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Discuss Sub panel: Underrated MCB? in the Electrical Forum area at ElectriciansForums.net

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Hello all

A quick question about an existing layout if I may.

The layout consists of a main CU connected to a sub panel via a single 63 amp MCB.

A Dorman Smith K1PC63 to be exact.

Total potential load on the sub panel is 25Kw.

Routinely all of that load is present on the sub panel throughout the day apart from two dishwashers (10 Kw) which are in service for about 25% of the time.

What do you think about that MCB? Underrated given the load or adequate?

I have my own opinion which is it should be considerably higher. Also, possibly, the cable that connects the two but I don't know about that because I haven't looked at it.

But I would like to hear your opinions if that's ok.

Cheers.
 
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SparkyChick

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Is the MCB tripping? Any signs of thermal damage? Have you clamped the cable to get an idea of actual current flow?

Sounds like a kitchen, in which case diversity may apply to a lot of the 25kW. The two dishwashers won't be pulling 5kW each all the time they are on, much of that is likely to be the wash water heating which will cycle reducing the overall load, depending on size, it could well be just a few hundred watts for control and pump power.

A full list of appliances and their loads may help provide a better answer, but if it's not tripping, there's no evidence of thermal damage to the cable or breaker, I'd be inclined to leave well alone pending a better assessment of the actual current that is likely to be flowing.
 
D

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From what the OP says, 75% of the time, the board is running at or close to the max.
For the 25% of the time, the board is running at or close to 1.6 x max.
Is this 3 phase, single phase or a mix of both?
 
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From what the OP says, 75% of the time, the board is running at or close to the max.
For the 25% of the time, the board is running at or close to 1.6 x max.
Is this 3 phase, single phase or a mix of both?
Yes. That's my concern. I feel it should be a 125 amp to limit it to about 80% of capacity.
 

darkwood

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I wouldn't be feeding a sub-main via an mcb anyway, it is poor design from the off unless the distribution of the sub-main are rated low enough to give the discrimination that should be factored into design.

If this is indeed wrong due to demand then as long as the cable is rated higher than the mcb after all factors applied then at least it is not an immediate fire risk, this of course do not mean it is correct.

From the info you provide it is likely the mcb is experiencing small overloads for extended periods of time which is definately a worry, if it was getting large overloads then it would be nuisance tripping but that has not been disclosed here so assume it isn't.
 
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I wouldn't be feeding a sub-main via an mcb anyway, it is poor design from the off unless the distribution of the sub-main are rated low enough to give the discrimination that should be factored into design.

If this is indeed wrong due to demand then as long as the cable is rated higher than the mcb after all factors applied then at least it is not an immediate fire risk, this of course do not mean it is correct.

From the info you provide it is likely the mcb is experiencing small overloads for extended periods of time which is definately a worry, if it was getting large overloads then it would be nuisance tripping but that has not been disclosed here so assume it isn't.
I concur. The puzzlement here is that this was all installed from scratch in a new building by a qualified electrician, part of a project team, who had access to layout drawings and a complete inventory of all equipment that would require electricity so the is no question that the full potential load was calculable. So why would he choose a 63 amp MCB under thos circumstances irrespective of whether a sub panel is teh best approach. And if he was happy to underrate that then it's not inconceivable he might apply the same policy to the cabling. That's wher it becomes problematic because it's one thing to upgrade the MCB but it's ane entirel different prospect when it comes to the cable. It's all buried too so it's teh worse case scenario from an overheating perspective.

There is a lot of other stuff as well with this installation. Most of the MCBs are the wrong way round in CU. Off is on and on is off. I know it doesn't make any difference functionally but you intuitively expect Green to indicate ON and Red OFF not the other way round. Confusing when you are looking for a tripped switch. To me that's just laziness.
 
D

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It is a poor design, no two ways about it.
Before making any decision about increasing the rating of the MCB, you should first establish the rating of the cable supplying the board.
I would also want to know what temperatures the MCB is running at, so as to consider what effect it is having on adjacent MCBs.
 
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It is a poor design, no two ways about it.
Before making any decision about increasing the rating of the MCB, you should first establish the rating of the cable supplying the board.
I would also want to know what temperatures the MCB is running at, so as to consider what effect it is having on adjacent MCBs.
Yes. As I have just replied to darkwood. The feedback so far from you guys suggests the way forward is to recommend a full inspection of the cable that connects the CU and sub panel with a proposal to increase the capacity from 63 amps to 125 amps (my estimation) whatever that involves. Thanks.
 

darkwood

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You say as part of a project team and also use the term Qualified Electrician too...

Was this a skills course teaching practical skills that should have been overseen by a competent person?

The term Qualified Electrician means nothing on its own, competent is the term one is looking for nowadays.

I personally wouldn't worry if the cable does need replacing either, you are not the designer or installer of this mess you are merely passing on your concerns and possible remedies to resolve them, if it does indeed need digging up then that will fall on the original designer and/or installer.
 
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It is a poor design, no two ways about it.
Before making any decision about increasing the rating of the MCB, you should first establish the rating of the cable supplying the board.
I would also want to know what temperatures the MCB is running at, so as to consider what effect it is having on adjacent MCBs.
I am actually looking to see if I can get hold of an IR camera so I can see it there are any hot spots on the panels or the walls where the trunking runs.
 

darkwood

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I am actually looking to see if I can get hold of an IR camera so I can see it there are any hot spots on the panels or the walls where the trunking runs.
Use the palm of your hand, its cheaper and instant results, the issue is any heat build up may not be constant but intermittent and it may come down to the fact you check it at the wrong time to get any real feedback..

Just look at the cable terminations, gauge its size and calculate its suitability for the job with all cable factors included, you need not go into testing or the like to establish this is a poor/dangerous install.
 

Andy78

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Yep, an awful design in all respects.
As to the why a 63A MCB was chosen I think this might be quite simple. It is clear that the person installing did not quite know what they were doing. They have determined for whatever misguided reason that a standard TPN board was sufficient to feed this distribution circuit. A standard TPN board usually has a maximum of 63A devices. I know some ranges go above this but I'm talking in general, I don't really know the Dorman Smith ranges.

If your capacity is only advisory I would advise a data logging exercise to ascertain the real usage of the circuit.
 
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