How to Specify a Panelboard

[charlie b]

A colleague (a consulting engineer on a project that I am reviewing) and I have differing opinions regarding what to show on a panel schedule. We agree that the intent is to give the manufacturer sufficient information to be able to construct what is needed. We disagree on the method of conveying short circuit withstand ratings. I have been on a long (and mostly fruitless) campaign to get rid of all references to the term “AIC” on panel schedules. My reasoning (and my colleague is in agreement with this) is that panels do not interrupt current, so with regard to the panel itself the term is meaningless. I prefer to show “Short Circuit Current Ratings,” or SCCR, on the schedule, believing that is all the manufacturer needs in order to select the appropriate item from their product line.

My colleague is willing to add “SCCR” to the panel schedule. But he contends that an AIC value should be added as well, with a note that clarifies that the AIC pertains to the breakers installed in the panel. We both agree that a fault immediately downstream of a breaker would see a fault current equal to the “Short Circuit Current Available,” or SCCA, at the panel’s location. We agree that any breaker installed in the panel must have an AIC rating equal to or higher than the SCCA.

So the essential question I wish to present is this: how much is enough? Is showing an AIC rating sufficient to tell the manufacturer that all breakers must have at least that rating, AND THAT the panel itself must have at least that high an SCCR rating? Would showing just an SCCR value on the panel schedule be enough to convey the same message? Shall I drop my losing campaign and work instead on something more important, like convincing people to stop using “emergency generator” as a generic descriptor or using “amps per phase” just before adding the three phase amps to get total amps?

 

[Strathead]

Since this is on of my weaker subjects, a question. Wouldn't the SCCR of a panel be equal to or higher than the highest AIC rated breakers allowed in the panel? If so, then who cares? (and that comes from someone who is anal, me.) In fact, The panel schedule is not just for the bus and the enclosure, it is an assembly including the breakers, and I am not aware of anyone who doesn't know that the minimum AIC rating "of a panel" refers to the breakers that are included in that panel, so the semantic are less, not more confusing.

 

[electrofelon]

I agree with strathead that the SCCR "follows" from the AIC required. When I spec a panelboard, I will state the required AIC for the breakers. I would think the manufacturer takes care of the SCCR (and I would conjecture that the SCCR will by default be high enough to cover pretty much any breaker that can go in it).

 

[jim dungar]

It would be hard to find a panel board from a major manufacturer that would not have a UL Listed SCCR rating based on the lowest AIC rating of the installed protective devices.

Switchboards and Motor Control Centers, and custom panels are a different beast, the bus bar bracing can be rated different than the protective device AIC.

Personally I would prefer to see the required SCCR of an assembly rather than the AIC of its components.

 

[dkidd]

From http://trcsupport.eaton.com/General...e-difference-between-AIC-and-SCCR-ratings.htm

What is the difference between AIC and SCCR ratings?

AIC (kAIC) applies to protective interrupting devices such as circuit breakers and fuses. The unit of measure for AIC is Amps RMS Symmetrical. This figure, which is typically in range of 5000 to 200,000 amps, describes the maximum fault current that the protective device can clear safely, without welding closed or causing damage to equipment or personnel.

SCCR applies to complete pieces of equipment such as motor control enclosures containing breakers/fuses, contactors, or solid state devices used for motor control or components combined into a control scheme. The unit of measure is also Amps RMS Symmetrical, and describes the maximum fault current that the (combination) equipment can withstand safely—or the maximum available fault current of the feeder to which the equipment can be safely connected.

 

[charlie b]

What is the difference between AIC and SCCR ratings?

Thanks for the effort. But my colleague and I already understood those concepts.

The question has to do with whether a panel schedule should show AIC ratings (i.e., that apply to the breakers), or SCCR ratings (i.e., that apply to the panel itself), or both. Put another way, if you were the EC, and you showed a panel schedule to your equipment supplier, and it had just AIC or just SCCR or both, would it make a difference to what your supplier understood to be your project requirements?

 

[Jraef]

In my opinion using the term SCCR allows for more flexibility from the suppliers in choosing series rated equipment. AIC is more finite; it either is or is not rated for that interrupting capacity. Now whether or not you WANT suppliers to be able to use series rated equipment is another matter. If not, then using AIC ratings might be interpretable as not allowing series ratings. I think the semantics will matter though and the best thing would be to just explicitly state that one way or the other.

 

[augie47]

Locally the terms are interchanged so much few folks make the distinction.
We seem to concentrate on the AIC rating of the devices and most often assume the panel SCCR is adequate for that number.

 

[electrofelon]

I wonder if it makes a difference when you are utilizing series ratings? For example say I have a branch panelboard containg 10k rated breakers that series rate with the feeder breaker, but the actual available fault current is 22k. I would think the SCCR of the panelboard needs to be 22k.

 

[jim dungar]

I wonder if it makes a difference when you are utilizing series ratings? For example say I have a branch panelboard containg 10k rated breakers that series rate with the feeder breaker, but the actual available fault current is 22k. I would think the SCCR of the panelboard needs to be 22k.

But we have a different problem with Motor Control Centers, the combination series rating for the motor starter and disconnect is often 65kA as standard, even while the AIC of the motor circuit breaker may only be 18kA.
However, the bus bar bracing is often only 42kA as standard.

Specifying an SCCR rating takes care of the need to understand all of these nuances.

 

[Jraef]

But we have a different problem with Motor Control Centers, the combination series rating for the motor starter and disconnect is often 65kA as standard, even while the AIC of the motor circuit breaker may only be 18kA.
However, the bus bar bracing is often only 42kA as standard.

Specifying an SCCR rating takes care of the need to understand all of these nuances.

That's what I feel too. If we go back a bit, we used to talk about "Withstand Ratings" and AIC separately. The reason for changing Withstand Ratings to SCCR was, as I understood it, because there was that potential disconnect between WR of the gear and AIC of the devices in it. SCCR has to include both together, but does so without regard to how you get there.

You know how sometimes you run into someone who spouts off about how a panelboard cannot have an AIC, because only the breakers in it actually interrupt? I'm that guy, and I've had a lot of people give me eye rolls for sure. But the thing is, it's true. Commonplace or not, AIC cannot by definition apply to the GEAR, only the DEVICES inside of it.

 

[d0nut]

I typically specify the available fault and require that the equipment ratings (SCCR or AIC as applicable) need to be 120% of that value to give a bit of a safety factor for the actual equipment purchased and how the circuits get run in the field. My specs will state whether the equipment needs to be fully rated, or if series rated is acceptable. This approach began when I started seeing manufacturer's ratings that were not the "standard" 10, 14, 22, 42, 65 kA values. If my available fault is 45 kA, why should I require 65 kA equipment if a manufacturer makes something rated at 55 kA, for example. By providing the fault value and my safety factor, I am not overly restricting the manufacturer from providing the most economical equipment that would still meet my criteria.