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

[HG_Phenom]

Along those lines, consider having 3 of the 30 tenants on a 25 kVA single-phase transformer fed by phases A-B (i.e., A and the high leg), and another 3 tenants on a 25 kVA fed by B-C. The remaining 24 tenants would be put on A-C of the 4-wire delta service. It appears this would reduce the line currents on A and C drawn by the apartments from 1040A down to 890A. There would be 181A on the high leg phase B feeding the two transformers.
Doing this would reduce the total required transformer kVA by a factor of 5 from that of a 240V delta - 208Y120V transformer that feeds all of the apartments.
The transformer kVA's mentioned above are minimum required values.

Hmmm.

But would this not go against the NEC code in order to apply demand factor on total line load?

 

[HG_Phenom]

Hmmm.

But would this not go against the NEC code in order to apply demand factor on total line load?

Now that I think about it, I do recall some general language on the department of building and safety electrical plan check corrections side requiring "reasonably" balanced load design across the system.

Anyway, I will still propose this tomorrow and see what everyone says.

Thanks again for all feedback and ideas so far.

 

[winnie]

I see where you are going on the demand factors, and it is an issue.

With the plan synchro proposed you would probably need to apply separate (less desirable) demand factors for each separate transformer group of tenants.

Going to the option of feeding 3 phase to each unit, did you remember that the water heater (and any pure 240V load) can be connected to the high leg? If you can get pure 240V ranges, these could also go on the high leg. Only loads that require the neutral would be on the 'single phase' part of the system.

Jon

 

[electrofelon]

Also from what you mentioned, unfortunately what you're missing is all these are including NEC 220.84 demand factor of 0.32. once you feed other units differently, you lose the low demand factor and assumed load goes much higher.

I don't follow this. I don't see that using a transformer on several units affects use of the 220.84 demand Factor. I see nothing in there Prohibiting feeding some units "differently".

Have you run the numbers with running the three phase high leg Delta to some units and loading up the high leg as much as possible on those? I don't know what your loads are, the water heater is definitely okay on the high leg, range is probably not unless you can find special ones, but what about electric heat and AC? Seems like that is by far the simplest way to go and shouldn't require too many units to peel off that 40 amps you need.

 

[zbang]

I don't recall seeing this, but how does DWP feel about single-phase loads on the high leg? (and if it's an open delta, putting any on the open side is not a good idea)

 

[winnie]

I don't follow this. I don't see that using a transformer on several units affects use of the 220.84 demand Factor. I see nothing in there Prohibiting feeding some units "differently".

IMHO the demand factor for a feeder only applies to the number of units served by that feeder. So if the OP 'peels off' 6 units from the main feeder (the 'single phase' part of the delta) then instead of using the demand factor for 30 units he has to use the demand factor for 24 units.

Because this plan creates an intentionally unbalanced load (bunch of single phase units on the single phase leg of the delta, other units using all legs) I'm not sure how you would apply the demand factors. On the other hand the demand factors change pretty slowly after the first few units, so perhaps this isn't a big deal.

I don't recall seeing this, but how does DWP feel about single-phase loads on the high leg? (and if it's an open delta, putting any on the open side is not a good idea)

Always good to check assumptions at the door!

I've been assuming that given this is intended for an apartment building, that DWP is providing something intended for single phase 240V loads and lots of 120V L-N loading on the 'single phase' leg of the Delta. But both of these assumptions bear confirmation. It may be the case that the OP is forced by the service characteristics to use transformers for the entire load.

 

[electrofelon]

IMHO the demand factor for a feeder only applies to the number of units served by that feeder. So if the OP 'peels off' 6 units from the main feeder (the 'single phase' part of the delta) then instead of using the demand factor for 30 units he has to use the demand factor for 24 units.

Because this plan creates an intentionally unbalanced load (bunch of single phase units on the single phase leg of the delta, other units using all legs) I'm not sure how you would apply the demand factors. On the other hand the demand factors change pretty slowly after the first few units, so perhaps this isn't a big deal.



.

Sure but it seems like the the big limitation in this case is the service size so as long as you can apply the entire demand factor to the service, you don't really care about the feeders

 

[electrofelon]

I don't recall seeing this, but how does DWP feel about single-phase loads on the high leg? (and if it's an open delta, putting any on the open side is not a good idea)

Now that's an interesting idea actually. If you could feed say the water heaters line to neutral off the high leg on these hypothetical several units that have three phase, you could transfer a bunch of load right to that high leg. Another small advantage is you could use regular 4500 watt at 240 elements and they would be about 3,400 Watts at 208. Do that to three or four of the units you'll be golden.

 

[HG_Phenom]

As a clear reference point on how skinny our load calc demands have gotten (some of which are not even feasible to find in the marketplace, but let's ignore that for now), I've attached a picture below. Maybe we missed something that others notice which can be cut down but I believe this is the absolute minimum power for each item or that may be required in the calc as per NEC.



Also, just to remind some of you of the minimum mandatory requirements and how NEC 220.84 works to allow for demand factor: 1 feeder to unit, electric stove/oven, and space-heat/ac required -- See code picture attached here.

Now that's an interesting idea actually. If you could feed say the water heaters line to neutral off the high leg on these hypothetical several units that have three phase, you could transfer a bunch of load right to that high leg. Another small advantage is you could use regular 4500 watt at 240 elements and they would be about 3,400 Watts at 208. Do that to three or four of the units you'll be golden.

Yea, we've already assumed we are putting a boiler in the building with gas so the water heater is now out of the equation with the demands previously mentioned we are at.

I don't recall seeing this, but how does DWP feel about single-phase loads on the high leg? (and if it's an open delta, putting any on the open side is not a good idea)

from what I recall, it is an open delta. They did however mention previously that nothing both an extremely nominal load can go on the high "wild" leg and only if properly protected. Will ask again though to confirm.

 

[wwhitney]

Due to significant site conditions and constraints, off-site transformation is needed from service coming from the utility company, DWP.

Is that to say that with on-site DWP transformation, the space required is greater, or the flexibility of where to put the transformers is less, compared with customer-owned transformers? Because some of the discussion has included customer-owned transformers.

If DWP's limit is insurmountable, then you need to start with the maximum allowable load on the center-tapped delta on each leg. That will tell you the allowable load in kVA. I like to think of it as a 3-phase delta service on top of a center-tapped single phase service. So if the maximums are, say, 1000A, 1000A, and 100A (high leg), it's the same as a 100A 3-wire delta service plus a 900A 3-wire center-tapped single phase service. Which would have an available kVA of 900*240 + 100 * 240 * sqrt(3) = 258 kVA.

Then if you can get your total kVA below that limit, and if you can allocate the load to the 3 different legs to stay within the limits of each leg, you have a solution without any on-site transformers. If you can get your total kVA below that limit, but can't distribute the loads among the different legs to meet all the individual limits, you should be able to use 1 or 2 hopefully small transformers to shift the load distribution to match the service limits. And if you can't get the total kVA below that limit, you have no solution.

Cheers, Wayne

 

[winnie]

from what I recall, it is an open delta. They did however mention previously that nothing both an extremely nominal load can go on the high "wild" leg and only if properly protected. Will ask again though to confirm.

Umm that changes the game _completely_.

The whole point of putting a 240V delta to 208V wye transformer on this setup is to put more load on the high leg. If you go this route and then balance your loads properly, then the load on the 'wild leg' will _equal_ the load on the main legs. You use a 240V delta to 208V wye if you have a true three phase service and need to balance its loading

What you describe is basically a single phase service with a bit of three phase capability tacked on.

-Jon

 

[winnie]

I think that you've done your maths a bit wrong in the load calc.

You calculated the VA required in the first portion of 'MSB'. You then divide by 208 and 1.732 to get amps. But you don't have 208V, you have _240V_, and you don't have to assume that your MSB loads are evenly distributed on the three phases.

For the motors on MSB you have amp numbers. You added all of these amp numbers up and then added them to your heavily loaded 'single phase' base. But you don't have a single phase service, you have an unbalanced three phase service with some (unknown) high leg capacity.

Call your phases A, B, C with C being the 'high leg'.

If you supply all of the tenants from the single phase base (A,B,N), your calculated load is 904A. This lets DPW separately meter every tenant.

Now the question is what to do with 'MSB'. I calculated that MSB has a total of 188A of 240V single phase loading.

I propose that MSB be the only three phase service, but with only single phase loads. You would connect these loads 240V A-C and B-C only, with _no_ A-B loading. Any 240V load simply gets connected, any 120V circuits would be fed via a transformer. This puts 84A on each of A-C and B-C.

The load on C is 84 * 1.732 = 146A (not the full 188A because of the phase angle difference between the A-C and B-C loads).

The load on A is 904 vector added to 84A at 30°, which works out to 978A (again less than 988A because of the phase angle difference).

So, can the C leg handle 146A? Can you get DPW to provide you with a 1000A, 1000A, 200A high leg service?

-Jon

 

[kwired]

Now that's an interesting idea actually. If you could feed say the water heaters line to neutral off the high leg on these hypothetical several units that have three phase, you could transfer a bunch of load right to that high leg. Another small advantage is you could use regular 4500 watt at 240 elements and they would be about 3,400 Watts at 208. Do that to three or four of the units you'll be golden.

You probably going to need 480/277 breakers and panel to find a single pole breaker that can be connected to 208 volts.

 

[electrofelon]

You probably going to need 480/277 breakers and panel to find a single pole breaker that can be connected to 208 volts.

Nah it's no biggy, they have breakers for that.
Probably cheapest solution is just use a three pole as they are all straight rated. Siemens has a straight rated 2 pole, i.e. Q230R. Might want to just stay away from home line, they might not have one as that is kind of a specifically residential line.

 

[synchro]

Call your phases A, B, C with C being the 'high leg'.

If you supply all of the tenants from the single phase base (A,B,N), your calculated load is 904A. This lets DPW separately meter every tenant.

Now the question is what to do with 'MSB'. I calculated that MSB has a total of 188A of 240V single phase loading.

I propose that MSB be the only three phase service, but with only single phase loads. You would connect these loads 240V A-C and B-C only, with _no_ A-B loading. Any 240V load simply gets connected, any 120V circuits would be fed via a transformer.

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.

 

[kwired]

Nah it's no biggy, they have breakers for that.
Probably cheapest solution is just use a three pole as they are all straight rated. Siemens has a straight rated 2 pole, i.e. Q230R. Might want to just stay away from home line, they might not have one as that is kind of a specifically residential line.

two and three pole yes, single pole I don't think you will find a straight 240 breaker (for commonly found loadcenters/panelboards) will need to jump to the 277V class breakers and panelboards.

 

[winnie]

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

 

[kwired]

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

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.

 

[electrofelon]

two and three pole yes, single pole I don't think you will find a straight 240 breaker (for commonly found loadcenters/panelboards) will need to jump to the 277V class breakers and panelboards.

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.

 

[kwired]

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.

Can, but you are burning one or two other spaces in the panel if you don't want common handle/trip for other circuits