3 phase Delta High Leg Single Phase Load
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
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.
Thanks
Jeff Neithammer
Electrical Engineer
Advanced Engineering, Inc.
- Location
- Lockport, IL
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- Retired Electrical Engineer
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.
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.
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
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
Senior Member
- Location
- London, England
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.
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
Senior Member
- Location
- bay area, ca
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
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
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." :roll:
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
Senior Member
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.
noted in post #5 ... as I recall that's the Code stumblimng block
- Location
- Wisconsin
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- PE (Retired) - Power Systems
The use of a 277/480 breaker would probably be okay for the 208V and it is likely that a 2P 240V, not slash rated, might work. But, I am not aware of any "listing" for a single pole breaker at 208V L-G.
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well first if you have a 120 /240 volt system how would one install a 208 volt breaker in the panel in the first place all the panels are rated at 240 volts ?
But let me try explain why its not done in a three phase delta the winding C is larger than A or B winding ampacity wise due to it carrying a larger current load it has the three phase loads and the 120 volt loads to handle .
If one added a high leg load to B phase and C had to carry that excess load its a increase current flow on the neutral which is a magor unbalance on transformer it can burn out not only the center tap but puts a extra load on the smaller windings on B . Its bad news B for bad
But let me try explain why its not done in a three phase delta the winding C is larger than A or B winding ampacity wise due to it carrying a larger current load it has the three phase loads and the 120 volt loads to handle .
If one added a high leg load to B phase and C had to carry that excess load its a increase current flow on the neutral which is a magor unbalance on transformer it can burn out not only the center tap but puts a extra load on the smaller windings on B . Its bad news B for bad
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- Location
- Illinois
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- retired electrician
Yes, even the ones we use on 208Y/120 volt systems are rated for 240 so that is not an issue. The only issue is a single pole breaker rated at 208 or above that will fit in the panel.
mivey
Senior Member
Both smaller windings would share the load, along with the lighting pot winding.
Any tranformer will burn up if overloaded enough, even with standard connections. Using the high leg would be fine if you considered the reduced capacity, much the same way you have to take into consideration the single phase loads on a three phase bank.
That said, it is not the best way to get things done.
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well just a tought
A WYE connected winding delivers the same power with less line current than a delta connected system meaning balanced wye .
Line voltage is equal to phase voltage line current is equal to phase current balanced.
with a delta two windings suffer reduced voltage while one is 208v meaning hi leg a wye connected load sees this and there would be a unbalance .
Resulting load burn out! one leg current is lower two legs current is higher balance is a problem on load and transformer .
But iam not a engineer so please help me out thats my way of looking into the future it may work but not effective way to go and thats why ?
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RICK NAPIER
Senior Member
- Location
- New Jersey
I believe using the high leg would be a violation of 110.3(B) as the transformer does not list 208 as one of it's voltages.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The windings in a delta connection do not suffer reduced voltage. They see the voltage they are designed to see.
You are correct that a true 3PH 4W wye load (which thankfully is rare) should not be connected to a 3PH 4W delta supply. And definitely, you should not run a nominal 208V load on a nominal 240V system. But, the discussion, up to now, has been on connecting a 1PH 208V load to the high leg and grounded conductors.
mivey
Senior Member
If you overload the transformer, it will be a problem. With the properly sized transformer, you could serve a 208 load from the high leg without damaging the transformer or causing significant issue with other loads served by the transformer. But that is the case with any transformer and mixed loads.
I guess we could draw out the currents and determine the proper size for a given set of loads, but you could always find an adequate size (within reason). Putting unbalanced loads on three phase transformers always causes issues, high-leg or not. Just because there is a high-leg load does not mean it will burn stuff up.
Hmm... I consider that rather vague on the matter.
I've never seen a listing, labeling, or instructions which says not to connect a 1? hi-leg to neutral circuit. However, I seldom read instructions in their entirety during commonplace equipment installations any more. It seems rather redundant after installing several
So that said, you show or point me to the documentation and I'll agree. Yet you will have to concede that if it only concerns a particular piece, or manufacturer's line of equipment, that does not mean all equipment of its kind.
Then there's...
If we have a 208V supply (I don't see where it is required to be a nominal 208V supply) and a 208V load, it's a 208V circuit... and with the equipment rated 240V or better, there's no problem here.
Up 'til now, everyones been talking about an adequately-rated (= or >208V) 1P breaker. What about a 240V 2P or 3P breaker with 2 or 3 individual circuits—or even 1 208V 1? circuit for that matter—as long as the breaker is rated for full 240 (i.e. not 120/240 rated)???
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