Over in the "Large residential service question" topic, using 208Y/120 three phase was mentioned for mega mansions that have a massive service drop. Often this is a utility requirement due to the limitation on single phase service, sometimes in some places as low as 200 amps, though usually at least 400 amps.

I have experienced multiple complications due to the higher voltage changing from 240 to 208. Among these is lower heat from resistive devices like ovens, and motor burnouts. I'm wondering how these things have been dealt with in cases you might have encountered.

Another issue in upgrades is conversion of existing single phase panels to three phase.

My grandfather got three phase power to new his home around 1960. The purpose was to support some three phase equipment in his wood shop. But it was all on a single service, so the house had 120/208V as well. I don't remember if it had 2 of the phases or all 3 running in the house directly. I do remember that my grandmother's electric stove was slow to heat up, and this had been complained about as an ongoing problem. One year, there was a brownout in the summer. Being already on low voltage at 208 volts when intended to be on 240 volts, the brownout caused the air conditioner to burn out.

I believe he was expecting to get 240 volt delta with center tap and ended up with 208Y/120. He had trouble with some of the woodshop equipment, too.

One alternative I see is "triple split phase". That is, 3 separate 120/240 volt systems. It would be derived from an ordinary 3 phase supply by 3 single phase transformers. Each is then fed to their respective panels. You'd have to keep these 3 systems reasonably balanced.

Another alternative I see to keep single voltage 2-wire 240 volts stuff running well would be to buck-boost the 208Y/120 system up to somewhere near 240Y/139 and operate line-to-line from that to get 240 volts, possibly on a per-load or per-circuit basis with small transformers.

Mega mansion owners often get "industrial style" appliances, such as for the kitchen. Many of these are either the exact models, or domestic variations of models, intended for commercial/industrial use. So these might already be adapted for 208 volts, such as 208V heating elements for water heaters.

Even in a large residence I can only see there being a handful of appliances that require "240V" but most of them like electric water heaters and motors rated "200-230V" (as are most central A/C units I have seen) will operate on 208V with no problems.

The only place I have actually seen a buck-boost transformer used on "residental" appliances would be when I was in school, where the cooking classes had them on the electric stoves so they would heat up within time, and in the office where they had a window shaker A/C rated at 230V and would not start at 208V. Electric water heaters and the like were connected right to the 208V supply.

I think I would work with the PoCo engineering department and try for the center tapped delta. You do have to be careful with the load balancing if they only give you 2 transformers. If they do this in "delta vee" you can drag the wild leg around with unbalanced L/L 1p loads.
If this goes in as 208 wye the customer will need to replace a lot of equipment, either now or when it burns up. There is not a whole lot of consumer grade stuff that is tappable to 208

In my opinion, there ought never be another 240/120 Delta system installed, ever. 208/120 Wye is the only way to go. You just can't beat having a real neutral.

Appliances and motors are, by NEMA conventions, intended to operate within 10% of their nameplate voltage. Combine this with the fact that most 208 services actually deliver closer to 212-215 volts, and a 230v rated appliance ought to do just fine.

Yes, heating elements will be affected, and in many cases the appliance maker will have 208v elements available. (I encountered this with a restaurant 'booster' heater, that had been ordered for the wrong voltage).

One blessing of three phase is that motors now require motor starters. I say this, because the new ones have electronic overloads ... and incorporate low voltage protection, phase imbalance protection, etc.

As for the panels .... what you do is determined by the service the PoCo gives you.

If they give you Delta, the original panel stays - but is fed by a new 'main' panel. You must do this to stay within the listing of (strangely enough) the three-phase panel; the fine print will tell you that UL doesn't want single-pole circuits in a delta-fed panel. I suppose they want to require a smarter idiot in order to mess things up.

If you're given Wye, then it's panel change-out time. You can't very well have a hot leg pass through the main panel, can you?

I'd avoid the 'triple split' arrangement like the plague. Apart from the balancing issues, you have the very real chance of some circuits - especially after a few remodels - having conductors (especially neutrals) divided among several panels.

I would agree. I would not want this kind of system. If I need to power 240 delta loads that can't do 208, I'd rather do it with 240Y/139.


Based on voltages many people have reported in various postings I've read online, this isn't quite true. There are cases where the actual voltage is slightly lower than 208.

There are other issues that make it necessary for motors to have some operating range. Services often vary in median voltage. Tariffs I have read give the PoCo a 5% leeway in most cases. And that's on the median voltage. The voltage can also vary hour to hour or day to day. And that's before we have cases like hot summer brownouts (which just end up burning out more air conditioners). 208 is on the very edge of the +/- 10% range centered around 230. That gives none of the leeway for other issues. Most things will try to work on 208. But sometimes they will fail.

This situation is what I had in a business I worked at, once. It was an office pad environment, with each unit having its own A/C system using single phase power. Ours was cooling a substantial computer room (about 60 computers) and one summer it failed. A blower motor had to be replaced. Then it failed again in 3 days requiring the same blower to be replaced. Over the course of 3 weeks, it and another motor were replaced several times. The A/C service company special ordered a couple 208 volt single phase motors and substituted those, and the failures stopped. This was in the peak of summer when the A/C system was forced to run continuous. That and the heat on the roof was killing it. The 240 (230?) volt motors worked on 208, but not well enough to work in the adverse conditions a true 208 volt motor would work in.

There should have been a 208 volt three phase A/C system there.


For commercial and industrial equipment this is common. In fact I found some cases where 240 volt was not an available option (even when 480 was an option).


That would certainly make things better, whether three phase or single phase.


And it can become a real mess if you have to change it all.


My understanding is that 240 delta with one side center tapped for 120/240 had a limitation that the single phase loads could be no more than 5% to 10% of the total transformer capacity. It's certainly not a choice for buildings that have all or nearly all single phase loads, but so much load that the PoCo puts them in the "must use 3 phase" category.

Consider the situation where a business has single phase service and lots of equipment that uses 240 volt single phase as the bulk of their load. One example that comes to mind are kilns for ceramics and glass work. They want to expand, but the additional loads requires three phase service. How do you avoid having to replace all the existing 240 volt kilns when they can't be changed in the field?

If the limit on single phase service is, to pick a number, 400 amps, then why not add additional service from another phase, up to 400 amps. If the expansion were a separate building, that would be workable.

BTW, larger single phase kilns typically can be rewired between 240 and 480. The 208 volt ones can be rewired between 208 and 416. Many smaller kilns, however, are single voltage (you specify the voltage when ordering).

I would not want to put boost transformers on all the kilns.


The problems are greatest when a change is forced. but there can even be problems for homes forced to use three phase power right from the start, due to lack of 208 volt products.


Are there technical issues with "triple split"? Or is it something that complicates things too much?

If I was wealthy enough to have one of those mega mansions and had to take three phase, I'd just insist on it being delivered as 480Y/277 and go with three 480 to 120/240 transformers.

In my home town (Oradell NJ) there's a neighborhood that has several residences fed by 240 delta with one side center tapped for 120/240. Built in the late 50's I think, a development of McMansions of the era, with this new feature called "central air". The builder must have used smaller commercial AC units that required 240V delta. There was a garage sale at one of these houses recently, and I saw a big main 3 phase panel, just below the kilowatthour meter, and a 2 phase subpanel, probably for normal household loads.
These houses were fed by 2 transformers, to create a "wild leg" at 208V from ground, and the 240V delta with a missing transformer.
I suppose one could connect a dryer to the "wild leg" and to the appropriate 120V line (so the dryer's motor sees 120V instead of 208V!) and also connect the dryer's 240V heating element to the 2nd transformer, and not bridging the pair of transformers, though is there any way to tell which of the 120V lines the 2nd transformer is connected to? Does the power company have a convention, maybe the 120/240 transformer always on the phase that leads the 2nd transformer? Like 120/240 on phase A, 2nd on B, and other places B then C and others C than A?

I think today's code says that any voltage higher than 120V to ground is not allowed in residences.

here the link what my POCO in Wisconsin listed for voltage and ampcapcity ratings

WPS voltage / amp info { PDF format }

I know some POCO have much more stricter requirement however as I put up the link there were some talking making some change along the way { the single phase size may shrink a little down from 800 to 600 A size }

Merci,Marc

We had several places with computer rooms (back in the olden days when we had a lot of 3p load) and class A office space on 240 center delta. It is not that hard to deal with. The single phase 120/240 panels, out where the civilians work, look the same and you have a 3 phase panel up stream. You get a fat single phase pig centertapped and a smaller single for the wild leg.
(something like 25kva and 37 or 50 on the 120/240 side)
As long as your 240v loads are 3p there is no issue at all. The only time you get in trouble is if you don't balance single phase l/l loads on the 3p panel.

You should be able to put single phase 240v loads across the 120/240 side with no problem, up to the total capacity allowed for all single phase loads. The 3p 240v loads could go up to the total capacity for the wild leg. But how much total capacity can a utility provide if all your load is on the single phase side of a 240 center delta? It's all on one side of their distribution. If you're a really big customer, they will want to get it balanced. In theory, if the loads don't need a neutral, and don't care if the neutral is "off the line" somewhat, you could get equal sized 240v transformers and balance all those loads on all three phases even with 240 delta.

But utilities are phasing that out and want everyone to use 208/120 or 480/277. Only one utility I have ever seen offers 240/139 and only a couple offered 416/240 or 400/230. I certainly understand and agree with wanting to move away from delta. But 416/240 is not hard to do for cases where the bulk of the load is 240 (then another transformer to get 120 out of that for general use). 240/139 would require an unusual voltage secondary, but 416/240 would not.

This comes down to the specific equipment. 95% of 240V equipment is rated for 208V, but there's always something that isn't. You'd have to check the specs of each 240V appliance to verify.

Given Watts = V^2/R, 1-phase heating elements will be about 30% cooler and take longer to cook/heat water/etc, it's just the nature of the beast. Get them to switch to 3-phase appliances

They can size the 120/240 center tapped transformer to what your expected load is. Typically it is a 37 or 50 in the installations I see and they put in something smaller for the 3p loads.


Where we got in trouble was some of the DP equipment had 3P line cords but contained a significant amount of single phase load. Tape controllers were the worst because the satellite drives could be plugged into any one of 8 receptacles in the controller. As long as you plugged them in in sequence or had a "full boat" they were fairly well balanced but if someone only had a couple drives and they weren't careful to balance them you could corrupt the red leg balance.

I still want to avoid three phase power at home. So I guess the thing to do is make sure I don't build a mega mansion. OTOH, if I were that rich, I could afford to bring in 480/277 and waste power transforming it to whatever I wanted

Actually, If I had that kind of money, I'd more likely be doing something off-grid, anyway.

That is why delta is the best choice. You can run your big loads 240/3p and your 120/240 panel stays the same.

Be careful with your words, not all deltas are centered tapped.
I was at a facility, in October, that had both a 240V delta grounded B-phase 3PH 3W and a 120/240 1Ph 3W services.

I assumed we were talking center tapped delta.
There are also times I could make a case for corner delta.
If your equipment is good for 208, then wye makes sense but you can have harmonic problems on the L/N loads.
All of these have good points and bad points depending on your customer. That's why the PoCo still sells them all.

The 5% loading restriction only exists on center-tapped delta transformers that share a common core. In recent years several manufacturers have starting building some center-tapped single phase units that do not have this 5% restriction.

Transformer banks created by using multiple single phase units may be sized to handle any mix of 3-phase and 1-phase loads.