ATS and Branch Size Limit

[mbrooke]

Going by a decrement curve an Emergency generator can produce 3x the rated current for a 3 phase fault 15 seconds, and 5 times the rated current for a L-G fault for 5 seconds.

1,500kw generator is rated 1,800 amps FLC. x3 = 5,400 amps, x5= 9000 amps.

Going by time current curves it takes 5x the current to open an OCPD in 10 seconds, 7x in 5 seconds.


9000/7=1,200 amps

Would it be safe to infer a 1000-1,200 amp feeder limit on the emergency switchboard during design to aid in selective coordination?

As is I like to limit ATSs to 800 amps max.

 

[xptpcrewx]

Going by a decrement curve an Emergency generator can produce 3x the rated current for a 3 phase fault 15 seconds, and 5 times the rated current for a L-G fault for 5 seconds.

1,500kw generator is rated 1,800 amps FLC. x3 = 5,400 amps, x5= 9000 amps.

Going by time current curves it takes 5x the current to open an OCPD in 10 seconds, 7x in 5 seconds.


9000/7=1,200 amps

Would it be safe to infer a 1000-1,200 amp feeder limit on the emergency switchboard during design to aid in selective coordination?

As is I like to limit ATSs to 800 amps max.

No two decrement curves are the same and the 300% field forcing you are referring to is a specific feature of a particular kind of AVR controller. It’s not an inherent property of the generator. So your question proposing such a limit makes no rational sense. Especially when the emergency system is designed as a last resort and the philosophy is run it to failure. Further, limiting the size of an OCPD is the last thing you want to do to make selective coordination easier, not all emergency systems use a switchboard, and emergency circuits are most likely feeders not branch circuits.

 

[mbrooke]

No two decrement curves are the same and the 300% field forcing you are referring to is a specific feature of a particular kind of AVR controller. It’s not an inherent property of the generator. So your question proposing such a limit makes no rational sense. Especially when the emergency system is designed as a last resort and the philosophy is run it to failure. Further, limiting the size of an OCPD is the last thing you want to do to make selective coordination easier, not all emergency systems use a switchboard, and emergency circuits are most likely feeders not branch circuits.

Correct, though I'm refereeing to a specific decrement curve vs generic rules.

If there is a fault in the equipment branch the last thing I'd want is the life safety or critical branch to go down.

 

[xptpcrewx]

Correct, though I'm refereeing to a specific decrement curve vs generic rules.

If there is a fault in the equipment branch the last thing I'd want is the life safety or critical branch to go down.

If that’s your goal, then why are you proposing to have a limit on the OCPD? Seems like 100% the opposite approach.

You don’t need to be conjuring up rules. Selective Coordination is already a thing for systems that need it, and “equipment branch” only applies to essential systems. Btw, the concept of FLC does not extend to generators.

 

[mbrooke]

If that’s your goal, then why are you proposing to have a limit on the OCPD? Seems like 100% the opposite approach.

You don’t need to be conjuring up rules. Selective Coordination is already a thing for systems that need it, and “equipment branch” only applies to essential systems. Btw, the concept of FLC does not extend to generators.

An 800 amp OCPD would clear faster than a 1,600 amp OCPD for the same current- increasing the odds of selective coordination and reducing winding heating of the stator.

 

[xptpcrewx]

An 800 amp OCPD would clear faster than a 1,600 amp OCPD for the same current- increasing the odds of selective coordination and reducing winding heating of the stator.

You have no idea what you’re talking about and it’s showing.

As is I like to limit ATSs to 800 amps max.

You are no designer.

I consider the matter addressed/closed.

 

[mbrooke]

You have no idea what you’re talking about and it’s showing.


You are no designer.

I consider the matter addressed/closed.

2:1 fuse selection ratio.

800 amp fuse will coordinate with a 2000 amp fuse.

1,600 amp fuse may not coordinate with a 2000 amp fuse.

 

[mbrooke]

Breaker, no such rule- not without being plotted, dials adjusted to cooperate with each other (if possible).

 

[Little Bill]

You have no idea what you’re talking about and it’s showing.


You are no designer.

I consider the matter addressed/closed.

Agree