Rant/question: How to determine bus bar ampacity when there is no label?

[photonboy]

This is one that has bugged me for years, I review projects all day where electrical panels no longer have their label and the installers put one in the drawings anyway and it almost always happens to be just what would be required to backfeed without a supply side tap or MPU.

When I was installing I was never able to find any permanent marking on panels and I could never figure out why they don't stamp the ampacity right on the bus during manufacturing so it will remain visible for the life of the panel.

Seems like an important safety issue, not having the right info could lead to overloading a buss and causing a fire.

Anyone out there know a secret I don't?

Thanks

 

[Carultch]

This is one that has bugged me for years, I review projects all day where electrical panels no longer have their label and the installers put one in the drawings anyway and it almost always happens to be just what would be required to backfeed without a supply side tap or MPU.

When I was installing I was never able to find any permanent marking on panels and I could never figure out why they don't stamp the ampacity right on the bus during manufacturing so it will remain visible for the life of the panel.

Seems like an important safety issue, not having the right info could lead to overloading a buss and causing a fire.

Anyone out there know a secret I don't?

Thanks

It's probably because for most applications in general, the main breaker matches the busbar. Distributed generation is the exception to the rule, where we may intentionally design to require an inconsistent busbar and main breaker, to meet the famous 120% rule. So an adhesive sticker would be considered "good enough", as frustrating as it may be for investigating old equipment.

 

[ggunn]

It's probably because for most applications in general, the main breaker matches the busbar. Distributed generation is the exception to the rule, where we may intentionally design to require an inconsistent busbar and main breaker, to meet the famous 120% rule. So an adhesive sticker would be considered "good enough", as frustrating as it may be for investigating old equipment.

And in the absence of the sticker we must assume the busbar ratings are equal to the MCB rating unless we have documentation to the contrary from the manufacturer that we can show the inspector. For example, we (the company I last worked for) had documentation from Eaton that says that their CH panels from 150A to 225A all have 225A busbars.

 

[jim dungar]

Panelboard ratings are predominately determined by heat rise not by bus bar size.
The same bussing could be used in a 125A fully sealed small enclosure or a 250A well ventilated large enclosure.

 

[Zee]

Maybe some photo AI might work like Google Lens?
That, and old electricians.

 

[wwhitney]

Panelboard ratings are predominately determined by heat rise not by bus bar size.
The same bussing could be used in a 125A fully sealed small enclosure or a 250A well ventilated large enclosure.

So ventilation can make a difference of a factor of 4 in heat rejection rate?

Cheers, Wayne

 

[jaggedben]

It's probably because for most applications in general, the main breaker matches the busbar. ...

No, in most applications the main breaker just doesn't *exceed* the busbar. I bet the majority of 125A busbars are protected by a smaller breaker, but one usually doesn't have labeling establishing that. The same may be true of 225A busbars as ggunn alluded to.

The thing is, outside of PV installations there is very little reason to care.

 

[Besoeker3]

This is one that has bugged me for years, I review projects all day where electrical panels no longer have their label and the installers put one in the drawings anyway and it almost always happens to be just what would be required to backfeed without a supply side tap or MPU.

When I was installing I was never able to find any permanent marking on panels and I could never figure out why they don't stamp the ampacity right on the bus during manufacturing so it will remain visible for the life of the panel.

Seems like an important safety issue, not having the right info could lead to overloading a buss and causing a fire.

Anyone out there know a secret I don't?

Thanks

We used Traffolyte labels and screwed them in.

https://imgur.com/lLZNUYD

That is my gate but for professional projects we included the fixing screws.

 

[Besoeker3]

Oops.......

 

[jim dungar]

So ventilation can make a difference of a factor of 4 in heat rejection rate?

Cheers, Wayne

It might not have to. What assumptions are you making?
Say the bus bar in open air has a rating of 275A.
In a well ventilated enclosure it is almost a full rating and is below the heat rise test, but in a poorly ventilated enclosure it reaches a much higher temperature, so the manufacturer assigns it a lower rating to provide a large safety margin while allowing for degredation in heat transfer due to poor housekeeping. Then there could also be heating issues based on the physical size of any main breaker which might be involved.

 

[wwhitney]

It might not have to. What assumptions are you making?

My assumptions are that the sources of heating are all current based, and the heating rate from each source behaves as I²*R for some R, and that heat rejection is proportional to the temperature difference.

So if the same components are good for 125A in a tightly sealed enclosure and 250A in a well ventilated enclosure, I infer the well ventilated enclosure can reject heat at approximately 4 times the rate for a given temperature difference. [Approximately because of rounding to standard ratings, etc.]

Cheers, Wayne

 

[jim dungar]

infer the well ventilated enclosure can reject heat at approximately 4 times the rate for a given temperature difference.

But there is no reason to think the bus temperature rise needs to be the same. Maybe the low rating runs hot while the large one runs cool.

 

[wwhitney]

But there is no reason to think the bus temperature rise needs to be the same. Maybe the low rating runs hot while the large one runs cool.

That would make the ventilated enclosure (the larger rating) have a heat rejection rate more than 4x that of the sealed enclosure, is that your point?

But I think I see what you mean, each product is likely not labeled with the highest ampere rating that would pass the test, just some standard rating that is desired for market purposes or whatever. So there's no reason to expect the final heat rise to be the same across different products.

Still, my last comment recognized that, and it seems like absent all other information, 4x the heat rejection rate is about what is required to get 2x the current rating.

Cheers, Wayne