220/440 Plasma Cutter Nameplate

[Mike Scarry]

I have a customer with a used plasma cutter that has a nameplate voltage listing of 220/440 (3-phase). We had to do a service change to get him 3 phase. The POCO is asking me 120/208 Y or 120/240 delta w/center tap & high leg. At first I was thinking 240 delta because I always thought that 220, 230, 240, & 250 V were all nominal 240V. Then, I was thinking 208 would be better since 220 falls closer to 208 in a 10% tolerance band. Does anyone have thoughts? I appreciate the insights in advance.

PS - I am not concerned about 120V loads, any single phase loads for that matter. We re-fed the old main single phase panel, and no other future loads are anticipated.

 

[hurk27]

Very surprised you can get a 240/120 4-wire delta (high leg) as our POCO has stopped allowing these for a while now.
What is the manufacture of this equipment, as the voltage rating sounds a little European, if so it could be a 50hz rating, and that could make a difference to which way you should go.

While 208/120 is the most common here, some equipment does not like 208, tanning beds with magnetic ballast are one, and high torque motors rated at 230 volts are another.

I generally like higher voltage then lower as any voltage drop can affect the load also, but the many problems with using a 4-wire delta can be a pain, as you have to show a load demand separately to the poco so they will know which tank to make larger, as you can have a larger center tap tank for a higher 120 volt load demand or if you have a higher 3-phase demand then the two other tanks can be larger, with a smaller center tap tank, also having the 208 volts from "B" phase to neutral can be scary if this place has other people also doing hook ups, so care must be taken to place warning labels in the panels, or set single phase sub panels that only have A and C phase to it, and mark the main panel for 3-phase loads only. but try to get more on tolerances from the manufacture and go with their recommendation, as they will know how this machine will operate, at least with the 208/120 you can buck boost up to the 220 volts, fairly easily and you don't have the unbalance loading of the service that happens with a 4-wire delta, or the high leg that if some one hit it with a 120 volt load and burns up equipment, which is another factor as many 240 volt loads that have a 120 volt controls will have to avoid having the controls on the B phase.

My vote is a 208/120 and if needed buck boost to the correct voltage for this machine.

But watch out for air compressors with 208, they will require a 200 volt rated motor as these are one of the high torque motor loads I was talking about, but most other 230 volt rated motors run fine on 208, and scroll type air compressors also dont have this problem, but care must be taken when ordering equipment.

 

[Joethemechanic]

I kinda like to lean towards the 120/240 4 wire delta (high leg)

Maybe because it's familiar to me because they were always so common in the philly area.

But here is what I see as the advantages.

240 Volts, I see lots more under-voltage motor failures than I do over-voltage motor failures.

And the second reason is that should you ever need it, you can run two phase loads from it. (although you will probably never need to)

The down side

First is the high ground to phase potential on "the high leg" many electricians/maintenance men are unfamiliar with it, and can get hurt or burn up a lot of 120 volt equipment if they make a mistake.

Second is, if you need to supply a lot of 120V loads, it is hard to keep all 3 phases balanced.

Although I still always seem to want a high leg delta if I can get it, I think it's the having the higher 240 volts that is always the deciding factor for me.

 

[kwired]

If you are leaving the existing single phase panel pretty much intact then probably go with the 120/240 three phase.

If you wish to balance the 120 volt loads out then you have to use 120/208.

Remember if balanced each phase is carrying 1/3 of the load. If you have a wild leg and mostly 120 volt loads you can not balance this and may need larger service to be able to handle the load. ( A 125 amp 120/208 feed can handle roughly same total VA as 200 amp 120/240 single phase) If you only have limited load on third phase you kind of have a single phase supply at the same expense as three phase equipment.