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Electricians step inside (3 phase panel question)

Fl-Krawler

Well-known member
Joined
Dec 3, 2020
Member Number
3107
Messages
81
Loc
Cookeville, Tennessee
So I am at the point I need to buy a panel for my shop. Power in my area is all single phase 240v. Let me preface by saying im a cheap ass, and right now my budget is non existent. Will running a 400amp 3 phase panel wired in a single phase application pass inspection? From the research I have done, everything I have read says yes as long as I label the legs at the panel but I would like the IBB's opinion. The reason I am wanting to do this is I have a ton of large amperage draw 3 phase CNC equipment and I'd rather not buy two 400amp panels. I have a 100HP rotary phase converter so my thoughts are to feed 2 legs of the panel with single phase, and then power the 3rd leg from the rotary phase converter to give me 3phase power for all my cnc and machining equipment. The plan would be to go through the initial inspection to get the power turned on with the minimum single phase items wired into the panel, and then post inspection add my phase converter and wiring for everything else just to avoid the inspector telling me I need a 2nd panel or some other gay shit like my building will burn down in an electrical fire:laughing::flipoff2:I just cant see the reason to buy 2 panels when I can accomplish what I need to with one
 
The plan of using a three phase panel wired single phase should work if everything is rated for the voltage and current you’re using.

The rotary converter plan is a bad one. Getting that extra leg 120 degree apart from the two from utility is impossible.

Just set another panel to run 3 phase loads. Put it in after inspections.
 
Do you really need 400 amp size for the second panel? I believe the answer is no.

Three phase stuff is 208 and your utility power is 240.

100 hp @ 208 3 phase is about 250 amps.

Your three phase panel could be 200 amp.

I'd put in two panels.
 
Your gonna need a breaker between the line in and phase converter. I have what you are asking in my shop. It goes line in 400amp single phase panel. A 200 amp breaker to a knife switch to phase perfect to transformer to 3phase panel. 3 phase stuff gets wired to the 3phase box single phase stuff gets wired to the single phase box.
 
Rangerrod said:
The plan of using a three phase panel wired single phase should work if everything is rated for the voltage and current you’re using.

The rotary converter plan is a bad one. Getting that extra leg 120 degree apart from the two from utility is impossible.

Just set another panel to run 3 phase loads. Put it in after inspections.
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I have already been using the rotary phase converter in Florida where it was powered off single phase and produced a 3rd leg. Only difference is I plugged my equipment into a plug on the phase converter, and it didnt go through an additional panel. I dont see how just running the 3rd leg off the phase converter into the empty leg on the panel. Only the 3ph equipment will run off that leg and ill just have to be mindful of which breakers I'm using for what inside the panel

Muckin_Slusher said:
Do you really need 400 amp size for the second panel? I believe the answer is no.

Three phase stuff is 208 and your utility power is 240.

100 hp @ 208 3 phase is about 250 amps.

Your three phase panel could be 200 amp.

I'd put in two panels.
Click to expand...
I have 7 cnc machines, 3 of which pull 100amps at full load. I had 400amps at my last shop and it proved sufficient although by the math a 600amp panel would probably be more appropriate. Going to a 600amp panel here creates a whole new set of headaches that I dont wanna deal with. My rotary phase converter produced 230v 3ph when I had it running in Florida. I will not be be able to run all the machines at the same time. Money is extremely tight right now so dropping an additional $400-$700 on a second panel just isnt in the cards at this time if I dont have to do it
 
Rangerrod said:
The rotary converter plan is a bad one. Getting that extra leg 120 degree apart from the two from utility is impossible.

Just set another panel to run 3 phase loads. Put it in after inspections.
Click to expand...

Wut? How is a second panel going to change anything? You're still using the 2 utility legs plus a generated leg.



ThePanzerFuhrer said:
Your gonna need a breaker between the line in and phase converter. I have what you are asking in my shop. It goes line in 400amp single phase panel. A 200 amp breaker to a knife switch to phase perfect to transformer to 3phase panel. 3 phase stuff gets wired to the 3phase box single phase stuff gets wired to the single phase box.
Click to expand...
Fl-Krawler said:
I have 7 cnc machines, 3 of which pull 100amps at full load. I had 400amps at my last shop and it proved sufficient although by the math a 600amp panel would probably be more appropriate. Going to a 600amp panel here creates a whole new set of headaches that I dont wanna deal with. My rotary phase converter produced 230v 3ph when I had it running in Florida. I will not be be able to run all the machines at the same time. Money is extremely tight right now so dropping an additional $400-$700 on a second panel just isnt in the cards at this time if I dont have to do it
Click to expand...

I just did this on a smaller scale. 30hp rotary and I managed to find a used 3p panel at the local surplus place for $120. 200a main panel on the right, 125a breaker to the 3p panel in the middle, 80a breaker from that to the RPC, which then back feeds the generated leg back to the 3p panel. I'm only running a small-ish VMC and lathe off of it plus a few random smaller 3p tools. Don't think I can quite run the mill and lathe at full load simultaneously, but I doubt that would ever happen any way. :grinpimp:
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May be worth checking CL, FB and any local salvage places to see if you can find an old spare 3 phase panel. If you can find a deal like I did, it'll probably be quite a bit cheaper to get a new singple phase panel to pass inspection a cheap used panel for the 3 phase side.
 
bgaidan said:
Wut? How is a second panel going to change anything? You're still using the 2 utility legs plus a generated leg.
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At first I didnt understand what he was saying but as I typed my reply I kinda did. On a 3ph panel, breaker slot 1 pulls off the first leg, 2nd breaker off the 2nd leg and 3rd breaker slot off the 3rd leg. Not being mindful of this could cause you to wire in single phase 240v to the dead leg when the phase converter isnt running and creates issues when it is due to the rotation of the 3ph (im not an electrician so that s the best explanation I can give:flipoff2:). As long as I am mindful of which breakers are being used for what purpose it shouldnt be an issue
 
You would run all three legs from the converter to the 2nd panel. Panel would only be energized from phase converter. Otherwise you can ensure phases are 120 degrees apart.
 
Fl-Krawler said:
At first I didnt understand what he was saying but as I typed my reply I kinda did. On a 3ph panel, breaker slot 1 pulls off the first leg, 2nd breaker off the 2nd leg and 3rd breaker slot off the 3rd leg. Not being mindful of this could cause you to wire in single phase 240v to the dead leg when the phase converter isnt running and creates issues when it is due to the rotation of the 3ph (im not an electrician so that s the best explanation I can give:flipoff2:). As long as I am mindful of which breakers are being used for what purpose it shouldnt be an issue
Click to expand...

Well, duh. I have a few single phase things wired in to my 3p panel because it already had the breakers. I'm just carful to put the double pole breakers on legs 1 & 2 for those.

It's a bigger dumb to forget to start up the RPC before turning on one of the 3 phase machines. :homer:
 
bgaidan said:
Well, duh. I have a few single phase things wired in to my 3p panel because it already had the breakers. I'm just carful to put the double pole breakers on legs 1 & 2 for those.

It's a bigger dumb to forget to start up the RPC before turning on one of the 3 phase machines. :homer:
Click to expand...

I have everything from a single 3-ph panel. Dads shop does as well. I have never heard of keeping things on legs 1/2. What is the danger?
 
Bacho said:
I have everything from a single 3-ph panel. Dads shop does as well. I have never heard of keeping things on legs 1/2. What is the danger?
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Is it an actual 3 phase service or a rotary?

If you're running that panel off a rotary, the third leg is only going to be hot when the rotary is running. So if you hook up a single phase load to legs 1 and 3, only one leg will be hot until you start up the RPC. There's also potential for a big voltage spike on the generated leg when the RPC is starting up and contactor for the caps is still closed. I fried a varistor in my Fadal when the power briefly went out, came back on and the RPC restarted with the Fadal's disconnect still closed.
 
bgaidan said:
Is it an actual 3 phase service or a rotary?

If you're running that panel off a rotary, the third leg is only going to be hot when the rotary is running. So if you hook up a single phase load to legs 1 and 3, only one leg will be hot until you start up the RPC. There's also potential for a big voltage spike on the generated leg when the RPC is starting up and contactor for the caps is still closed. I fried a varistor in my Fadal when the power briefly went out, came back on and the RPC restarted with the Fadal's disconnect still closed.
Click to expand...

I have 3-ph service. I guess I didn't catch that context.
 
aczlan said:
Depends on how your service is set up, some can have a wild leg on the C phase where the voltage can spike significantly above what the others are at.
Apparently the official name is High Leg Delta: High-leg delta - Wikipedia


Aaron Z
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Usually B leg at least in our area. A dead give away is an every third breaker is missing in the panel unless it’s a three phase load. That leg will be labeled with orange tape. It was used in machine shops long time ago. 3 phase 240 volt single phase 120 on a+c.
 
Open Delta is not a long time ago with Central Hudson utility. The Utility does it with 2 tubs on the service instead of three. C phase in the meter and B phase in the panel. Orange tape on the high leg. Commonly referred to as a "power" service as opposed to "Lighting " service. All the older 3phase motors were wound for 230 volts not 208.
 
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