UPS Breaker Rating

[EE2019]

Hi! Just want to be be enlightened, if the scenario is ECB to UPS to another Main CB, should the ECB and Main CB ratings' the same?

 

[jim dungar]

Hi! Just want to be be enlightened, if the scenario is ECB to UPS to another Main CB, should the ECB and Main CB ratings' the same?

Sure. Why not?

Please do not assume that acronyms are universally understood.
I assumed ECB stands for Enclosed Circuit Breaker. But it won't be the first time I have been wrong today.

 

[topgone]

Hi! Just want to be be enlightened, if the scenario is ECB to UPS to another Main CB, should the ECB and Main CB ratings' the same?

Not necessarily. If the main breaker has loads other than your UPS (which is more likely than not), that other loads determines what size the source breaker will be..

 

[EE2019]

Sure. Why not?

Please do not assume that acronyms are universally understood.
I assumed ECB stands for Enclosed Circuit Breaker. But it won't be the first time I have been wrong today.

Yes ECB stands for Enclosed Circuit Breaker.

 

[EE2019]

Not necessarily. If the main breaker has loads other than your UPS (which is more likely than not), that other loads determines what size the source breaker will be..

Hypothetically, if the UPS is the only load. This means that the source breaker should be of higher rating than the breaker after the UPS, right?

 

[jim dungar]

Hypothetically, if the UPS is the only load. This means that the source breaker should be of higher rating than the breaker after the UPS, right?

Why?

 

[ron]

Although the input CB to a UPS has to support the load, UPS losses and battery recharge, where the UPS output is only carrying the load, depending on standard breaker sizes available, they may be the same. Often if the sizes work out the input will be larger due to the battery recharge and losses.

 

[jim dungar]

Although the input CB to a UPS has to support the load, UPS losses and battery recharge, where the UPS output is only carrying the load, depending on standard breaker sizes available, they may be the same. Often if the sizes work out the input will be larger due to the battery recharge and losses.

Maybe. But not on the ones I have seen, input breaker, bypass breakers(s), and output breaker have always been the same.

 

[petersonra]

Hypothetically, if the UPS is the only load. This means that the source breaker should be of higher rating than the breaker after the UPS, right?

Maybe. The breaker feeding the UPS may have to be able to take the full load the UPS is designed to provide in bypass mode plus whatever load the battery charger is using to recharge the batteries. The breaker after the UPS only has to handle the load downstream of the breaker.

 

[topgone]

Although the input CB to a UPS has to support the load, UPS losses and battery recharge, where the UPS output is only carrying the load, depending on standard breaker sizes available, they may be the same. Often if the sizes work out the input will be larger due to the battery recharge and losses.

This! After a power interruption, the UPS batteries will have expended some of its stored energy and will be recharged upon restoration of normal power source. UPS input circuit breaker should consider these:

a. The UPS rated power and overload characteristics.
b. The Battery charging current.
c. Must withstand the prospective short circuit from the most powerful of sources (utility transformer).
d. Must trip the prospective short circuit delivered by the least powerful of sources (typically the generator).