Series Rated???

[klubie]

I am feeding a new panel in a building from an adjacent building through an underground sleeve we installed between the two buildings. The panelboard manufacturer is telling me I want the feeder to be series rated.

What does this mean?

Thanks.

 

[davidbeach]

Breakers can be series rated. Never heard of a series rated feeder. You need to ask the manufacturer for a clarification.

 

[klubie]

My apologies. What is a series rated breaker?

 

[davidbeach]

Series rating is a method that splits the fault interrupting between two (or more) devices to allow a lower rated device to be used in a location where, when considered alone, it would not have a high enough interrupting rating. For instance, you could have 20kA of fault current at your existing panel, and 16kA at the new panel. To use fully rated breakers you would need 22kA breakers in the new panel. On the other hand, there are listed combinations where a 22kA breaker in the existing panel can be combined with a 10kA breaker in the new panel to provide the necessary protection. Series ratings are established by test and, for breakers, only exist for combinations where both breakers are from the same manufacturer. If there are motors connected between the two breakers under consideration, it is unlikely that the series rating will be allowed.

Series rating inherently require both the upstream and downstream devices to trip to interrupt the fault. This is often pointed out in a manner that suggests that using a fully rated combination would not have both devices tripping. That is not a true conclusion; it is often impossible to distinguish a series rated combination from a fully rated combination if only looking at the trip characteristics of the two devices.

 

[DanDel]

davidbeach has touched on the main reason why series-rating is not as desirable as fully-rated gear: in a series-rated system, the upstream device MUST trip to protect the under-rated downstream device. This can be troublesome if the tripped upstream device takes out your whole switchboard.
A fully-rated system allows coordinated settings so that this can be avoided.

 

[klubie]

with all than in mind, should I suspect the manufacturer is only suggesting the series rating to insure I install his equipment in the new building or is this a code or safety issue? basically, we're feeding a small 60A emergency panel from an adjacent building emergency panel to avoid having to purchase a new little generator for the new building....

 

[DanDel]

The other main difference between series-rated and fully-rated: fully-rated costs more.
Some systems/circuits are quite suited to series-rated, since the components are cheaper and maybe the loads aren't that critical. Some aren't.
If your manufacturer is doing this to save YOU money, then you only have to decide how important the loads are. If he's doing it to save HIMSELF money (like he already gave you a price, but didn't indicate that series-rated equipment was to be used) that's another story.

 

[jraef]

It's strange that he is suggesting this, it only serves as a way of reducing costs IF you can get away with it. If you just use a breaker with the same rating (AIC) as the main one, no problem at all. I doubt if there is much cost savings at all on a 60A service panel anyway.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[klubie]

ok, well this load is very important.....the new emergency panel in the new building is for a new server room.

I am afraid that it will be an adder to go with series rated equipment, but you're telling me otherwise, correct, so I should ask him for a credit, correct?

 

[DanDel]

Fully-rated costs more than series-rated.
Fully-rated gives you the possibility of isolating a fault on, say a 20A C/B supplying a lighting circuit, to that 20A C/B instead of knowing that the main device must trip for a major fault on that 20A circuit if the system were series-rated.
Or, this may be an example of where a design engineer should be retained.

 

[klubie]

Bottom line is unless we asked for series rated, we should get fully rated, correct?

I suspect he is trying to save a few bucks and in turn get his equipment installed. I'm not interested in saving a few bucks to go this route, primarily because I really don't care to go with his equipment.

My biggest concern is that there's some inherent risk in not going series rated....but you all are telling me otherwise.

 

[davidbeach]

No risk in fully rated other than spending more money than might be necessary. I find DanDel's comments excessively optimistic about gaining selectivity. Selectivity will usually require a) fuses, b) solid state trip units, c) very low available fault currents, or some combination of the above. Two thermal-magnetic circuit breakers in series will not allow coordination unless the available fault current at the downstream breaker is less than the instantaneous setting of the upstream breaker. So, if you are feeding a 60A panel, through a 60A breaker, you will not have selectivity, using thermal-magnetic trips, if the available fault current at the downstream breaker is more than 600A. Highly unlikely if the manufacturer is even talking about series rated.

I am all in favor of selective coordination where it can be readily achieved, and all selectively coordinated combinations will be fully rated, but the vast majority of all fully rated combinations ARE NOT selectively coordinated.

If you truly need selective coordination at lower load currents, say less than 400A, and have reasonable fault currents, say above 5kA, fuses are about the only way to get there. But then you have fuses to deal with rather than circuit breakers, and fuses are not the answer to all electrical protection issues as a certain fuse manufacturer would have you believe.

If you did not specify either fully rated or series rated, the manufacturer is free to supply which ever as either will meet the interrupting duty requirements.

 

[EEJaime]

I would require the contractor to furnish a fully rated system if the construction documents support this. You are talking about a critical load. You do not want to depend on a piece of equipment in another building to trip to provide the protection for this building. You need to furnish a main c/b for this building anyway. There is not going to be an excessively large amount of available fault current from a system fed from a generator, especially when it is only a 60 Amp feed. This is probably a 208V/3ph feed? Don't forget that this will be considered a seperate service and the main device needs to be as close to the point of entry into the building as possible and should either be co-located with the normal power service or have identification on the panel warning that this panel will be live when the building main is tripped. Inspectors will ding you on that.

 

[waross]

I suggest that if you look up the impedance of the feeders. There is a good possibility that the available fault current at the second panel will be low enough that the problem will go away.
If the impedance check even gets you close, route the feeder so as to use enough cable length so that the cable impedance is enough to limit the available fault current to an acceptable value.
respectfully

 

[dpc]

I agree with David Beach that selective coordination between molded case circuit breakers is not generally possible, whether series rated or fully rated.

But if at some point in the future, someone replaces that upstream breaker with another brand, there goes your series rating. Fully-rated systems probably reduce your liability and risk to some degree.

 

[davidbeach]

dpc, I agree with your second paragraph; that's why the NEC requires labeling at the upstream breaker that it is part of a listed series rated combination.

 

[resqcapt19]

If this is an Atricle 700 Emergency System or a an Article 701 Legally Required Standby System, then the 2005 code would prohibit the use series rated breakers and may prohibt the use of any breaker. Selective coordination is required by 700.27 and 701.18
Don