UL bus rating vs overcurrent device

[mcgyvr]

This just doesn't seem right to me and wanted to see what others think. We use a product (DC fuse panel) with a bus rating of 100A. Per UL we need to protect it with a max. 125A overcurrent device. However they only thermal test that product to the bus rating of 100A. With a 125A overcurrent device that product could hum along for ever at 125A without the overcurrent device ever tripping.

I would think that you should protect it with an overcurrent device no greater than your bus rating. So bus rating = 100A, overcurrent device = 100A. Is this yet another hole in the standards that hasn't been plugged?

 

[DRWeig]

mcgyvr, is this under UL 508 or UL 508A, or another standard?

DW

 

[mcgyvr]

For example (see general specifications..Electrical)

http://www.telect.com/

http://www/Products/LearnMore.aspx?...ent+GMT+Fuse+Panels_pid1137&VariantId=HPGMT10

I have a feeling this is just incorrect and that it should be a 100A bus rating, a 100A interupt device with the wiring sized to 125A.

 

[mcgyvr]

UL 60950

 

[DRWeig]

Rats, that standard isn't in my library, so I can't see it in context.

Could it be that there's an allowance for loads that have inrush current (capacitive things I guess)? Same way we protect motor circuit conductors with overcurrent protection set higher than listed ampacity.

I can see how that might be needed. Each circuit coming off your bus has its own fuse protection, so if properly designed it might be OK.

Personally, I'd put a 100% protector on it (or less, depending on my loads)... The 125% is a maximum after all.

Anyway, I'm just thinking while typing --

Anybody else out there?

Good on ya,

Goober Dave

 

[mcgyvr]

Yes each circuit has its own fuse. This usually feeds only resistive loads. The product is basically a large input connection which feeds multiple fuses that provide protection to multiple pieces of equipment. Just like any other fuse/breaker panel in your house,etc... It just bothers me that the products are only tested by UL to 100A but then allowed to be fused at 125% so the panel could be running over its tested rating for an indefinite time which can easily create a fire hazard,etc... Then it is also recommended to use a 2 AWG wire to feed the panel which is only rated to 115A per NEC 310.16 75 deg C column.

It just doesn't seem right.

 

[mcgyvr]

I guess this comes from requirements to size overcurrent device at 125% of the continuous rating. So the continuous rating of that panel is 100A. But I would still think UL should test the product to 125A since it "could' run at 125A for an extended period with a 100% rated overcurrent device. 125A is worst case scenario and not 100A.. Just another hole in the testing/safety world IMO

 

[SteveSmith]

mcgyvr,

You have to remember that UL comes in thinking about fire safety 1st and above all other concerns. Electrical safety, while important is a second class citizen. I've worked with UL on products under UL 1778, 60950, & 508A and it's always the same. I belive if you check the NEC standards (again a fire safety standard), you'll see the 125% is pretty common.

Steve Smith

 

[DRWeig]

I would still agree with McGyver that UL's bus rating process should include extended-time overcurrent at 125% of what they're ultimately going to rate it.

You should see the tests they do for small transformers, it's a hoot.

Goober Dave

 

[SteveSmith]

I also agree that you would expect them to test the product at the 125%, but I have never seen it happen. As long as the fuse/breaker trips during a dead short they seem to be happy.

No thanks on the transformer test. I've had my fill with UPS, Power Factor Correction equipment, & UL 924 Emergeny Power Systems.

Steve Smith

 

[mcgyvr]

oh well..can't change the world, but I can make my little piece of it safer.. Panel will have overcurrent device rated at 100A.
Thanks guys...

 

[dpc]

Just keep in mind that a 100 A molded case circuit breaker is only rated to carry 80 A on a continuous basis, "continuous" being defined as three hours or more. Beyond that, it may trip.

So your 100 A bus is now only good for 80 A on a continuous basis.



David Castor

http://www.cvoes.com

 

[mcgyvr]

The ones I'm using are rated to carry 100% continuous. But yes I am aware of that on other models. Thanks.