3 phase Delta, phase B high leg with A & C center tapped.
Grouped service disconnects,
1. 400a two pole breaker on phase A & B serving a 400a single phase lighting and appliance board.

2. 100a 3 phase breaker serving the HVAC panel.

Service conductors are (2) parallel 250 Kcmil on A & C and a #3 on the high leg B

http://esteroriverheights.com/electrical/3p_delta_vee_downsize_b.jpg

Off the top, it looks good to me.

Panels are generally not listed for serving single phase circuits when used with a delta service. Using the panel to feed a separate single phase panel is just one code compliant way .... splitting the feed at the meter, or using a gutter, have their own issues.

Which brings up wire sizing. I can't imagine any way the #3 would draw more than 100 amps.

Side note: Every such service I've seen has actually had only two transformers (open delta). The (much smaller) second transformer also limits the power available to that 208v wire.

Good point on the service disconnect, thanks


Try it now

Looks OK to me .... especially if there are only 2 transformers, there's no way that small wire is ever going to pull more than 100 amps.

Indeed ... and I know this sounds 'backwards' .. but the PoCo might not want parallel 250's on the 'high' leg.

On 'this side' of the meter, the main concern - it seems to me - is that we prevent any possible accidental 208v single-pole circuits. There's no issue with 2-pole circuits ... those will be 240v, no matter what buss they come from. Because of the difference in transformer sizes, a conscientious sparky would make sure to draw his 240 single-phase loads from "A" and "C" legs - but I don't think that's a requirement of any code.

I suppose one could connect a dryer to "A" and "B", but you would have to make sure that the dryer's 120V motor sees the "A" phase, and not the "B" phase. And that would be a problem when the dryer is replaced years from now. 240V only baseboard electric heaters would not have that problem, and should work fine. Another issue would be that the power company would probably perfer similar 240V loads not be across both transfomers, but just be across the actual high leg transformer. Any way for a sparky to know which pair of wires to select, for in this example "A" to "B" or "B" to "C", to only load the high leg transformer? "A" to "C" would be obvious, but that's boring...

I suppose you've hit on why things are the way that they are ....

Three phase is pretty unusual in residential areas. It's almost unheard of to a house itself; before the PoCo will supply three phase to a house, there is a tremendous amount of paperwork.

When three phase is an afterthought - that is, the PoCo didn't expect for there to be a need for three phase at the site, and the three phase loads are specific - then the PoCo is likely to save a few dollars, and create it using only two transformers. Again, a large part of the paperwork the PoCo will want will focus on exactly how the delivered power will be distributed.

Unless he can see the transformers up on a pole, there is no way for a sparky to tell if he is dealing with a "delta" or "open delta" service. The only indication that a leg is overloaded will be the voltage drop during peak loads.

For economy, the PoCo likes 'open delta' three phase services. That's one less transformer to buy. By any other criteria, the "Wye" style is much preferred : three transformers, every leg the same voltage to ground.

For the "dryer" example, we look back to the difference between an "appliance" panel and a"power" panel. A "power" panel, in practice, lacks a neutral - so there will be no 120v loads. This is one case where it pays to read all that fine print on the panel labels

The setup is like many older commercial services.

SoCA Edison hasn't used them in decades, but many are still in service.

It is very unusual for a house to have 3 phase. But there's an older development in my town that was built in the late 50s, and the houses had this new feature called "central air". High end houses. I suppose the only equipment that was avaliable to do that job was equipment intended for commercial office buildings, and that needed 240V delta. So they have 208V wild leg inside the house.

Things would get a little "interesting" if a DIYer homeowner there picks the wrong slot of the wrong panel to run a new circuit...

I guess this is the place to bring this up:

Most old timers (of which I am one) would not install a single phase breaker in a 3 Phase delta panel where one of the phases is center taped and you have a high leg with 208 to ground.. There are several reasons for this other than the obvious. First of all it's not a violation when you don't involve the high leg. Second of all, you will have a problem if the circuit is 240v and you use the high leg. (see 240.85). I typically would install a 3 Phase panel rated 240/120v for the 3 phase loads and then a single Phase pane; rated 120/240v for the single phase loads and then you will never have a problem with the high leg.
If you have a single phase 240v. breaker with a "slash rating" the higher voltage is the voltage line to line and the lower voltage is phase to ground. If this helps.

There is a neighborhood in sw Houston built in the 1950's that has lots of homes with 3p. I see lots of odd breaker panels. like a panel with a 3p main lug but with only 1 3p breaker spot, 4 2pole breakers and a split buss sub feed for lighting. I also seen mostly 2p main lug (also split buss) with a 30 or 60a fused switch for the 3p. Most of these homes used to have AC with a water chiller (tower) outside and pumps and compressor (non hermetic) in the utility room. Homes are only single story ranch 2000-3100 sf. Lots of the AC units been replaced with 1p unit mostly on undersize wiring due to replacing a 3p with a single phase using the existing 3 p ckt like 5ton unit on 12.

These are hard to replace the service (expensive) The City requires all homes to have only 1 main disconnect. You must install a 3 pole disconnect to handle the full load and then separate panels for single and 3P.

I've also seen quite a few 120/240 Delta installations in Northern NJ, in fact I learned the hard way on my first experience with one in Park Ridge at a lighting store.

These services are also popular in North Carolina in 50's era homes. As a kid, I was always curious as to why they had two 4/0, a 2/0 neutral and a #2 for the third leg. Now I know why. The GP circuits, range, etc. are fed from the single phase 200A panel and the electric heat/HWH loads run from a separate panel using only the high leg. I am sure that dates back to the days when lower rates were offered for heat/HWH.

That is one beast that you need to tame before you touch it for sure.

With today's proliferation of DIY shows on TV and home centers, I seriously doubt that we'll ever see a new 3-phase service in a residence again. Even in high-rise condos, I'm sure that single-phase is all that we'll ever see. There's too much risk with Harry Homeowner.