Looking for input on adding circuits

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James L

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
Looking for input. I went with a GC to look at a small job at a small nutrition supplement manufacturer. The job was a recent tenant finish in an industrial park, and previous electrician is done and has final inspection passed. Now I'm here to hook up some equipment. It's all cut and dry, except this....

They have a whole host of HEPA filters running through control boxes protruding into a mezzanine. I guess the owner gave specs to the previous electrician for 120v 20a units, so there are a half dozen circuits wired with MC cable into the walls above

But the control boxes that got installed are 120v and 28 amps. There are 5 of those control boxes. So I have to add 5 circuits. No biggie.

There is a 200 amp 42 space panel that's full, almost all small 3-phase loads (most are 20 amp breakers). Nothing is labeled, but I know there are 2 air compressor circuits on 3-pole 30 amp breakers. Equipment is not there yet

To the left is a newly added 24 space panel that's full of mostly single breakers. Nothing is labeled. That panel is powered by a 2-pole 100-amp breaker in spaces 1 & 3 of the main panel.

Here's what I told GC....
The main panel is out of space, so I can't add the 5 there. I have 4 spaces in the subpanel, but....

Considering the main panel is full of 3-phase loads, previous electrician should have never added a single phase panel. How could it ever be balanced?

There's substantial load in those HEPA filters. If I add 3 on one leg, thats 84 amps. I don't know what all those other single circuits are powering, and can't measure amperage until they're up and running. But they can't get up and running without the filters.

I think it needs to have that subpanel changed to a 30-space 3-ph panel. In order to do that, I would need to put a tandem breaker for the 2 single pole circuits in the main panel, then shift a 3-pole breaker down one space to make room for a 3-pole 100 amp breaker. I don't think the panel is rated for more than 42 circuits.

Either that, or add a second subpanel to the left of the what's in the pic. Then I'd have to move a 3-pole circuit from the main panel into new subpanel.

I think I've been clear about my preference, but what do you think? With that kind of info, what would you suggest?
 

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May or may not have any bearing on the "problem" but what are the larger cables going out the bottom of the "main" as they don't seem to be fed off the "main" ?
 
augie47 said:
May or may not have any bearing on the "problem" but what are the larger cables going out the bottom of the "main" as they don't seem to be fed off the "main" ?
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That's the main feed coming in from the bottom. It's hidden on the left behind all the branch circuits
 
I agree with Larry about replacing the existing subpanel with a larger 3-phase one. If you add a smaller 3-phase subpanel to the existing single-phase one, then you 'd really have to put three of your five 28A loads on the one leg that the single-phase panel isn't using. Consolidating into one 3-phase subpanel would give more flexibility to balance the loads. Even so a 100A subpanel seems like it could be a little light, although the total loading on both panels would have to be figured out to see if it can work.
 
With the existing service at best the customer will squeak by with your taking the steps shown above.
Have no fear, when a problem arises it will be YOUR fault :)
 
synchro said:
I agree with Larry about replacing the existing subpanel with a larger 3-phase one. If you add a smaller 3-phase subpanel to the existing single-phase one, then you 'd really have to put three of your five 28A loads on the one leg that the single-phase panel isn't using. Consolidating into one 3-phase subpanel would give more flexibility to balance the loads. Even so a 100A subpanel seems like it could be a little light, although the total loading on both panels would have to be figured out to see if it can work.
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Yeah, I was thinking if I leave subpanel as is, having to put 3 filters on C leg to offset the existing subpanel being on A + B legs. Just seems like a lot of extraneous effort, imo.

I also agree that 100 amps seems light.

Doesn't seem like there's any really good way to approach this. Thanks for your input
 
hillbilly1 said:
Or you could move 4 circuits that are lightly or no load to the sub panel, (if they are single phase) and put the filters in the main panel.
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I thought about that, but couldn't come up with a way to get 3 of those filters on leg C to offset the imbalance caused by the subpanel being on legs A + B. All those 3-pole breakers seems to remove that as an option

My eye sockets started to hurt after a while from trying to out-think this one 😓
 
there is no code rule that says the phases have to be balanced. Even if you try to do so there is no real reason to expect that they would be balanced when things are running.

my inclination given the lack of breakers for future circuits that will almost certainly be needed is to add a 3 phase panelboard. If it makes you feel better you can move the circuits from the single phase box to the new one but I see no good reason to do so.
 
hillbilly1 said:
That would be the best for future growth, but from the sounds of it, the customer is going as cheap as possible.
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Yeah, I'm not sure who's gonna be footing the bill.
GC seems to be on the hook, but I mentioned that single phase subpanel being a big miss (and why), and now he's reeling.

But it seems to me the owner and previous electrician are the cause of the issues. And I'm sure neither of them will offer to pay 😜
 
What is load calculation for the supply? is the supply a service or a feeder? Looks like separate neutral and EGC so presuming a feeder.

If enough capacity still left in the feeder replace the single phase panel with three phase panel with same capacity as the existing three phase and tap the feeder ahead of main. Install main if you wish in the additional panel. This would allow for easier future conversion to a larger feeder when the time becomes necessary, or to convert to two individual feeders.
 
>I would need to put a tandem breaker for the 2 single pole circuits in the main panel, then shift a 3-pole breaker
>down one space to make room for a 3-pole 100 amp breaker.

In the main panel below the 100 amp 2 pole breaker and the 3 pole breaker is a single pole breaker. Pull the single pole breaker, shift the 3 pole down one space and replace the 2 pole 100 amp with a 3 pole. Replace the two phase subpanel with a 3 phase panel and move the single pole breaker into the new 3 phase subpanel.
 
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