Distribution board should not exceed it's rating and incomer rating, the size of MCBs is regardless, your answer is not to exceed the Distribution board rating as listed by the manufacturer, get a grip man


Very helpful reply to a forum newcomer!!

I was a little hasty when writing the OP and as a result it doesn't read too well.

Am i correct in thinking the total sum of the mcbs must not exceed the rating of the breaker protecting the board.

Example. Board is on a 125amp trip, it's 3 phase. I have mcbs rated 63amp 50amp and 40amp. The sum of those is 153amp.

Is is this bad practice?
 
Last edited by a moderator:
Example. Board is on a 125amp trip, it's 3 phase. I have mcbs rated 63amp 50amp and 40amp. The sum of those is 153amp.

Is is this bad practice?

In this example, are these three phase MCBs or are they single phase MCBs on three different phases?

What are the loads on each of these circuits?
 
Example. Board is on a 125amp trip, it's 3 phase. I have mcbs rated 63amp 50amp and 40amp. The sum of those is 153amp.

Is is this bad practice?

No

You'd have to be very lucky to get loads matching the MCB exactly.
Mcb's are always higher rated than the expected load for that circuit, then make use of those Ex Factor dancers.
 
Blimey. This is pretty basic stuff. The same applies to single phase installations - these days the circuits are segregated down so much that adding up the MCBs will give a meaningless (and often quite large) value.
 
I could see some bl**dy big main switches around.....not to mention tails...be getting up to 220 amps with a CU in an average semi.
 
If you're asking a question like this, then the HNC you say you have, isn't in electrical Engineering!!
So you're either an apprentice or another wannabe Electrical Trainee electrician!!
 
Hi farraday. Welcome to the forum
First of all I'm presuming you mean a 250A isolator and not a 250A breaker.
Your question was answered in post 2 by Marvo. Read post 2 again.
Also depends on when the 3 phase board is feeding and if you can apply diversity.
Not wanting to sound negative (but always end up doing so) but why do you need to know and I'm kind of going along with Eng54 first sentence in post 17.
 
Hi farraday. Welcome to the forum
First of all I'm presuming you mean a 250A isolator and not a 250A breaker.
Your question was answered in post 2 by Marvo. Read post 2 again.
Also depends on when the 3 phase board is feeding and if you can apply diversity.
Not wanting to sound negative (but always end up doing so) but why do you need to know and I'm kind of going along with Eng54 first sentence in post 17.

Thanks for the sensible reply mate, very much appreciated.

Regarding my HND, I did not say it was electrical so there's no need to question my qualifications. It's in refrigeration design.

I understand that forums are close knot communities as I use several others. Some of the responses in here to a new member and someone wanting to learn are pretty shocking g and certainly doesn't paint the board in a good light. We you not all just starting out and trying to learn at some point?
 

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