3 phase Delta High Leg Single Phase Load

[neity]

If you have a 3 phase 120/240 center tapped delta high leg service, giving you 208V to neutral on the high leg, is this an acceptable way to feed a single phase 208V load using only a single pole breaker?

[LarryFine]

Welcome to the forum!

If you have a 3 phase 120/240 center tapped delta high leg service, giving you 208V to neutral on the high leg, is this an acceptable way to feed a single phase 208V load using only a single pole breaker?

Simple answer: Definitely not.

[neity]

I've heard this simple answer more than once. Besides a load requiring 3 wires with a neutral to grab 120 from a phase to neutral, and grounding issues, what difference would a load see between a 208V polyphase source and a single pole to neutral, given equal RMS values. If you were to plot the absolute RMS value at any given point, with the 2 pole starting at a phase crossing and the high leg at a zero crossing, how would this differ? I read through article 450 and can't find anything in there that says you can't do this either. Besides being unconventional, I would really truly like to know why you cannot do this.

Thanks

Jeff Neithammer
Electrical Engineer

Advanced Engineering, Inc.

[charlie b]

I have never encountered a high leg delta system, either in school or in real life. So I don't know much about them. But the question does seem to be a good one. If the voltage between the high point and the neutral is 208 volts, and you connect a single pole breaker to that leg and take a grounded wire along with it, the load being fed in this manner would not be able to discern that it was not being fed from a two pole breaker in a 120/208 system. So let me ask, Larry, is there a code prohibition against such a branch circuit? If so, can you tell me where it might be found?

The only difference I can see, with regard to safety considerations, is that such a branch circuit crosses the line drawn in 110.26. If this circuit hits a disconnect switch before it goes on to a motor, then the working clearance for that disconnect switch would have to be increased, since you would have to use the "151-600 volts to ground" row of Table 110.26(A)(1).

Is there another consideration I have missed?

By the way, Jeff, welcome to the forum.

[augie47]

This has been discussed here previously but I can't find the thread offhand. memory tells me there may be a problem concerning transformer loading, but that memory is vague.
however.............
Unless you are using a higher voltage panelboard (higher than 240), your single pole breaker is probably rated at 120 volts, so using it on a 208 load would be a violation of 240.85

[broadgage]

Although 208 volts is present between the high leg and the neutral, a 120/240 delta service is not intended for operation of 208 volt loads.

If the 208 volt load is small in relation to the service then it should work fine, the load does not "know" whether it is wired phase to neutral on a 120/208 service, or hot leg to neutral on a 120/240 delta service.
The single pole breaker is likewise used within its ratings and would give the desired protection.

If however the 208 volt load is significant in relation to the service, then the voltage regulation will be poor because the current is passed through two sets of transformer windings, with voltage drop in each.
Transformer losses will be increased, and must be paid for by someone.
The transformers may be overloaded, despite a total load that appears to be within the rating.

If it is expediant to supply some relatively small 208 volt load, I would not worry. But for any substantial load it would be best avoided.

[TOOL_5150]

If you have a 3 phase 120/240 center tapped delta high leg service, giving you 208V to neutral on the high leg, is this an acceptable way to feed a single phase 208V load using only a single pole breaker?

No, I believe the reason is because that 208V to ground is "unstable" I was told to never use that voltage on a delta transformer.

~Matt

[augie47]

broadgage, I'm impressed !!!

[LarryFine]

So let me ask, Larry, is there a code prohibition against such a branch circuit? If so, can you tell me where it might be found?

I'm not sure, as I don't have my NEC available right now, as my D drive isn't in my tower; I'm 'hosting' PJ's D drive while her tower is "on vacation."

I vaguely recall, though, reading something about not using "the phase with the higher voltage to the neutral connection" or something like that.

Technically speaking, with an open Delta, only half of the center-tapped secondary will be loaded (but the same could be said about one with any 120v load).

With a full Delta, the high-leg's 1ph current through each half of the center-tapped secondary will be opposing the current in the other half, relative to usual operation.

Whether that might cause overheating and/or reduced ampacity is something the more technical-minded forum members will have to respond to. I dunno.

[mivey]

No, I believe the reason is because that 208V to ground is "unstable" I was told to never use that voltage on a delta transformer.

~Matt

But it doesn't have to be with the right size transformer. I don't see why it could not be used in limited capacity but I don't recall ever having seen an application that did it.

Seems I remember Jim Dungar posting about there not being breakers rated for this or something like that.