3 phase question

Can anyone tell me how to get 208/230 3 phase from a older panel that has 2-120 bars and 1-240 bar? I've never seen a system like this. Do I need a transformer? Thanks

Chances are you don't even have three phase on your street let alone in your box. Go out and check the power lines on the street. The top ones are for elec.power, if there are three there is three phase up there, if only two you don't have it available. If you have it on the street you need to contact your utility about a service upgrade.

I have 3 phase and it's a older 3 phase panel from 70's.

Did you check the lugs in the panel each leg to ground with a meter and got these readings if so you have a high leg three phase. You don't see that much anymore but if that is what you have all three phase equipment is wired as usual but you have to watch out to not use the high leg on anything single phase

You have a high leg delta system. If you really need 208V out of that service, you will have to get a transformer. What ampacity is the load you are feeding?

Mick, I will try to answer your question and help you but such first requires some educating you, so listen, here goes:


"Can anyone tell me how to get 208/230 3 phase from a older panel that has 2-120 bars and 1-240 bar?"


FIRST NOTE: You can get 208 or 230/240 but NOT BOTH. See SECOND NOTE below::


1) Ifffffffff you REALLY have three phase service (there would be four wires out in the air from the utility to start with, NOT just single phase with two wires you know) to your panel in the first place and ifffffffff you have two legs of 120 with respect to Neutral and another leg YOU CALL 240 (which I suspect is actually 208) with respect to Neutral.

a) There would have to be FOUR WIRES coming from the utility transformer to the panel. Three UnGrounded HOTS, One Neutral

b) Ifffffff that's what you have??? Such is a 120/240 volt Three Phase Four Wire High/Red Leg Center Tapped DELTA Service.

c) Iffffffff you have that, you have Two legs of 120 volts with respect to the Grounded Conductor (Neutral via the center tapped transformer) and the third Red/High Leg would be 208 volts (NOT 240) with respect to the Grounded Conductor (Neutral)

2) You can get 240 Volts Three Phase from such a service, simply from the Three Hot UnGrounded Phase conductors, as its 240 Volts Three Phase on the three corners of the Delta. Leg A to Leg B to Leg C is 240 volt Three Phase.

3) If you indeed have a) Three Phase and indeed have b) Four wires on the aerial HV Primary and Four Wires from the utility transformer to your panel and indeed c) Its 120 volts from two legs to Neutral but 208 (NOT 240) to Neutral on the Red/High leg THATS 240 VOLT THREE PHASE FOUR WIRE CENTYER TAPPED RED/HIGH LEG DELTA and the one transformer is center tapped with the center tap earth grounded and the Neutral.

FINALLY AND AGAIN (if this is what you have) you get 240 volt Three Phase by use of a three pole breaker where each of the three phases is connected to the three panel buss bars through the three pole breaker.

SECOND NOTE: You are throwing around 208 and 230 TOGETHER ???, you can have a A) 208 Y 120 Volt Three Phase Four Wire Service orrrrrrrrrr B) a 120/240 Volt Three Phase Four Wire Center Tapped Delta Red/High Leg Service BUT NOT BOTH TO YOUR PANEL

If your service were 208 Y 120 Volt Three Phase Four Wire you would still have four incoming wires and it would be 120 volts to Neutral on alllllllll phases (3 Hot Legs) or 208 volt (NOT 240) Three Phase line to line to line.

SOOOOOOOOOOOO use a Three pole breaker (has a connection to each of the three legs) and that will give you 240 volt (NOT 208) Three Phase.

From your post Im assuming you don't just have typical residential 120/240 volt single Phase Three Wire Service. That gives you two legs of 120 or one leg of 240 line to line.

Okay got it??

Im sure I missed something here in a hurry, but if so hopefully the other electricians and engineers can add to this.

John T Long retired electrical engineer and latest code rusty BUT I DONT THINK THEY HAVE CHANGED THE ABOVE TYPES OF SERVICES not to my understanding lol

Just hook up to a normal 3 ph breaker. Your 208 3 ph load should be fine.You have a delta 3 phase or high leg system now.

Yep, a 208 load will most likely work if he serves it with 240 (what I suspect he has) while a 240 volt load may or may not work quite as well fed at only 208, depends on the equipment design and tolerances etc etc. Over the years I saw some that did fine (feeding only 208 to a 240 device) and others that didn't do so well.

Always fun sparky chattin with ya dr

John T

Iirc with the wild and crazy grounded Delta systems . There should be three phase 240 from bus bar, to bus bar to bus bar.
Hook it up and let'er rip.

We always called that set up crazy leg or wild leg 3 phase.