Sub panel: Underrated MCB?

[Ray Pooley]

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.

 

[Ray Pooley]

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.

I don't know. It was a new build and managed by external project manaf=ge

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.

Ok. Cheers. BTW what's the normal life expectancy of these MCBs. These have been in service for just over 4 years. Reason I ask is because one of the wall sockets on a circuit off the main CU overheated and actually melted and I would have thought the associated MCB would trip before it got to that point. It didn't. So I am wondering whether to recommend replacing at least the ones I suspect might be malfuntioning. That's another issue with this install. It is very difficult not to overload the socket curcuits off the main CU. There are 16 x 13 amp outlets and only 3 x 32 amp MCBs. Total potential load is 208 amps. Like I said. Bewildering.

 

[Ray Pooley]

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.

Thanks. I will make a note of that.

 

[Ray Pooley]

Can you put a load logger on it?

I guess so. Thanks. I will make a note.

 

[Ray Pooley]

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.

Thanks for your input. will go back and try that.

 

[Ray Pooley]

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.

Thanks. A few of you have suggested that so it's on the to do list.

 

[Ray Pooley]

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.

Not on the sub panel circuitry. But on the main CU circuitry. Melted socket internally but no tripping of MCB which is odd. I suspect the MCB may be faulty so you can't take it for granted that there is no tripping. It's a busy cafe. But even without the dishwashers the MCB is at full capacity. I don't know about the cable. Other have sugested data logging exercise so that would appear to be the next logical step.

 

[Toneyz]

I am a bit surprised that the kitchen was not a 3 phase supply and the load balanced as best as practical.

 

[Darkwood]

MCB's detect current overload and short circuit, a loose connection overheating will not be seen by the mcb (unless it damages the cable or socket enough it shorts out), I really think this install needs a detailed test and inspection as it sounds like you have a list of issues and concerns here.
With a full electrical report you can then approach the original contractor and present your concerns and a good case if the report reveals anything.

 

[Pete999]

MCB's detect current overload and short circuit, a loose connection overheating will not be seen by the mcb (unless it damages the cable or socket enough it shorts out), I really think this install needs a detailed test and inspection as it sounds like you have a list of issues and concerns here.
With a full electrical report you can then approach the original contractor and present your concerns and a good case if the report reveals anything.

Absolutely.

 

[Ray Pooley]

MCB's detect current overload and short circuit, a loose connection overheating will not be seen by the mcb (unless it damages the cable or socket enough it shorts out), I really think this install needs a detailed test and inspection as it sounds like you have a list of issues and concerns here.
With a full electrical report you can then approach the original contractor and present your concerns and a good case if the report reveals anything.

That's reasonable. I believe I have enough ammo to make that recommendation but I think the conclusions will be much the same as mine ie: it needs changing. Sooner rather than later. It's just a matter of defining the scope.

 

[westward10]

I am a bit lost with this. You have been asked to look at a situation in an advisory capacity but you come on an internet forum asking advice which says to me you don't have the necessary knowledge or experience to give advice?

 

[Deleted member 26818]

I am a bit lost with this. You have been asked to look at a situation in an advisory capacity but you come on an internet forum asking advice which says to me you don't have the necessary knowledge or experience to give advice?

He’s just getting second opinions.

 

[davesparks]

It is very difficult not to overload the socket curcuits off the main CU. There are 16 x 13 amp outlets and only 3 x 32 amp MCBs. Total potential load is 208 amps. Like I said. Bewildering.

Are these general use sockets or are they intended for specific appliances?

16x 13A outlets spread across 3x 32A ring circuits seems well provisioned for general use outlets

 

[Ray Pooley]

I am a bit lost with this. You have been asked to look at a situation in an advisory capacity but you come on an internet forum asking advice which says to me you don't have the necessary knowledge or experience to give advice?

I have plenty of knowledge and experience. It's called "seeking a second opinion". The reason I have come here is because I am bewildered at what I am seeing given that it was designed and installed by someone who you would probably be happy to describe as having the "necessary knowledge and experience". Everything I see is way beyond the boundaries of what I would expect from someone with the "necessary knowledge and experience" required to install safe systems. So, I have come here to see if there are others on here who find this as alarming as I do. That would appear to be the case and I am grateful for their indulgence.

 

[westward10]

Fair enough but what are you finding that is so alarming.