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]

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

They are all general sockets. 8 x 2 port 13 amp. But it's more to do with the layout. The list of equipment detailed 4 main items along with a few other peripheral devices. A grill and three hotplates. A total of 50 amps for those items alone. I would have liked to see dedicated outlets for these 4 devices with their own single 64 amp MCB and circuit. At this point in time there is nothing to stop someone plugging 8 x 13 amp devices into one of these segments while it only has a 32 amp MCB. You can't expect the end user to know the dangers of doing so. The layout should be designed to prevent it in my opinion. As far as is reasonably possible at least. If you tskr this to its extreme it is possible to demand 208 amps from the CU through three segments each with a 32 amp MCB.

 

[SparkyChick]

It is not possible to demand 208 amps from the CU... if this is the sub-board, it's protected by a 63A MCB which will allow a small overload for some time, but not a significant one, and if it's all on one phase (the fact the MCB is a single phase unit - unless the model number is wrong), unless it's a 200A supply, I would expect the suppliers fuse to rupture.

So why is this being investigated now? What reason has the business had to get so concerned?

 

[Ray Pooley]

It is not possible to demand 208 amps from the CU... if this is the sub-board, it's protected by a 63A MCB which will allow a small overload for some time, but not a significant one, and if it's all on one phase (the fact the MCB is a single phase unit - unless the model number is wrong), unless it's a 200A supply, I would expect the suppliers fuse to rupture.

So why is this being investigated now? What reason has the business had to get so concerned?

Trippings and internally melted sockets. All equipment recently PAT tested and deemed fit for purpose.

 

[SparkyChick]

Is it the 63A MCB supplying the sub-board? If so, how long has it been going on? I ask how long because if it's significantly less than 4 years one might assume that something has changed. If it's not that MCB, what is supplied by the MCB(s) that trip?

As for the melted sockets, that's not an indicator the design is bad... could be a loose connection in the plug, loose fuseholder, cheap socket outlet, substandard fitted plug (on a piece of cheap equipment maybe). There are plenty of reasons a plug can melt that are nothing to do with the design.

 

[westward10]

We used to do a lot in fast food restaurant kitchens and there was a practice of multiple radial circuits to single point sockets. The layout was often mirrored especially the drive-through outlets and a socket served a particular piece of equipment and it had nothing to do with the current load of the equipment. The chain of thought was losing a multiple outlet radial or ring final through whatever reason meant the loss of several items of equipment and possible closure of the outlet. Lose a single point radial was not likely to prevent trading.

 

[Ray Pooley]

It is not possible to demand 208 amps from the CU... if this is the sub-board, it's protected by a 63A MCB which will allow a small overload for some time, but not a significant one, and if it's all on one phase (the fact the MCB is a single phase unit - unless the model number is wrong), unless it's a 200A supply, I would expect the suppliers fuse to rupture.

So why is this being investigated now? What reason has the business had to get so concerned?

I'm not sure what you mean by "not possible to demand 208 amps". If I walk in there now and plug a 3Kw kettle into each of the 16 outlets and switch them on what do you suggest would happen?

 

[Toneyz]

We used to do a lot in fast food restaurant kitchens and there was a practice of multiple radial circuits to single point sockets. The layout was often mirrored especially the drive-through outlets and a socket served a particular piece of equipment and it had nothing to do with the current load of the equipment. The chain of thought was losing a multiple outlet radial or ring final through whatever reason meant the loss of several items of equipment and possible closure of the outlet. Lose a single point radial was not likely to prevent trading.

I would have thought that this would have been a more suitable design.

 

[Toneyz]

I'm not sure what you mean by "not possible to demand 208 amps". If I walk in there now and plug a 3Kw kettle into each of the 16 outlets and switch them on what do you suggest would happen?

But that would not happen in the real world. Have you got a list of equipment that they are using?

 

[SparkyChick]

I'm not sure what you mean by "not possible to demand 208 amps". If I walk in there now and plug a 3Kw kettle into each of the 16 outlets and switch them on what do you suggest would happen?

I would expect the OCPDs protecting the circuit or if this on the sub-board (protected by a 63A MCB) to trip at the very least, and unless the suppliers OCPD is over 100A, I'd expect that to rupture.

 

[Ray Pooley]

But that would not happen in the real world. Have you got a list of equipment that they are using?

I doubt you would get away with that excuse if you found yourself on the end of a corporate manslaughter charge through negligence. The way I see it is this. You can't do a great deal with existing layouts other than enhance and improve. But when you start off with a blank sheet it should never be possible at sign off for the end user to demand more from the system than it is capable of supplying. That's the whole purpose of the legislation. You start off with a list of the demands that need to be catered for safely at the outset. Then you can discuss issues of scalability with the client. For example you might only need 3 hotplates today. Tomorrow you might need 6 if business goes well. So you design for expansion if that arises. Whatever the total demand is then that's what the supply should be capable of delivering. Ideally at 80% capacity. Certainly not 50% less.

 

[westward10]

So what are these alarming issues.

 

[Ray Pooley]

So what are these alarming issues.

The numbers.

 

[westward10]

The numbers.

Sorry don't get that.

 

[SparkyChick]

Unfortunately, in reality, it's almost always possible for an end user to demand more than their supply is capable of handling, which is why OCPDs are installed. Take a look around your home and ask yourself what would happen if I plugged a 3kW kettle into every socket in my kitchen.

The OCPDs in the board are there to protect the installation wiring and little else and the suppliers OCPD is there to ensure the install as a whole doesn't exceed it's capabilities.

But it's still not clear what's tripping, and when the tripping started. You're just basically pointing the finger at the spark who you've been told designed and installed this 4 years ago, so I'll say this... if these tripping problems were not present when the installation was signed off 4 years ago (if they were, they should have been reported to the original contractor), and only started in say the last 18 months or so I'd suggest the client has changed something and introduced equipment that pushes the demand beyond the capabilities of the installation (as designed 4 years ago based on the information available to the spark at the time).

I actively try and encourage my clients to include some additional capacity 'for the future' but there comes a point where it's not practicable or financially viable. If it was a new business, they possibly had budget constraints so the spark may have been financially impeded. There's so many factors that could have played a part in his decision making process and you can pretty much guarantee you won't get the negative points from the client since they obviously want you on their side.

 

[westward10]

Unless I have missed it SC there is no mention of tripping circuits so I am still confused what he has been asked to advise on.