3 tier series rating

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hhsting

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Glen bunie, md, us
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I understand I had similar post before but at that time I was not sure about 3 tier series rating.

I have high rise that has bunch of meter stacks fed from switchboard with 65kaic rating. Meterstacks then feed individual resident panel.

Attached sketch shows one of the meterstacks fed from switchboard feeder breaker. No meters are shown at meter stacks for clarity. Meterstack has 22kAIC rating and so does the panelboards. There are 26 feeder breakers in the meter stacks which feed 26 dwelling unit panelboards that have HVAC load

Each dwelling unit panelboard has hvac load FLA of 16A. Assume their is 3-tier series rating between switchboard feeder breaker, meterstack feeder breaker and panelboard feeder breaker

I did trace fault current path, did calcs and sum of all motor FLA currents I got are greater then 1% of 22kaic at fault points F1,F2,F3 attached sketch. However I am not sure about it or if I did it correctly.


Assume their is fault at point F1, F2, or F3 attached sketch or at any other location then according to NEC 2017 section 240.86 one cannot do 3-tier series rating or any tier series rating? Would this be correct or incorrect?

cd39b9bf062e7b3921d64216d64a8752.jpg
 
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petersonra

petersonra

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Northern illinois
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engineer
Is there any actual chance of regeneration from the HVAC system? Most of them these days have some kind of drive on them that would prevent any backfeeding from the motor back on the line.
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
hhsting said:
Attached sketch shows one of the meterstacks fed from switchboard feeder breaker. No meters are shown at meter stacks for clarity. Meterstack has 22kAIC rating and so does the panelboards. There are 26 feeder breakers in the meter stacks which feed 26 dwelling unit panelboards that have HVAC load

Each dwelling unit panelboard has hvac load FLA of 16A. Assume their is 3-tier series rating between switchboard feeder breaker, meterstack feeder breaker and panelboard feeder breaker
Click to expand...

Doesn't sound like a three tier system to me.

Also what is the available fault current at the switchboard? You say it "has a 65k rating" but it's not the rating that matters it's what the actual AFC IS.
 
H

Hv&Lv

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I’m probably lost here and not quite sure what you asking but…

What makes you ASSUME three tier rating between these components?
isn’t that a rating for all three devices together?
 
Fred B

Fred B

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Location
Upstate, NY
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Electrician
hhsting said:
Why? Meterstacks usually have 3 tier series rating
Click to expand...
No, the meter stack as a whole must handle whatever full AFC is provided. What down stream would be allowed might be a portion of the full rating provided the distribution of current allows for it.
electrofelon said:
Doesn't sound like a three tier system to me.

Also what is the available fault current at the switchboard? You say it "has a 65k rating" but it's not the rating that matters it's what the actual AFC IS.
Click to expand...
And that number near impossible to get from POCO, they are noncomittle as to actual AFC being supplied from them. Another thread had this discussion as to the POCO reasoning and hesitancy to provide a number.
Also not just the breaker and buss affect kaic at any given point but also length of run of wire. VD as a result of conductor resistance helps in reducing the AFC at equipment and at any given point. VD is usually seen as a bad thing, something to avoid, but in this instance it can be helpful .
Had a IAEI training recently on this topic and it was even suggested that if the AFC was to close to exceeding limitations of equipment that adding length to the feeders can reduce the AFC availability at the equipment and can be calculated into the AFC calculation for series ratings as one more piece of the series. Another reason POCO won't commit to number. Same xfer located 500ft from customer will provide a different AFC than one located 3/4 mile away, and different than 1.5 mile away.
Bottom line, get the AFC from POCO (if they'll commit) then you can do a realistic calculation otherwise it's just an exercise in "what ifs". The AFC that matters, AFA the hvac, is that from the individual panel board to hvac. That breaker has to withstand what might be available at that location, and everything before it in line can help to reduce what is available.
And the 1% requirements is you don't want motor contribution to be able to create a cascade event upstream from the last end use equipment failure, and why the series rating not backwards calculatable from a motor load and the 1%, thus the need to properly size the breaker kaic value. So each point of the series must be able to handle what is available at that point, and what is ahead of it effects what is available at each point down stream closer to end use equipment.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
hhsting said:
Why? Meterstacks usually have 3 tier series rating
Click to expand...
Ok I see what you mean. It's hard to understand because your wording is sloppy sometimes. "The breakers in the meter stack are part of a three tier series rating" is what you mean I think.

I still don't think you have a three-tier system though. Read thru this:


Other Applications of Series Ratings

Series ratings can also be applied under the following
guidelines:
■ Any FULLY RATED breaker can be applied upstream,
downstream, or in the middle of any of the series ratings
stated in the tables
■ Any series rating stated in the tables may have additional
series rated branch breakers of the EXACT SAME TYPE
further downstream in that rating
COMBINING SERIES RATINGS are allowed under certain
conditions. Main and branch ratings may be combined if:
■ Breakers A, B, and C are in series respectively from
main to branch. Breakers A and B series rate together.
Breakers A and C series rate at the same interrupting level
(or higher). It is allowable to use A, B, and C together at
the A-B series rating
It is improper to combine series ratings under the following
condition:
■ Breakers A, B, and C are in series respectively from main
to branch. Breakers A and B series rate together. Breakers
B and C series rate at the Breaker B interrupting rating
level. It is not allowable to use A, B, and C together at
the A-B series rating. However, combining multiple over-
current devices as in this example, can be accomplished
if all devices in the series combination have been tested
together and listed in triple rating Table 13
 
H

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
electrofelon said:
Ok I see what you mean. It's hard to understand because your wording is sloppy sometimes. "The breakers in the meter stack are part of a three tier series rating" is what you mean I think.

I still don't think you have a three-tier system though. Read thru this:


Other Applications of Series Ratings

Series ratings can also be applied under the following
guidelines:
■ Any FULLY RATED breaker can be applied upstream,
downstream, or in the middle of any of the series ratings
stated in the tables
■ Any series rating stated in the tables may have additional
series rated branch breakers of the EXACT SAME TYPE
further downstream in that rating
COMBINING SERIES RATINGS are allowed under certain
conditions. Main and branch ratings may be combined if:
■ Breakers A, B, and C are in series respectively from
main to branch. Breakers A and B series rate together.
Breakers A and C series rate at the same interrupting level
(or higher). It is allowable to use A, B, and C together at
the A-B series rating
It is improper to combine series ratings under the following
condition:
■ Breakers A, B, and C are in series respectively from main
to branch. Breakers A and B series rate together. Breakers
B and C series rate at the Breaker B interrupting rating
level. It is not allowable to use A, B, and C together at
the A-B series rating. However, combining multiple over-
current devices as in this example, can be accomplished
if all devices in the series combination have been tested
together and listed in triple rating Table 13
Click to expand...

Yes i understand 3-tier series would have to be tested by manufacturer. However, IT is 3-tier tested by manufacturer. Question post#1 is not about it is considered 3-tier its about NEC 2017 section 240.87(C) apply or not due to heavy hvac motor loads. See post #1 and its sketch. Meterstack 22kaic rating is Without series rating.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
hhsting said:
Yes i understand 3-tier series would have to be tested by manufacturer. However, IT is 3-tier tested by manufacturer. Question post#1 is not about it is considered 3-tier its about NEC 2017 section 240.87(C) apply or not due to heavy hvac motor loads. See post #1 and its sketch. Meterstack 22kaic rating is Without series rating.
Click to expand...
My point is you may not have a three tier system. I am not really clear what you have, but my thought is you have a situation covered by the first bullet after "combining series ratings"

How about this. Can you clarify, at each OCPD point A,B, and C, what the available fault current is and what the AIC of each one is? We need to establish and clarify where series ratings are used and not used to answer your question.
 
H

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
electrofelon said:
My point is you may not have a three tier system. I am not really clear what you have, but my thought is you have a situation covered by the first bullet after "combining series ratings"

How about this. Can you clarify, at each OCPD point A,B, and C, what the available fault current is and what the AIC of each one is? We need to establish and clarify where series ratings are used and not used to answer your question.
Click to expand...

I see what you are saying.

Available calculated fault currents at the following locations:

1. Swbd: 70kA
2. Meter stack: 40ka
3. Panelboard closest one: 30kA
 
J

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
hhsting said:
Available calculated fault currents at the following locations:

1. Swbd: 70kA
2. Meter stack: 40ka
3. Panelboard closest one: 30kA
Click to expand...
There is no point in this discussion.
Your OP said the switchboard was rated for 65kA, but you have 70kA available.

Select some new equipment and then comeback.
 
H

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
jim dungar said:
There is no point in this discussion.
Your OP said the switchboard was rated for 65kA, but you have 70kA available.

Select some new equipment and then comeback.
Click to expand...

Ok say lets say they selected 100ka switchboard instead of 65ka still 3 tier series rated downstream with meter stack and panelboard

Would it comply with NEC 2017 section 240.83(C) still going back to post #1 question?
 
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