Is old capacitor needed for 208v service?

[Lbartowski]

I work for a "design-build" contractor, which I've found means the engineering decisions get put on me (electrician not an engineer). Yes, technically we have a PE who signs our drawings, but he's not always available or capable, when it comes to what I encounter.

Anyway, I'm working on a 9-story multi-unit housing facility (old folks home). I need a switch in the main switchboard for a new 200 amp ERU on the roof, but there are no spares. One potential option is to disconnect the 30 year old capacitor (400 amp switch), fuse it down, and use it for the new mechanical equipment.

Do you see any reason why this capacitor would be necessary? Do capacitors, over time, lose their ability to function? My experience is that large capacitors are used in industrial facilities for PF correction due to extensive motor use, but this is an "old folks" home that doesn't have a lot of motors.

I wonder if it has something to do with the 50HP fire pump? The owner doesn't have a clue, and likely doesn't even know what they have, and so looking to them for direction, doesn't work.

I don't have a problem with telling them that their service is not sufficient, I just need to make sure that I know what I'm talking about.

Thanks for the help.

 

[petersonra]

I work for a "design-build" contractor, which I've found means the engineering decisions get put on me (electrician not an engineer). Yes, technically we have a PE who signs our drawings, but he's not always available or capable, when it comes to what I encounter.

This might well be considered unethical behaviour on the part of a PE, although it is hard to know just what you are suggesting is being done.

Do you see any reason why this capacitor would be necessary? Do capacitors, over time, lose their ability to function? My experience is that large capacitors are used in industrial facilities for PF correction due to extensive motor use, but this is an "old folks" home that doesn't have a lot of motors.

Most often capacitors are there to improve the power factor. They can indeed fail over time. But, if you just remove it, the building may well get hit with a power factor penalty by the POCO.

There should be some drawings that describe what this capacitor is being used for that might be of assistance. Is the capacitor switched or online all the time?

 

[Jraef]

Don't ASSume...

That capacitor might be part of a starting scheme for the fire pump, there was an old way of starting larger motors on weaker systems called "capacitor Assisted Starting". It is very tricky to get right, and runs the risk of damaging a motor if done all the time, so it was not used much until you get into really huge MV motors. But it was sometimes used on smaller Fire Pumps because they were rarely started; just for tests, or of course for a fire.

Could also be a phase adder system if your facility only has a single phase service and they had a 3 phase fire pump.

A cap that is fed by a 400A disconnect switch seems way too big for PF correction for a facility with only a few motors, but you don't say what size the cap actually is.

 

[GoldDigger]

I suppose that it is also remotely possible that the breaker once fed both a motor and an associated cap and the motor was removed but the cap left behind.
A cap that is not switched along with its associated motor can lead to an equally bad power factor part of the time, just capacitive instead of inductive.

 

[kwired]

If it were for the fire pump you would think it would need to be on the same service equipment (that is supposed to be remote from the regular service equipment) that supplies the fire pump - especially if it is needed to help start the fire pump.

Otherwise a single capacitor/bank of capacitors is usually for power factor correction of either a single large motor and is controlled with that motor, or a group of motors where power factor remains fairly constant or at least is shut down when the loads are shut down - more typical in a industrial plant then in a high rise with most motor load only being HVAC equipment.

I'd probably find out, first if the thing is still working. If not it may be ok to remove it.

Second if the POCO is penalizing the customer if they have poor power factor, if not it probalby isn't needed all that much.

Third if power factor is really poor for some reason maybe it does serve some useful purpose - but don't seem too likely.