NEC 2020 Article 110.16

[wbdvt]

What is everyone doing to meet Article 110.16 in reference to labeling the service entrance equipment with the available fault current when the utility will only provide the infinite bus short circuit current?

 

[ron]

Use that value to calculate the available current at the service entrance equipment considering the conductor impedance add other source contribution such as energy storage, generation and PV and downstream motor contribution as needed and label with that.
FWIW, this is not new in 2020

 

[wbdvt]

That does not give me the available short circuit current only the results of the infinite bus fault current at the transformer terminals reduced by the impedance of the secondary cables to the service entrance equipment. Not the same thing.

For example: A 1500 kVA, 480Y/277V sec, 5% Z would have an infinite bus short circuit of ~36 kA at the terminals. Depending on cable impedance, let's say it is reduced to 33.5 kA. However, if the utility available fault current is low, this value could be as low as ~21 kA. That could make a big difference on the clearing time of the service overcurrent protective devices.

The other factor is for older existing systems that have be expanded over the years, it could make the difference in equipment being overdutied or not and the level of incident energy to determine the proper arc rated PPE.

 

[kwired]

From personnel safety perspective wouldn't you rather be in error on the high side, I think many times such calculations are done assuming infinite supply capabilities and start reducing with the impedance of the transformer. It is the only thing that is more likely to remain constant as well because a few years down the road the utility could re-route lines, change their source transformer, etc and throw those more precise results off and nobody even knows about it anyhow. Even having double fed points on the utility side for the maintenance or other reasons can change the results if something is bypassed or alternately routed.

 

[ron]

That does not give me the available short circuit current only the results of the infinite bus fault current at the transformer terminals reduced by the impedance of the secondary cables to the service entrance equipment. Not the same thing.

For example: A 1500 kVA, 480Y/277V sec, 5% Z would have an infinite bus short circuit of ~36 kA at the terminals. Depending on cable impedance, let's say it is reduced to 33.5 kA. However, if the utility available fault current is low, this value could be as low as ~21 kA. That could make a big difference on the clearing time of the service overcurrent protective devices.

The other factor is for older existing systems that have be expanded over the years, it could make the difference in equipment being overdutied or not and the level of incident energy to determine the proper arc rated PPE.

My method gives you the maximum for comparing short circuit ratings and code compliance as your post requested.

You are correct though that it does leave you short in most cases for arc flash calculations and will result in incorrect PPE selection. But for code compliance, you are all set.

 

[wbdvt]

From personnel safety perspective wouldn't you rather be in error on the high side, I think many times such calculations are done assuming infinite supply capabilities and start reducing with the impedance of the transformer. It is the only thing that is more likely to remain constant as well because a few years down the road the utility could re-route lines, change their source transformer, etc and throw those more precise results off and nobody even knows about it anyhow. Even having double fed points on the utility side for the maintenance or other reasons can change the results if something is bypassed or alternately routed.

Not always the case. For example: using an infinite bus value can place the arcing current value (different than the bolted value) in the instantaneous trip region of the breaker or fast range of a fuse. This will result in a lower value of incident energy. However it is possible that the available scc value places the arcing current in the short time trip region which will result in a higher value of incident energy.

So for the example above, using the infinite bus could result in an incident energy value of 4 cal/cm2 while the available value results in an incident energy value of 15 cal/cm2. So to err on the higher incident energy side, you would have to use the available fault current not the infinite bus value.

Which study would you rather base your arc rated PPE selection on?

 

[kwired]

Not always the case. For example: using an infinite bus value can place the arcing current value (different than the bolted value) in the instantaneous trip region of the breaker or fast range of a fuse. This will result in a lower value of incident energy. However it is possible that the available scc value places the arcing current in the short time trip region which will result in a higher value of incident energy.

So for the example above, using the infinite bus could result in an incident energy value of 4 cal/cm2 while the available value results in an incident energy value of 15 cal/cm2. So to err on the higher incident energy side, you would have to use the available fault current not the infinite bus value.

Which study would you rather base your arc rated PPE selection on?

Makes sense.

What are you to do when POCO doesn't give you an actual figure?

What good does any particular posted value do when POCO changes things in their distribution that might effect that value, and on top of that you may not even know it changed? Their final transformer before your service is often the only thing that is obvious if it would be changed. Your available current can even change when they operate a few switches to bypass a section of their system for maintenance or improvements and your supply takes an alternate route from what it usually takes.

 

[Hv&Lv]

Makes sense.

What are you to do when POCO doesn't give you an actual figure?

What good does any particular posted value do when POCO changes things in their distribution that might effect that value, and on top of that you may not even know it changed? Their final transformer before your service is often the only thing that is obvious if it would be changed. Your available current can even change when they operate a few switches to bypass a section of their system for maintenance or improvements and your supply takes an alternate route from what it usually takes.

Ding! Ding! Ding!

:thumbsup::thumbsup:

I’ve always been told never give an actual number.

 

[kwired]

Ding! Ding! Ding!

:thumbsup::thumbsup:

I’ve always been told never give an actual number.

I think it is probably best in most cases to use infinite bus to determine interrupt/withstand ratings of equipment.

To find actual potential incident energy I think might be almost impossible without some assumptions, and when you do so you still should probably pick variables that can exist that will give worst case.

ADD: this leaves the information required by 110.16 to be somewhat useless IMO as well. If anything it helps inspector determine if you have high enough rating on equipment, and that presumes you came up with the correct figures, but not much else is useful about it.

 

[Hv&Lv]

I think it is probably best in most cases to use infinite bus to determine interrupt/withstand ratings of equipment.

To find actual potential incident energy I think might be almost impossible without some assumptions, and when you do so you still should probably pick variables that can exist that will give worst case.

ADD: this leaves the information required by 110.16 to be somewhat useless IMO as well. If anything it helps inspector determine if you have high enough rating on equipment, and that presumes you came up with the correct figures, but not much else is useful about it.

A downfall is the infinite bus method could potentially cause you to fail something because you have to use say a 22kA breaker rather than a 10

 

[kwired]

A downfall is the infinite bus method could potentially cause you to fail something because you have to use say a 22kA breaker rather than a 10

True.

But also pretty common to see 22 kA main breakers in a panel and 10kA branch breakers that are series rated for use with the 22kA main.