Panel Ampacity, short circuit, voltage drop calculation

[Designer69]

I have a local I&C panel that is being fed from a switchgear feeder. This panel connects various field devices and provides instrumentation/control power.

would my Ampacity, short circuit and voltage drop calculation be just for the feeder connecting the switchgear and panel or also from the panel to local field devices.

I would think just the feeder from switchgear to panel since the panel vendor would consider field instrumentation connections/trips etc.

appreciate your input

 

[charlie b]

When I do such calculations, I show the power sources, the panels, and the conductors that connect them. I only show the amount of load on any given panel; I need that for the voltage drop calc. I have not calculated any short circuit numbers downstream of the panel. I am not sure if that answers your question.

 

[kingpb]

I have a local I&C panel that is being fed from a switchgear feeder. This panel connects various field devices and provides instrumentation/control power.

would my Ampacity, short circuit and voltage drop calculation be just for the feeder connecting the switchgear and panel or also from the panel to local field devices.

I would think just the feeder from switchgear to panel since the panel vendor would consider field instrumentation connections/trips etc.

appreciate your input

There will not be any short circuit contribution from the field devices. Calculate at the panel, if the fault is in the cable to the device it will be very low and limited by the small impedance in the cable to the device.

 

[Designer69]

There will not be any short circuit contribution from the field devices. Calculate at the panel, if the fault is in the cable to the device it will be very low and limited by the small impedance in the cable to the device.

this is a great comment, what I was expecting. Are you able to elaborate on this a little...

There will not be any short circuit contribution from the field devices.

I just want to have a basis as to why they don't contribute, thanks so much

 

[Designer69]

There will not be any short circuit contribution from the field devices. Calculate at the panel, if the fault is in the cable to the device it will be very low and limited by the small impedance in the cable to the device.

Also when you say, Calculate at the panel, do you mean total short circuit contribution from the panel to the upstream Switchgear?

So what I would think the calculation should cover is ampacity and voltage drop of the feeder from the switchgear to the panel and short circuit contribution from the panel to switchgear.

thanks again

 

[charlie b]

I just want to have a basis as to why they don't contribute, thanks so much

An item cannot contribute energy to a fault, unless it has some energy stored within itself. Motors do. They are still spinning when the fault occurs, and that represents energy. During the first fraction of a second, a motor will act as a generator, feeding energy into the fault point. Instrumentation and control devices have nothing similar, nothing that can send energy back into the electrical distribution system.

 

[kingpb]

Also when you say, Calculate at the panel, do you mean total short circuit contribution from the panel to the upstream Switchgear?

So what I would think the calculation should cover is ampacity and voltage drop of the feeder from the switchgear to the panel and short circuit contribution from the panel to switchgear.

thanks again

I think you'll want to know the fault current in both locations. You have a contribution from the panel to the switchgear and vice versa; albeit that is assuming that the panel has load on it that can provide fault current. See CB's post, it explains it. If the panel only has control devices and resistive loads, then the fault current at the panel will simply be the fault current at the switchgear minus reduction due to the cable.

 

[Designer69]

thanks, do solenoids provide fault current? seems like this panel is supplying power to a bunch of valve solenoids

 

[Designer69]

thanks, do solenoids provide fault current? seems like this panel is supplying power to a bunch of valve solenoids

just wanted to re-ask this question, I would not think solenoid valves contribute to short circuit current but they are inductive so would appreciate some help.

thanks

 

[petersonra]

thanks, do solenoids provide fault current? seems like this panel is supplying power to a bunch of valve solenoids

It would have to be a huge solenoid to make any substantial difference.