Is there a code reference regarding single phase load on the high leg of 240v 3ph delta?
I was asked to defend my stand and could not quote or find anything in the NEC

Are you asking if the single Ø 208-grnd is usable? I have never seen any 240V 4 wire that has used a hi-leg by its self. I suppose its possible but what would it be used for?

The situation you'd run into with the 208V to Neutral would be that standard 1 pole C/B's would not be rated for the voltage, as they're rated at 120V
If you were to use a C/B with a higher rating (277V), or use a fuse with a 250V rating, then I don't think that there's anything to prohibit it in the Code.

Thanks for the replies. I was not as clear as I could have been. I am asking about single phase 240 using the high leg with either 'A' or 'C'. I was aware of the 208 and breaker rating mentioned.
In my area it is an 'understood rule' that only 3ph. goes on the high leg. Transformers and services are sized accordingly. Is it a code issue or just a local thing?

If you think about it, a three phase load is made up of three single phase loads that work together to make advantage of the 120 degree phasing of the source. Each load of a three phase load, taken by itself, is indistinguishable from a single phase load.

Given that phase B is the high leg, adding 240 volt 2-wire loads to A-B and B-C will shift load off the A-C 120/240 side of the delta.

Nothing in the NEC encourages or discourages this placement of load on a 120/240 volt 4-wire delta.

The first thing I would wonder is what the local PoCo's method is for sizing the transformer(s) that supply phase B, the high leg? Do they put the smallest possible can(s) on the pole?

Al

I am not sure if this addresses you're Question specificly, atricle 240-85 mentions breakers with a slash designation 120/240 cannot be used between the high leg and another phase (a-b)you need a breaker with a straight 240v rating.

Since sometimes people take code rules and put them where they don't belong (Ya know, I remember the code sez....) someone may be confusing 455-9 for phase convertors where it says single phase loads are not to be put on the derived phase, but this has nothing to do with delta high legs (which are also getting pretty rare).

To ElectricAl
Power Co. sizes 'B' for three phase only.

To therain4
That jogs something in my foggy brain. I remembered it was something indirect. 240 delta is still very common here. Is this uncommon in other areas?

PoCo here, covers several states, refuses all 120/240 4-wire 3-phase requests since the mid '90s. 120/208 only.

As I understand it, they're trying to shift their generator load balancing down to the consumer.

Is the service you are considering old and has 3-phase load been shed, freeing transformer capacity?

Existing service. Someone has added single phase 240 load to high leg and now I am questioning my understanding.

[This message has been edited by wolfdog (edited 10-24-2001).]

As to your L-L connections using the "Wild Phase" of the 4 wire Delta ["A" to "B", or "B" to "C"], it should be acceptable with the NEC, since this is like any other type of L-L single phase connection and load.
As long as you use 2 pole units with a 1 pole rating of 240 VAC, there is really no problem. Please note that the SINGLE POLE RATING needs to be 240 VAC, not just simply the complete unit, or 2 pole rating. This is to assure that the device can withstand a L-G fault from the "Wild Phase" and not fail from exceeding the 1 pole fault rating.

This type of connection will not imbalance the system unless a majority of L-L loads were connected only between "B" and a certain Ungrounded Conductor - such as "A", and there was little or no L-G [L-N] and / or 3 phase L-L-L loads.

A uniform balance of loads, regardless of which "Phase" is used, will be the best method of connecting L-L loads on the 4 wire Delta.

As far as an L-G connection using the "Wild Phase" ["B"], it would work just fine to run a 208 volt 1 phase load, but the Grounded Neutral [Common Grounded Conductor] would not be able to "Balance" the current as it would between the Ungrounded Conductors on that coil ["A" and "C"]. This would cause the complete load current to flow from the "B Phase", through that "Phase's" Conductor, through the load, through the Grounded Neutral Conductor, through the Center Tapped point on the "Lower Winding", then through the "Lower Winding's" coil to either position "A" or position "C" where it will return to the coil of it's origin.
Once again, the OCPD used will need a 1 pole rating of 240 VAC, and since this will be a 1 pole device, it needs to have a rating of at least 10KAIC @ 240 VAC for the single pole breaker. Fuses will easilly conform to this rating, since by default they are listed at the L-L system voltage for the AIC rating [unless incorrectly used - such as if 240 VAC fuses are used on a 480 VAC system. even if the system was a 480Y/277 3 phase 4 wire Wye, the L-G voltage of 277 VAC is higher than 240 VAC].

For these reasons, I believe the NEC will not allow any loads connected L-G on the "Wild Phase" - be it either "B" as normal outside of metering sections, or "C" within metering sections.

I seem to remember a while back, the NEC allowed Welders to be connected L-G using the "Wild Phase" of a 4 wire delta. Not sure which NEC version this was, or which Article it was found in - just remember my Father talking about it being allowed at one time.
One of the members whom is more fluent with the NEC and older versions can verify this
[hint-hint to Don, Bill, Steve, Virgil, Scott W!!!].

Hope this is helpful and makes sense. Feel free to post any questions or comments you might have regarding this techno-babble.

Scott SET

I am a licensed electrician, so it isn't techno-babble to me yet. I am in the same boat you are, I can think of reasons why it isn't a good idea. and I want to say the NEC doesn't allow it, but I can't find a reference other than the above mentioned ones. Maybe the "wise ones" you spoke of will jump into this.

There is nothing wrong with connecting 1PH 240 loads from A-B or B-C. As stated above the breaker must be rated at straight 240V. As far as balancing the transformers I don’t think one load, unless it is very large, is going to be a problem. When balancing loads from a delta bank you can do one of two things with 1PH 240 loads. You could put them all on A-C and size the 1PH transformer accordingly. (This transformer is already larger. A lot of these loads would make it huge!) Or you can count the total KVA as three phase load and balance them accordingly. Closed delta banks are sized as follows. Take 67% of the entire 1PH load and add it to 33% of the entire 3PH load. This gives you the size of the mid tap transformer. The two power transformers are found by taking 33% of the total single and three phase loads.