LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

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Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

I came in late....

Is it 240V delta, corner ground

or 240V delta with a high leg??

That's important...

If it's corner ground...you can only use Phases A,C for single phase loads, because phase B is grounded.

If it's a high leg, you can only use the A,C for single phase loads, because the B phase is at 208V.

Mainly this system is for three phase loads, however for the high leg, 240/1ph loads are ok to use
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

To get the most useage out of this type of 120/240 3 phase delta electrical system, I have seen electricians use one of the 120 volt legs and the power leg (208v to ground) to feed a single phase 240 volt (with no nuetral) load. This puts some load on the power leg and takes some load off the 120 leg (phase A or Phase B, assuming Phase C is stinger leg; sometimes phase B marked as the power leg). The risk of this type of wiring configuration is that in event of a ground fault, you now has 208V to ground.

On another occasion I saw an installation of this voltage setup where someone grabbed a circuit to feed a single phase motor be putting a single pole breaker on the power leg. The nuetral was used for Line-2 making it a 208 volt circuit. The motor worked fine but I always questioned that installation because I wasn't sure the breaker was rated for that voltage. The breaker was a Square D QO 20/1.
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Lady,
If it's corner ground...you can only use Phases A,C for single phase loads, because phase B is grounded.
Click to expand...
I don't understand why this would be. Why can't a single phase 240 volt load be fed by A and B in a corner grounded 3 wire delta system? You would use a single pole 240 volt breaker and one of the feed wires to the load would be identifed per 200.6
Don
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Originally posted by don_resqcapt19:
Lady,
If it's corner ground...you can only use Phases A,C for single phase loads, because phase B is grounded.
Click to expand...
I don't understand why this would be. Why can't a single phase 240 volt load be fed by A and B in a corner grounded 3 wire delta system? You would use a single pole 240 volt breaker and one of the feed wires to the load would be identifed per 200.6
Don
Click to expand...
When I said single phase for A,C, I meant 120V single phase, because if you put a single phase, 1 pole breaker on the B phase, then you can't get any current because it is grounded. Yes, you are right you can use the A and B for a single phase 240V, but not just B for 120V.


Lady :)

[ June 03, 2004, 11:34 AM: Message edited by: lady sparks lover ]
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Lady -

How do you get 120V single phase from a 240V corner grounded Delta? This would be a new one on me.
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Originally posted by wanderer20001us:
Lady -

How do you get 120V single phase from a 240V corner grounded Delta? This would be a new one on me.
Click to expand...
I can see you guys are going to have fun watching me explain myself, but oh well.... :)

For 240V delta corner ground, it's 240V three phase, or 240V single phase, what I said was that you can't use the B phase as a single phase anyway, because it's ground, therefore your potential is zero!

For 240V delta high leg, you can get 120V, 240V, and 208V (only single pole). Only the high leg will produces 120V, single pole power.

If you don't understand this, try this website...
it's a little unclear, but it's gets the point or also try seimens, they are good at explain.

diagram
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Patricia,
Only the high leg will produces 120V, single pole power.
Click to expand...
the high leg is 208 as you already stated.

The center grounded winding will produce "end to end" voltage on the winding of 240 or 480 v (or other) depending on the transformer.

Now think of this winding by itself and measure voltage at the center point, this will be half of the "end to end" voltage, and no different than any single phase transformer.

The corner grounded Delta is simply "end to end" voltage of the windings with the common point of two windings referenced to ground.

Roger
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Lady,
For 240V delta corner ground, it's 240V three phase, or 240V single phase, what I said was that you can't use the B phase as a single phase anyway, because it's ground, therefore your potential is zero!
Click to expand...
The only way to get single pahse power from a corner grounded delta is to use two of the three legs. Any combination of two legs will produce 240 volt single phase power. The only difference is when using the grounded phase as one of the legs is that you only need a single pole breaker and the grounded conductor will be white or gray. When using the ungrounded legs for the 240 volt single phase circuit, you need a double pole breaker and neither conductor is white or gray. If you mean that you can't use the grounded phase without using another conductor, of course that is true, and it applies to all systems. A power circuit will require at least 2 conductors to serve a load.
Don
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

And to think this all started with a Load Calculation question..............

:D
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Originally posted by don_resqcapt19:
Lady,
For 240V delta corner ground, it's 240V three phase, or 240V single phase, what I said was that you can't use the B phase as a single phase anyway, because it's ground, therefore your potential is zero!
Click to expand...
The only way to get single pahse power from a corner grounded delta is to use two of the three legs. Any combination of two legs will produce 240 volt single phase power. The only difference is when using the grounded phase as one of the legs is that you only need a single pole breaker and the grounded conductor will be white or gray. When using the ungrounded legs for the 240 volt single phase circuit, you need a double pole breaker and neither conductor is white or gray. If you mean that you can't use the grounded phase without using another conductor, of course that is true, and it applies to all systems. A power circuit will require at least 2 conductors to serve a load.
Don
Click to expand...
I already said that Don, if you carefully looked at my post. I just didn't go into detail like you did, that's all. ;)

Let me make it plain....

240V corner ground delta:
240V/1phase (any combo of phases)
240V/3phase

240V high leg
120V/1phase (only of phases A,C)
240V/1phase (any combo of phases)
240V/3phase
208V/1phase/1 pole (usually on phase B)

Seesh, I may not have the time in or the experience like all of you, but I do know this system, because I've had to deal with it on a number of occassions. ;)
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Greg,

Originally posted by ca_c10:
And to think this all started with a Load Calculation question.............. :D
Click to expand...
with the exception of me being a jerk, this is a good thread. :D

Don,
Not that I can remember something like this, I just don't see a code reason why you couldn't do it.
Click to expand...
I can't remember ever seeing a 4 wire Delta connected piece of equipment either, and maybe my statement of not being code compliant was a little strong, but I was thinking more on the terms of cord and plug connected.

Roger
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Using the diagrams m'Lady referred to in her earlier post, none of them actually show the installation as originally described. The 240V Delta-Stinger is very close. Assuming from the diagram that the node opposite the grounded center-tap is "B," an "open-delta" is created by removing one of the transformer coils between "B" and either of the other "phase" nodes; i.e., only two transformer windings are used.

This is still a "High-Leg Delta" application.("B" phase is still identified with orange.)

The application for this configuration is where the total single-phase load is much larger than the three-phase load. A single, much smaller, "stinger" coil can be selected to accommodate the three-phase loads.
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Maybe this will help!

4delta.jpg

The single-phase voltage on each side of the half-tap is one-half the voltage available in the normal phase-to-phase relationship. This provides the same half- or full-voltage arrangement seen in the normal Edison scheme with a grounded neutral. Notice also that the legs coming from the corners of the Delta would have a normal ungrounded appearance if it were not for the center tap of one phase. Thus, at any given location in the system, either three-phase power at full voltage or single-phase power with half or full voltage is equally possible. However, there are several strict precautions that must be observed in the operation of this system. First, all loads must be carefully balanced on both the single-phase and three-phase legs. Second, because the voltage between one leg and the grounded neutral is considerably higher than the rest of the single-phase system, a measurement between the neutral and the phase must be taken to identify the "high leg," or "bastard voltage." Last, the "high leg" is never used as a single-phase source because no ground or grounded neutral exists for this circuit.
Click to expand...
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Originally posted by rbalex:
Using the diagrams m'Lady referred to in her earlier post, none of them actually show the installation as originally described. The 240V Delta-Stinger is very close. Assuming from the diagram that the node opposite the grounded center-tap is "B," an "open-delta" is created by removing one of the transformer coils between "B" and either of the other "phase" nodes; i.e., only two transformer windings are used.

This is still a "High-Leg Delta" application.("B" phase is still identified with orange.)

The application for this configuration is where the total single-phase load is much larger than the three-phase load. A single, much smaller, "stinger" coil can be selected to accommodate the three-phase loads.
Click to expand...
I think I explained 240V/ 3phase (High leg and corner ground) in my other post, the limitation of these systems, and what types of loads can be connected to them and which phases. If I'm wrong in what I wrote, correct me.... :)

[ June 04, 2004, 05:22 PM: Message edited by: lady sparks lover ]
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

No M'Lady,

There was never a question that you knew what you were talking about.

And if I'd known how to do it yesterday I would have simply added this diagram and kept my mouth shut ;) :

Open%20Delta.gif
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

At the risk of being boring, this is what I wrote in our training module on transformers.

LOADING AND CONVERSIONS

Splitting the load on open delta and closed delta banks is not intuitive; however it is simple to do. The following formulae and examples show how to split the load on open delta and closed delta banks, how to convert from one to the other, and how to convert measured amperes for a service to load open delta and closed delta banks.

Loading an open delta bank

Place all of the single phase load on the lighting (larger) transformer and split the three phase load by multiplying it by the inverse of the square root of three (.577) to add to the lighter and the kicker (smaller).

Example:

75 kVA of single phase and 30 kVA of three phase load.

Lighter Kicker
Single phase 75.0 kVA 0.0 kVA
Three phase 17.3 kVA 17.3 kVA
Totals 92.3 kVA 17.3 kVA

Loading a closed delta bank

Split the single phase load by placing two thirds on the lighting (larger) transformer and one third on the kicker (smaller) transformer. Split the three phase load by placing one third on the lighter and each of the kickers.

Example:

30 kVA of single phase and 60 kVA of three phase load.

Kicker Lighter Kicker
Single phase 10.0 kVA 20.0 kVA 10.0 kVA
Three phase 20.0 kVA 20.0 kVA 20.0 kVA
Totals 30.0 kVA 40.0 kVA 30.0 kVA

Converting from an open to a closed delta

1. Using test data, subtract the kicker (smaller transformer) load from the lighter (larger transformer) load to get the total single phase load.

2. Multiply the kicker load by the square root of three to get the three phase load.

Use the results for calculating the loading for a closed delta bank.

Example:

From a test on an open delta bank, the lighter has 60 kVA and the kicker has 21.6 kVA of load.

Subtracting the kicker load from the lighter
60 kVA ? 21.6 kVA = 38.4 kVA

Multiply the kicker load by the square root of three
(Sq. Rt. 3)(21.6 kVA) = 37.4 kVA

Your loads are 38.4 kVA single phase and 37.4 kVA three phase.

Converting from a closed to an open delta

1. Using test data, subtract the kicker (smaller transformer) load from the lighter (larger transformer) load, then multiply the resultant by three to get the total single phase load.

2. Subtract one third of the single phase load from the kicker, then multiply the resultant by three to get the three phase load.

Use the results for calculating the loading for an open delta bank.

Example:

From a test on an open delta bank, the lighter has 76.8 kVA and the kickers have 64.8 kVA of load each.

Subtracting a kicker load from the lighter and multiplying by three
(3)(76.8 kVA ? 64.8 kVA) = 36.0 kVA

Subtracting one third of the single phase load from the kicker and then multiplying the resultant by three
(3)(64.8 kVA ? 12.0 kVA) = 158.4 kVA

Your loads are 36.0 kVA single phase and 158.4 kVA three phase.

This, of course, could not be served with an open delta bank because of the size of the loads.
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Originally posted by rbalex:
No M'Lady,

There was never a question that you knew what you were talking about.

And if I'd known how to do it yesterday I would have simply added this diagram and kept my mouth shut :)
Click to expand...
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

Is it safe to pick up a branch circuit for HID lighting or even for a single phase motor by grabbing the power leg (208V) and the neutral and using a plug on single pole breaker? This would be one way of using some of those blank breaker positions in a 3 phase-4 wire panel.
 
Re: LOAD CALCULATIONS FOR 120/240, 3 PHASE DELTA SERVICE???

From Dave's (Websparky) quote: Last, the "high leg" is never used as a single-phase source because no ground or grounded neutral exists for this circuit.
Click to expand...
Additionally, you would need a full voltage rated single pole circuit breaker.

Lastly, you would be putting something together that would be confusing to another electrician sometime in the future. Generally speaking, use the KISS method for installing electrical circuits and it will be easier to troubleshoot and safer for the next guy. :D
 
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