Another 705.12 Question! Busbar Connection at Reduced Section Bus

[cyriousn]

Looking to make a bus connection at the end of the 3000A switchboard to get 120% out of the bus. However, the distribution section bus is only rated for 2000A. I'm assuming my starting point in my calculation would be 2000A which would make it impossible to add any load connection via the bus since we have a 3000A main circuit breaker?

There is a 1200A breaker in between the Main Circuit Breaker and the Dist Section which is how there is only a 2000A bus on the dist. section with the 3000A Main but if that 1200A load was removed wouldn't there be a potential issue with too much available current from the utility?

With similar logic couldn't we use whatever is left for amperage once the load of the 1200A breaker would be taken into account?

So if the load off that 1200A breaker was 1000A or less we would have 2000A * 1.20 worth of capacity at the dist. section or 2400A that we could put on that bus?

If the load was somewhere between 1000A and 1200A then we would have somewhere between 1800A-2000A * 1.2 of bus capacity left to hook into.

I know that's not the way it's written but just trying to think through the intent here.

 

[wwhitney]

You need to treat the conductors between the 3000A main breaker and the 1200A breaker (call those conductors B) separately from the 2000A busbar. So what are conductors B and what is their rating/ampacity?

Your description is a 3000A main breaker, conductors B, 1200A breaker, 2000A busbar, presumably some distribution breakers (no interconnections), and then an interconnection at the end of the bus. With the 120% rule, you could interconnect up to 960A of continuous current power production at the end of a 2000A busbar protected at 1200A. Or you could connect up to 640A of continuous current power production anywhere on the 2000A busbar.

As for conductors B, the details are missing. If it's a 3000A busbar, then you'd only be able to interconnect 480A of continuous current power production under the 120% rule.

Cheers, Wayne

 

[cyriousn]

Re-reading my previous post I realize I worded that very poorly. The first section is 3000A MCB, the second section the 3000A horizontal bus continues and has a single 1200A breaker on it, the 3rd section has the 3000A horizontal bus continue with a 2000A vertical bus that the dist. breakers are tied into.

 

[wwhitney]

Not so familiar with switchgear--so there's nothing protecting the 2000A vertical bus at 2000A? If you installed over 2000A of distribution breakers on it you could cause more than 2000A of current on it?

In that case I'm not sure how to apply 705.12, I would have thought the 2000A vertical bus would require a 2000A breaker protecting it. But 3000A > 120% * 2000A, so the only rule I could see using is the 100% rule. If the distribution breakers add up to, say, 1500A, you could use a 500A breaker for a 400A continuous current interconnection.

Or can you interconnect in the second section somehow?

Cheers, Wayne

 

[cyriousn]

This is an old Square D switchboard from what looks like the 1980's. Correct, there is nothing protecting that bus...a spicy meatball!

I've sent over some pics to my local rep to see if shop drawings can be dug up from the grave and see what we can do in that 2nd section. I really like the exposed bus....

 

[jim dungar]

This is an old Square D switchboard from what looks like the 1980's. Correct, there is nothing protecting that bus...a spicy meatball!

I've sent over some pics to my local rep to see if shop drawings can be dug up from the grave and see what we can do in that 2nd section. I really like the exposed bus....

You have a factory built switchboard, so it is possible they employed a reduced capacity cross bus. Having a vertical bus of only 2000A while the cross bus is 3000A is acceptable because it is a UL tested configuration, and is very common with the I-Line construction.

You will need to look at the bussing, they did not always maintain the assembly drawings for the internal bussing, especially on 3 decade old equipment. In those days the assembler used to have some leeway in how they accomplished the internal construction as opposed to needing to follow rigid assembly details. I remember having to look inside of two year old gear before adding additional sections, because as built drawings of the interior were not available.

 

[wwhitney]

Having a vertical bus of only 2000A while the cross bus is 3000A is acceptable because it is a UL tested configuration, and is very common with the I-Line construction.

While different rules apply to factory built equipment, for field assembled equipment wouldn't it also be allowed under the 10 ft tap rule 240.21(B)(1), if the sum of the breakers on the 2000A bus is under 2000A? I'm a little unclear on the use of the plural "overcurrent device(s)" in that text..

Cheers, Wayne

 

[Dennis Alwon]

While different rules apply to factory built equipment, for field assembled equipment wouldn't it also be allowed under the 10 ft tap rule 240.21(B)(1), if the sum of the breakers on the 2000A bus is under 2000A? I'm a little unclear on the use of the plural "overcurrent device(s)" in that text..

Cheers, Wayne

I guess that would depend if it ended in an overcurrent protective device. Sounds like there is no main breaker in the distribution panel.

 

[wwhitney]

I guess that would depend if it ended in an overcurrent protective device. Sounds like there is no main breaker in the distribution panel.

Right, but does 240.21(B)(1) require that that the tap end in a single overcurrent device, the way 240.21(B)(2) does? The former uses the plural form "overcurrent device(s)" and refers to the rating of the equipment, and it does not have the clear language of 240.21(B)(2)(2).

Cheers, Wayne

 

[jim dungar]

While different rules apply to factory built equipment, for field assembled equipment wouldn't it also be allowed under the 10 ft tap rule 240.21(B)(1), if the sum of the breakers on the 2000A bus is under 2000A? I'm a little unclear on the use of the plural "overcurrent device(s)" in that text..

Cheers, Wayne

No.
The 2000A vertical bus would be a tap from the 3000A cross bus made under the UL procedures for that gear.

The ability to feed main lug panels was removed from the general tap rules more than 20 years ago.

 

[wwhitney]

No.
You cannot tap a tap. The 2000A vertical bus would already be a tap from the 3000A cross bus.

I don't see how it involves tapping a tap. The 2000A vertical bus would indeed be a tap from the 3000A cross bus, which is properly protected by the 3000A main breaker (if I understand the connectivity correctly). And the 2000A vertical bus would be protected by having the sum of the breakers supplied not exceed 2000A, a possibility that it appears to me the language of 240.21(B)(1) allows, in contrast to 240.21(B)(2).

Cheers, Wayne

 

[jim dungar]

I don't see how it involves tapping a tap. The 2000A vertical bus would indeed be a tap from the 3000A cross bus, which is properly protected by the 3000A main breaker (if I understand the connectivity correctly). And the 2000A vertical bus would be protected by having the sum of the breakers supplied not exceed 2000A, a possibility that it appears to me the language of 240.21(B)(1) allows, in contrast to 240.21(B)(2).

Cheers, Wayne

Oops I was editing as you were reading.

 

[wwhitney]

The ability to feed main lug panels was removed from the general tap rules more than 20 years ago.

So what does this phrase from (2017) 240.21(B)(1)(1) mean: "the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors"?

How does a tap conductor supply more than one overcurrent device other than by the equivalent of an MLO panel?

Cheers, Wayne

 

[jim dungar]

So what does this phrase from (2017) 240.21(B)(1)(1) mean: "the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors"?

How does a tap conductor supply more than one overcurrent device other than by the equivalent of an MLO panel?

Cheers, Wayne

I view that as referring to a piece of listed equipment which may contain multiple overcurrent devices, not to what are effectively panel busbars. I think it applies to something like an automatic transfer switch integral to the switchboard or maybe a power quality/voltage metering compartment.

 

[wwhitney]

I view that as referring to a piece of listed equipment which may contain multiple overcurrent devices, not to what are effectively panel busbars. I think it applies to something like an automatic transfer switch integral to the switchboard or maybe a power quality/voltage metering compartment.

I'm not particularly familiar with the 240.21(B)(1) so on the one hand I'm inclined to accept your understanding based on much greater experience. On the other hand the text I quoted seems to perfectly describe an MLO panelboard. The rating of the MLO panelboard's bus would be the "rating of the equipment" and it contains "overcurrent devices supplied by the tap conductors."

So I'm at a bit of a loss to know what's correct.

Cheers, Wayne

P.S. You mentioned an over 20 year old change, so I tried looking up the 1978 NEC, and I got lucky as it showed it was the first year with new language, so I also looked at 1971 (the previous version available at nfpa.org). What is currently (2017) 240.21(B)(1)(a) is basically unchanged since 1971. And then we have:

1971 240-15 Exception 5 (b)(2) not less than the ampere rating of the switchboard or panelboard supplied by the tap conductors.

1978 240-21 Exception 2 (b)(2a) not less than the rating of the device supplied by the tap conductors, or (2b) not less than the rating of the overcurrent protective device at the termination of the tap conductors

2017 240.21(B)(1)(1)(b) Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors

I'm not sure what to make of the changing the language. The 1971 language clearly does not prohibit an MLO panelboard, and I was hoping the change in 1978 would clearly prohibit MLO panelboards, but I don't find it much different than the current language.