Breaker compatibility w/inrush

[thewirenut91]

Hey all. First time poster, long time watcher here.
I am in the enviable position of starting to set up my own EC company, with all the attendant uncertainty etc. Ive been tasked with tidying up a 480v delta feeder circuit that is feeding various loads that are just tapped off the feeder w/polaris lugs and run underground to motors, etc, around a farmyard. It is a very farmyard setup. Intending to use the feeder to feed a distribution panelboard and install OCPD's (3 pole 480v Breakers) to protect the various branch circuits, i got quotes from several vendors for 480v panelboards. The most reasonable quote, and therefore the one id like to go with, is a Siemens P1 panelboard with the 3VA41 series CB's. Looking at the specs, I am increasingly uncomfortable with installing those breakers to feed the loads in the yard. A 20 HP 3 phase motor and a 45 KVA Xfrmr are the loads of highest concern. The reason I am worried is that the 3VA41 series are a non adjustable magnetic/thermal trip unit breaker with a 700 amp instantaneous trip rating. I would hate to install this panelboard, which i dont believe accepts a more heavy duty breaker, and subsequently discover the inrush on the xfrmr and motors trip their breakers every time. I realize that the approximate inrush is calculable according to the design specs on the units, but for future use, etc, it makes me a little queasy. Opinions?

 

[ptonsparky]

Hey all. First time poster, long time watcher here.
I am in the enviable position of starting to set up my own EC company, with all the attendant uncertainty etc. Ive been tasked with tidying up a 480v delta feeder circuit that is feeding various loads that are just tapped off the feeder w/polaris lugs and run underground to motors, etc, around a farmyard. It is a very farmyard setup. Intending to use the feeder to feed a distribution panelboard and install OCPD's (3 pole 480v Breakers) to protect the various branch circuits, i got quotes from several vendors for 480v panelboards. The most reasonable quote, and therefore the one id like to go with, is a Siemens P1 panelboard with the 3VA41 series CB's. Looking at the specs, I am increasingly uncomfortable with installing those breakers to feed the loads in the yard. A 20 HP 3 phase motor and a 45 KVA Xfrmr are the loads of highest concern. The reason I am worried is that the 3VA41 series are a non adjustable magnetic/thermal trip unit breaker with a 700 amp instantaneous trip rating. I would hate to install this panelboard, which i dont believe accepts a more heavy duty breaker, and subsequently discover the inrush on the xfrmr and motors trip their breakers every time. I realize that the approximate inrush is calculable according to the design specs on the units, but for future use, etc, it makes me a little queasy. Opinions?

I wouldn't worry about it at all. You would need to be starting approximately 100 HP to come close to the 700 amps. The transformer only has an issue at start and typically is not cycled once On. Distance from the POCO transformer to the panelboard, then to the loads etc will reduce the inrush of the transformer and the starting current of motors.

Do a Fault Current Calculation from the POCO transformers all the way to your loads. You may be surprised at the results. You should know the FC prior to selecting the Panelboard anyway.

Welcome to the Forum.

 

[don_resqcapt19]

I wouldn't worry about it at all. You would need to be starting approximately 100 HP to come close to the 700 amps. The transformer only has an issue at start and typically is not cycled once On. Distance from the POCO transformer to the panelboard, then to the loads etc will reduce the inrush of the transformer and the starting current of motors.

Do a Fault Current Calculation from the POCO transformers all the way to your loads. You may be surprised at the results. You should know the FC prior to selecting the Panelboard anyway.

Welcome to the Forum.

Yet Table 430.52 permits an instantaneous trip breaker to be set at 827 amps for this motor.

 

[thewirenut91]

Thanks guys!