Hi-Leg Delta Panel

[bobsydnor]

Contracted to add several 208v 1ph circuits for new HVAC equipment in a 100+ year old funeral home.

Went to take a look @ the job and found a 208/120v hi-leg delta configured service. Checked a fairly current (installed in 2000) Siemens 400a pnl with plenty of "B" slots open (imagine that!). The available hi leg slots would be perfect for the new circuits. Noted Siemens "BL" ckt brkrs. No problem.....I think.

It's time to buy the breakers. I go to the supply house and ask for Siemens 1pole, 20amp "BL" breakers. The guy brings the breakers to the counter. I happen to look at the breakers and notice they're rated 120/240v. I tell him that I can't use slash rated breakers, I need to use them on a hi-leg and that they need to be rated 240v. He looks at me as if I have 2 heads and informs me that they don't come in straight 240v and shows me in the Siemens catalog. He thinks I'm looking for the wrong breaker and I think so also.

I return to the job to confirm my selection. To my surprise, I notice that the name plate on the panel specifies 120/208Y(not delta), 3p, 4w, 400a. No wonder I can't find breakers! I'm thinking that the contractor that installed the panel selected the wrong equipment... what a mistake!

In any event, I switched some breakers around and used 2pole breakers rated at 240v for the new equipment.

Any of you guys run across this situation in your travels?

See any problems with the solution?

Bob

 

[bdarnell]

I have to admit, I'm confused. I read your post several times and still don't get it.

First of all, a high leg delta service is 120/240, not 208/120. Second, why were you looking for single pole breakers to put in the high leg positions ?

If your equipment is rated at 208 only, you have another problem.

The panel may have been factory assembled and labled 120/208 and then installed on this system, and I agree that this was missaplied.

Can you clarify a little on what it is that you are connecting, voltage and phase ?

 

[infinity]

I'm with Brad. If you need a 208 volt circuit you're out of luck. Your choices are single phase 240, three phase 240, or single phase 120.

 

[bobsydnor]

Brad,
Sorry, I switched 208/120 for 120/208.
The 120/208 voltage specified for a hi-leg delta is nominal and is correct.
It is calculated by multipyling 120 x the sq root of 3(1.732)=208, the same as 277x1.732=480......277/480volt system.
The hi-leg is obtained by tapping the mid-point of one of the legs of a delta configured winding(xfmr) to ground.
Maybe you were thinking of some other configuration.
I was looking for 1pole breakers because the "B" phase slots were readilly available. A hi-leg provides 208volts to ground on the "B" phase.
208v is the specified voltage for the equipment(fractinal hp air handlers).
Sorry my post was confusing, hope this clears it up.

Bob

 

[bobsydnor]

I was also thinking that there may be geographical differences in nominal voltages.

 

[bobsydnor]

Another correction:
The voltage range on the equipment is actually 200-240v single phase per nameplate.

 

[don_resqcapt19]

The 120/208 voltage specified for a hi-leg delta is nominal and is correct.

While you are correct that the high leg voltage is 208 to the neutral, it is not called a 120/208 volt system. It is a 3 phase, 4 wire 120/240 volt system.
Don

 

[james wuebker]

Another correction:
The voltage range on the equipment is actually 200-240v single phase per nameplate.

So the equipment is actually 200-240 single phase. So you take either A-B,B-C, A-C to get your single phase for the units. Maybe I'm missing something.
Jim

 

[bobsydnor]

don,

thanks for the clarification, i see your point.


James,

Your suggestion is exactly what I ultimately did, utilize a 2pole breaker phase to phase to feed the equipment.

The problem was with my initial plan, thinking (I use that term loosely) that I could use a 1pole breaker on the B phase (the hi-leg 240v to ground ) and satisfy the equipment requirements.

In other words I was going to run a length of 12/2MC from the panel (black attached to a single pole breaker on B phase, white=neutral) to the equipment and provide 240 volts hot to neutral.

I don't understand the purpose of the hi-leg if it has no use.
What am I missing here?

Bob

 

[roger]

Bob, check out this thread, there is some good information on this subject. Be sure to read the last post by Scott Thompson and see the graphic three post before the Scotts post.

In short, the high leg has no purpose and can not be used for any circuit connected to the grounded conductor.

Roger

 

[infinity]

The high leg is a resultant of the 120/240, 4 wire Delta system. Is doesn't have any real use as a single pole 208 volt circuit. You would need to use, as you've mentioned, a 2 pole CB at 240 volts with this type of system to supply your load.

 

[infinity]

Here is a graphic:

 

[augie47]

Hi-Leg

Hi-Leg

Bob, If you are not familiar with a hi-leg system, you might also want to refer to NEC 240.85, which, I believe, will require you to use a full rated two-pole breaker if you connect to the hi-leg. Breakers with a 12/240 v slash rating are not permissable for this application.

 

[LarryFine]

Bob, check out this thread, there is some good information on this subject. Be sure to read the last post by Scott Thompson and see the graphic three post before the Scotts post.

Yes, do, and read mine also, including the last one I just posted; sort of a recap. The OP was still confused. If anyone wants, I'll repost them here.

 

[snewman24]

In short, the high leg has no purpose and can not be used for any circuit connected to the grounded conductor.
Roger

So why is the high leg without purpose? Can't it be used for a single-phase 208V load? If not, why not?

 

[LarryFine]

Because the reason we get 208v, and not 120v, from the high leg to ground is that the 240v and 120v, with the 120-deg. angle thrown in, algebraically add the same way a buck-boost transformer bucks, but the two (actually, 1 & 1/2) windings don't share a common core, or primary, and can cause transformer overheating, even when otherwise loaded within specs.

Okay, you can inhale now.

 

[George Stolz]

Re: Hi-Leg

Re: Hi-Leg

Bob, If you are not familiar with a hi-leg system, you might also want to refer to NEC 240.85, which, I believe, will require you to use a full rated two-pole breaker if you connect to the hi-leg. Breakers with a 12/240 v slash rating are not permissable for this application.

But a 277V single pole breaker would be in compliance with 240.85, wouldn't it?

In short, the high leg has no purpose and can not be used for any circuit connected to the grounded conductor.

So why is the high leg without purpose?

It's not without purpose. Any two-pole (line-to-line load) or three phase load wouldn't see the high leg. The distance between all three phases is the same.

 

[frenchelectrican]

before it get out of hand just stop for a sec what about short current rating i did heard few diffrent conflet some saying 5 K some say more than 15 K amps for short circuit so i will like to heard what you guys saying about this ??

i dealt with wild leg before but i know short circuit do very funny way with wildleg so just ask you guys what is the scoop.

btw i will not even think about using wild leg for any kind of load at all

Merci , Marc

 

[bobsydnor]

Bob, check out this thread, there is some good information on this subject. Be sure to read the last post by Scott Thompson and see the graphic three post before the Scotts post.

Yes, do, and read mine also, including the last one I just posted; sort of a recap. The OP was still confused. If anyone wants, I'll repost them here.

Thanks, very helpful.

 

[bobsydnor]

Because the reason we get 208v, and not 120v, from the high leg to ground is that the 240v and 120v, with the 120-deg. angle thrown in, algebraically add the same way a buck-boost transformer bucks, but the two (actually, 1 & 1/2) windings don't share a common core, or primary, and can cause transformer overheating, even when otherwise loaded within specs.

Okay, you can inhale now. [/quote}

Larry,
Had my blinders on! I was focused on the branch circuit, everything downstream from the CB to the load. Never did I consider what was happening upstream (@ the xfmr) until your post.

Thanks again,
Bob