240V Delta/120V 3-Phase, 4-Wire to 208Y/120V 3-Phase, 4-Wire

[wwhitney]

I like your idea of putting the MSB single phase loads across just A-C and B-C. These load currents I_A-C ≈ I_B-C would each be 60° from the 904A on A-C. Because each of these load currents will be substantially less than I_A-B = 904A, their component in quadrature (at 90°) to I_A-B will end up phase shifting the total line currents on A and B a small amount, but add little to its magnitude. The components of the load currents I_A-C and I_B-C that are in-phase with I_A-B will each be down by a factor of sin(30°) = 1/2. And if these two in-phase current components are equal, they will flow through one load across to the other without impacting the current on C. And so their current component in-phase with I_A-B = 904A will be the same as if these two loads were in connected in series across the 240V on A-B, and not connected to high leg phase C. So, for example, if I_A-C = I_B-C = 100A, then they would contribute a total of 50A to the I_A-B current.

That's a lengthy way of saying that when vector X and Y are 60 degrees apart, and |Y| << |X|, then |X+Y| = |X| + 0.5 |Y|, to good approximation. Since |X+Y| = sqrt( (|X|+0.5|Y|)² + 0.75|Y|² ), and |Y|² << |X|², so the last term is negligible.

If C is the high leg, and the supply is open delta with, say, A-C open, what is best as far as the relative loading of A-C vs B-C? 1:1, or more heavily load B-C, which has the transformer? Assuming all single phase loads that may not be concurrent. [Versus a 3 phase load that will load A-C and B-C equally.]

Cheers, Wayne

 

[winnie]

Right you probably won't find a straight rated single pole, but you just use one pole of a two or three pole, no need to go to 277 / 480 class equipment.

But why? If you are using a 2 pole breaker why use only 208V rather than the 240V from the breaker you are using?

-Jon

 

[kwired]

But why? If you are using a 2 pole breaker why use only 208V rather than the 240V from the breaker you are using?

-Jon

That too.

Exception would be something that must have 208 instead of 240. Not too many items but there are some out there. Cove heaters is one I have encountered, at least from one particular manufacturer, they have 208 240 and even 277 volt versions in same size/watts. Warranty is void if you connected a 208 unit to 240 volts.

 

[electrofelon]

But why? If you are using a 2 pole breaker why use only 208V rather than the 240V from the breaker you are using?

-Jon

See posts 45 and 48.

 

[winnie]

See posts 45 and 48.

See post 57

If you run a load from C to N you don't load the A or B wires, but you do load the AB transformer coil. So you don't actually get any additional service capacity unless the limiting factor is the _wire_ from the transformer.

-Jon

 

[electrofelon]

See post 57

If you run a load from C to N you don't load the A or B wires, but you do load the AB transformer coil. So you don't actually get any additional service capacity unless the limiting factor is the _wire_ from the transformer.

-Jon

Right well of course you cant put load ONLY on the C phase. Part of this was theoretical in response to claims that there was no equipment available without going up to 277/480 class equipment. I guess one practical advantage in the case of the OP is the reduced wattage that would result from powering a resistive load at 208 instead of 240. Possibly this could result in the less wattage the OP needs without having to change out water heater elements or order less common equipment.

 

[HG_Phenom]

Thanks all for the continued feedback and support.

Call with DWP district Supervisor went better than expected . He provided us with the following from his handbook A) "A combination of 240-volt, three-phase and 240/120 volt, single-phase service will normally be furnished through four Wires. The fourth Wire will be grounded. The single-phase portion of the load shall be reasonably balanced between the phase Conductors with respect to the neutral Wire. The three-phase portion of the load shall be reasonably balanced between phases. When service is supplied in this manner, large motors, welders, or x-ray equipment, whether single- phase or three-phase, may cause Circuit Voltage fluctuation (light flicker)." B) "For 120/240 volt 3 4-wire delta services, the power leg (measuring 208 volts-to-ground) bus shall be identified with an orange color, or by tagging or other effective means."

He actually wasn't 100% sure if it was open or closed Delta service they would provide but said he's "pretty sure it's open".

At the end, he left us with "I am starting to understand the dilemma and let me see what I can do for YOU on capacity and switchboard thershold..." We don't want to count our chickens, but this is more hopeful that anything we have heard from them in 1.25 years

I like your idea of putting the MSB single phase loads across just A-C and B-C. These load currents I_A-C ≈ I_B-C would each be 60° from the 904A on A-C. Because each of these load currents will be substantially less than I_A-B = 904A, their component in quadrature (at 90°) to I_A-B will end up phase shifting the total line currents on A and B a small amount, but add little to its magnitude. The components of the load currents I_A-C and I_B-C that are in-phase with I_A-B will each be down by a factor of sin(30°) = 1/2. And if these two in-phase current components are equal, they will flow through one load across to the other without impacting the current on C. And so their current component in-phase with I_A-B = 904A will be the same as if these two loads were in connected in series across the 240V on A-B, and not connected to high leg phase C. So, for example, if I_A-C = I_B-C = 100A, then they would contribute a total of 50A to the I_A-B current. The current on phase C would be 2 x cos(30°) x 100A = 173A.

I think the elevator current might still need to be 3-phase, but the OP would need to confirm this.
Your point about the supply voltage being 240A vs. 208V for loads MSB suggests that these loads should be reviewed to make sure they are compatible voltage-wise.

The traction elevators these days needed are all running at 208V 3-phase. TKE has some from Europe that will run at 230V but wanted to use a converter that didn't make sense

There is generally no good reason to connect loads high leg to neutral.

Yes it gets you 208V without adding load to the A and B phase wires, but it loads the AB leg of the transformer, so you don't really increase capacity. Your grounded conductor is no longer a neutral and must be increased in size. About the only benefit is using a single pole breaker rather than double pole.

IMHO if the OP uses the high leg it should be for two and three pole loads.

Jon

yup, definitely agree with you on that

The branch circuit grounded conductor carries same current as the ungrounded conductor, it would be the grounded service and feeder conductors that very possibly would need increased in size as they are no longer "neutral" in relation to all the ungrounded conductors.

How much effect this current has can be worse on an open delta secondary than on a closed delta. Open delta there is only one current path for high leg to neutral conductor - the closed side. Full delta it can split both ways through the delta.

From our conversation with the department supervisor on Thursday, seems to be an open delta service but will find out more next week

 

[hillbilly1]

The last traction elevator I done was 240 single phase, I had 240 three phase in the slab, when the elevator arrived, it was 240 single phase, and used a freq drive to convert it to three phase! So now my feed was too small, and I had to run it overhead 200’!

 

[kwired]

Thanks all for the continued feedback and support.

Call with DWP district Supervisor went better than expected . He provided us with the following from his handbook A) "A combination of 240-volt, three-phase and 240/120 volt, single-phase service will normally be furnished through four Wires. The fourth Wire will be grounded. The single-phase portion of the load shall be reasonably balanced between the phase Conductors with respect to the neutral Wire. The three-phase portion of the load shall be reasonably balanced between phases. When service is supplied in this manner, large motors, welders, or x-ray equipment, whether single- phase or three-phase, may cause Circuit Voltage fluctuation (light flicker)." B) "For 120/240 volt 3 4-wire delta services, the power leg (measuring 208 volts-to-ground) bus shall be identified with an orange color, or by tagging or other effective means."

He actually wasn't 100% sure if it was open or closed Delta service they would provide but said he's "pretty sure it's open".

At the end, he left us with "I am starting to understand the dilemma and let me see what I can do for YOU on capacity and switchboard thershold..." We don't want to count our chickens, but this is more hopeful that anything we have heard from them in 1.25 years


The traction elevators these days needed are all running at 208V 3-phase. TKE has some from Europe that will run at 230V but wanted to use a converter that didn't make sense

View attachment 2560957



yup, definitely agree with you on that


From our conversation with the department supervisor on Thursday, seems to be an open delta service but will find out more next week

There is only two advantages to using open delta.

First is for when majority of load is single phase yet there is some three phase loads to supply.

Second is generally out in rural locations when there is need for three phase, you have relatively small capacity of service and you don't have all three primary phases nearby, you can build a line with just two phase conductors and neutral and supply an open delta bank.

I guess a third situation that we do have around here is limited capacity three phase loads where POCO would rather use two instead of three transformers in the bank even though all three primary phase conductors are on the same pole as the bank. Lot of center pivot irrigation services are just that, but the service capacity of those is usually 30 amps @ 480 volts. Demand load is even less, guys running same systems on generators driven by the diesel motor for the well pump are typically only using about a 10 or 12 kVA generator.

 

[HG_Phenom]

There is only two advantages to using open delta.

First is for when majority of load is single phase yet there is some three phase loads to supply.

Second is generally out in rural locations when there is need for three phase, you have relatively small capacity of service and you don't have all three primary phases nearby, you can build a line with just two phase conductors and neutral and supply an open delta bank.

I guess a third situation that we do have around here is limited capacity three phase loads where POCO would rather use two instead of three transformers in the bank even though all three primary phase conductors are on the same pole as the bank. Lot of center pivot irrigation services are just that, but the service capacity of those is usually 30 amps @ 480 volts. Demand load is even less, guys running same systems on generators driven by the diesel motor for the well pump are typically only using about a 10 or 12 kVA generator.

gotcha, and yes, if this ends up being the case where they confirm they are providing open Delta, then it seems like our best bet would be to really push them up on the single-phase load they are currently stuck at providing us through off-site transformation to make this possibly work.

Initially, they would only allow us on this 240V Delta service a total 800A (600A 1-phase and 200A 3-phase). Currently, they are at 1000A (800A 1-phase and 200A 3-phase) and say this is the max we will do for you since you guys have been waiting for nearly 2 years on this. Hoping they will come around and allow us 1200A (1000A 1-phase and 200A 3-phase) and then I think we can somehow make that work.

The last traction elevator I done was 240 single phase, I had 240 three phase in the slab, when the elevator arrived, it was 240 single phase, and used a freq drive to convert it to three phase! So now my feed was too small, and I had to run it overhead 200’!

Can you share this model spec or cut sheet with me?? We have not been able to find any 15HP or 30HP traction elevators like you have mentioned. If we go the full single-phase route at 240V/120V (no Delta) and they do end up allowing us to get to 1200A, then this elevator you are mentioning may be the last piece of the puzzle to make this work

 

[augie47]

Have you discussed them providing you with (2) services. If they can provide you with 1200 amp single phase perhaps they can also give you a smaller 3 phase service even if it originates and terminates at a different location

 

[HG_Phenom]

Have you discussed them providing you with (2) services. If they can provide you with 1200 amp single phase perhaps they can also give you a smaller 3 phase service even if it originates and terminates at a different location

what do you mean by originates and terminates at a different location?

 

[winnie]

If you now have the engineer on side working with you, another possibility is two services, one the originally offered 600+200 delta, and a second 600A single phase.

You don't need a single unified service; your loads naturally divide.

Jon

 

[synchro]

If you now have the engineer on side working with you, another possibility is two services, one the originally offered 600+200 delta, and a second 600A single phase.

You don't need a single unified service; your loads naturally divide.

Jon

Two smaller services will also likely have lower available fault currents to contend with, an issue that you brought up earlier in this thread.

 

[wwhitney]

But two services with the units split between them will each have a lower diversity factor than a single large service, correct?

Cheers, Wayne

 

[winnie]

But two services with the units split between them will each have a lower diversity factor than a single large service, correct?

Cheers, Wayne

IMHO yes, if by lower you mean 'closer to 100%'.

The demand factor for 31 units is 32%. The demand factor for 15 units is 40%.

So if you need 1000A to serve 31 units you would need 15/31 * 40/32 * 1000 = 605A to serve 15 units and 629A for the other 16 units.

Jon

 

[kwired]

gotcha, and yes, if this ends up being the case where they confirm they are providing open Delta, then it seems like our best bet would be to really push them up on the single-phase load they are currently stuck at providing us through off-site transformation to make this possibly work.

Initially, they would only allow us on this 240V Delta service a total 800A (600A 1-phase and 200A 3-phase). Currently, they are at 1000A (800A 1-phase and 200A 3-phase) and say this is the max we will do for you since you guys have been waiting for nearly 2 years on this. Hoping they will come around and allow us 1200A (1000A 1-phase and 200A 3-phase) and then I think we can somehow make that work.

Since they appear to be giving you limits on single and three phase loads, seems they likely do have open delta or even closed delta but with larger transformer on the A-N-C side of the delta.

Might be that is largest transformer they want to go with, however maybe they could supply with more than one service, that would specifically be one the conditions in NEC to allow for more than one service - 230.2(C)(2) in particular.