NEC 2014 section 240.86(C)

[hhsting]

Please see attached sketch. I have enclosed circuit breaker 0 which feed meter stack. The meter stack has feeder breakers 18 of them AIC rating of 10kA which feed MLO panelboards 18 of them in apartment each dwelling unit. The MLO panelboard feed feeder breaker 10ka hvac load MCA of 20A.

Meter stack feeder breaker is suppose to be series rated with MLO panelboard feeder breaker. Also the meter stack feeder breaker is series rated with ECB Bkr 0.

However the way I see it is series rating would Not be allowed between meter stack feeder breaker and panelboard feeder breaker due to NEC 2014 section 240.86(C). HVAC MCA is 20A. 20Ax17=340A which is greater than 1% of 10kA.

Following questions:


1. Am I correct or incorrect that NEC 2014 section 240.86( C) would not allow series rating between feeder breaker and MLO panelboard feeder breaker?

2. Would series rating apply to ECB breaker 0 and meter stack feeder breaker or not NEC 2014 section 240.86(C)?

 

[augie47]

Be interesting to see what some of our engineer members have to say but I think questioning the series rating between "O" and the feeder breakers might be valid based on 240.86(C)

 

[mayanees]

Isn't everything 10kA rated such that series ratings don't apply? If "ECB BKR 0" was 18kA and it was series rated with the downstream breakers then I think it would be noncompliant because of the motors, if the fault current exceeded 10kA. What is the available fault current, or what size transformer is it fed from?

 

[hhsting]

Isn't everything 10kA rated such that series ratings don't apply? If "ECB BKR 0" was 18kA and it was series rated with the downstream breakers then I think it would be noncompliant because of the motors, if the fault current exceeded 10kA. What is the available fault current, or what size transformer is it fed from?

Right ECB bkr 0 is 22kA. ECB bkr 0 is fed from service switchboard bkr 65kA which then is fed from poco xfmr. Available fault current at poco xfmr is 50kA.

I would imagine that service switchboard bkr cannot be series rated with ECB bkr 0 either?

 

[mayanees]

I think that's correct. But it all comes down to the fault current calculation, taking into account actual Utility contribution and feeder lengths. There's also some level of diversity on the 17 HVAC units such that they're all not running together that makes it less of a concern than if there were heavy motor loads that exceed the 100-amp limit.

 

[hhsting]

I think that's correct. But it all comes down to the fault current calculation, taking into account actual Utility contribution and feeder lengths. There's also some level of diversity on the 17 HVAC units such that they're all not running together that makes it less of a concern than if there were heavy motor loads that exceed the 100-amp limit.

What you are saying is demand load. I dont see in 240.86(C) anything abut demand load. It just says sum of the motor full load current. Does not say demand load. So I take it that NEC means to take full load currents sum not look at demand factors?

 

[mayanees]

What you are saying is demand load. I dont see in 240.86(C) anything abut demand load. It just says sum of the motor full load current. Does not say demand load. So I take it that NEC means to take full load currents sum not look at demand factors?

No, I'm saying that realistically only so many motors will be running at any given time, so if a fault were to occur, then the contributions will only be from those motors that are running.
I don't think it's a good design practice to follow, but if it's an existing installation and the offense is 1.2% versus 0.99% of connected motor loads, then it's not egregious enough to condemn it. And of course that's just my opinion.

 

[hhsting]

No, I'm saying that realistically only so many motors will be running at any given time, so if a fault were to occur, then the contributions will only be from those motors that are running.
I don't think it's a good design practice to follow, but if it's an existing installation and the offense is 1.2% versus 0.99% of connected motor loads, then it's not egregious enough to condemn it. And of course that's just my opinion.

240.86(C) does Not say to look at demand load or to look at what motors are on or off. It just says to sum up motor FLA not exceed 1%. Thats where I am getting at.

Worse case All motors can be on (18) apartment complex dwelling unit hvac hot or winter time

 

[mayanees]

240.86(C) does Not say to look at demand load or to look at what motors are on or off. It just says to sum up motor FLA not exceed 1%. Thats where I am getting at.

Worse case All motors can be on (18) apartment complex dwelling unit hvac hot or winter time

I have to agree with you given the application.

 

[don_resqcapt19]

I agree that there is a series rating issue, however you would not use the MCA for this calculation as that current exceeds the full load current. You would use the actual full load current for this purpose, however, that is will probably not change anything for this application.

 

[electrofelon]

I agree that there is a series rating issue, however you would not use the MCA for this calculation as that current exceeds the full load current. You would use the actual full load current for this purpose, however, that is will probably not change anything for this application.

I find actual current to be very close to 66% of MCA with remarkable consistency. However, I agree still too high.

 

[hhsting]

If one does short circuit fault current calculation for post #1 attachment then one also has to take into account motor contributions as well in that calculation correct?

 

[electrofelon]

To add another wrench in the works.........The feeder breakers in the meter stacks, are they fully rated or series rated with breaker 0? Sounds like you may need a triple series rating unless there is enough conductor length to make a series rating not necessary. We really would need to see a drawing that has the fault current levels at each point on the system and if a circuit breaker is not fully rated, documentation of the proper double or triple series rating.

 

[hhsting]

To add another wrench in the works.........The feeder breakers in the meter stacks, are they fully rated or series rated with breaker 0? Sounds like you may need a triple series rating unless there is enough conductor length to make a series rating not necessary. We really would need to see a drawing that has the fault current levels at each point on the system and if a circuit breaker is not fully rated, documentation of the proper double or triple series rating.

Feeder breakers in meter stacks were going to be series rated with breaker 0. I let the designer know of 240.86(C) and now he is Not doing series rating. Instead he is going to do the short circuit fault current calculation (SSFCC) at each location and provide fully rated breakers based on the SSFCC.

Only thing is while doing SSFC they would also need to do motor contribution study as well?