Main Panel Question.... Says 3PH 4W

[AWinston]

I am doing a redesign for a new restaurant TI job in an old building. The main 400A panel says 3PH/4W Delta 120/240. I am a bit confused as I've only seen Wye systems. I understand that VLL is 240V but there is no VLN. In order to get 120V, is the voltage being read between Line and and ground? Seems like a basic question but I don't want to assume.

 

[augie47]

3PH/4W Delta 120/240 is, IMO, not as popular a system as it once was but still common. Your L-L voltage will be 240 on all phases. Your L-N voltage will be 120 on Phases A & C and 208 on B to Neutral.
You need to take care is selecting breakers (see 240.85).

 

[AWinston]

3PH/4W Delta 120/240 is, IMO, not as popular a system as it once was but still common. Your L-L voltage will be 240 on all phases. Your L-N voltage will be 120 on Phases A & C and 208 on B to Neutral.
You need to take care is selecting breakers (see 240.85).

Just to be clear, when you said L-N voltage, that really meant L - ground correct?

 

[Smart $]

Just to be clear, when you said L-N voltage, that really meant L - ground correct?

Neutral is the grounded conductor. It is a midpoint winding tap of the SDS secondary winding connected A to C... so it is neutral to A & C, and not B.

 

[augie47]

True! "Fine line" terminology which I had not given thought to, but accurate . On a 3phase 4 wire 240/120 delta, your "neutral" conductor is in fact between A & C phase and will be 120v between those phases. It is also your grounded conductor and there would be 208v between B and "grounded conductor".( if it is a 240/120v 4 wire delta.)
In the original post you advised the was "no VLN" which would indicate a 3 wire ungrounded delta.

 

[kwired]

Since you are in the process of redesigning this you may want to take a few things into consideration.

If this 3 phase 4 wire delta system only uses a two winding(open delta) transformer bank to derive the system there may be limited capacity from the source. It depends what it was initially designed to serve. You could have a situation where there was limited three phase loads and a lot of 120/240 single phase load and the A-C windings are a larger unit then the high leg winding.

There are other cases when a full delta transformer is used where there maybe is a lot of three phase load and having that extra 240 over 208 volts has advantages. I mostly find this desirable where there is a lot of motor load and limited 120/240 single phase load.

You also run into situations where the facility is remotely located and POCO doesn't have all three primary phases in the area - they can still derive a three phase system where needed with an open delta transformer bank.

If you are doing a major renovation, and is more desirable to have a 208/120 wye system, maybe see what it will take for the POCO to supply it - may be worth it in the end. Existing gear and conductors should work with either system. You will need to adjust any existing motor overloads/ confirm that any existing 240 volt loads are still usable @ 208 volts but most will be fine. Something easy to forget- HVAC systems may have control transformer that needs taps changed from 240 to 208 supply.

 

[ggunn]

I am doing a redesign for a new restaurant TI job in an old building. The main 400A panel says 3PH/4W Delta 120/240. I am a bit confused as I've only seen Wye systems. I understand that VLL is 240V but there is no VLN. In order to get 120V, is the voltage being read between Line and and ground? Seems like a basic question but I don't want to assume.

It's what is commonly called a "high leg" or "stinger" system. As others have said, the phases are 240V phase to phase, and two phases (usually A and C) are 120V to the grounded neutral while the third (B) phase is 208V to neutral. For single phase loads you must be careful when stabbing breakers, obviously; feeding 208V to a 120V load would not be good.

I've never seen 240V Wye, though it might exist. The phases would be 138.6V to neutral/ground.

 

[Smart $]

It's what is commonly called a "high leg" or "stinger" system....

"Stinger" is a slang term referring to the A-B or C-B transformer of an open delta system. As such, a "stinger" system should be an open delta transformer secondary configuration.

 

[BIMsparky]

Quick note: Utility will typically land the high leg on the C phase for thier metering. You'll have to switch it over to B phase for the rest of the panels etc. Had an inspector call me on having the high leg on the B phase with the customer right there, I got to correct him, pointing to 408.3(E). Made me look smart in front of the customer who still remembers that to this day!

Sent from my SM-G900T using Tapatalk

 

[jrohe]

"Stinger" is a slang term referring to the A-B or C-B transformer of an open delta system. As such, a "stinger" system should be an open delta transformer secondary configuration.

I thought the slang for the two trannies (more slang) in an open delta system were the "lighter" and the "kicker"?

 

[ggunn]

I thought the slang for the two trannies (more slang) in an open delta system were the "lighter" and the "kicker"?

That's the thing about slang, innit? By definition there can be no "correct" term.

 

[GoldDigger]

And be very careful about using "tranny" in public.

 

[Smart $]

I thought the slang for the two trannies (more slang) in an open delta system were the "lighter" and the "kicker"?

I concur with ggunn's comment.

With multi-term slang comes misunderstandings. In a two transformer open delta scenario, some refer to the leading phase pot as the "lighter" and the lagging phase pot as the "kicker". I believe this terminology stems from the utility side of the electrical trade. Seems mind-boggling when you try to reconcile that paradigm with the "lighter, stinger" terminology on the consumer side of the transformer bank.

 

[Iron_Ben]

At the poco where I worked, we used "lighting pot" and "wing pot" for the larger, 120/240 transformer and smaller 240 transformer, respectively.