Unusual home voltage

[Robert.Boston]

I was puzzled that my Model S was reporting about 204v when I plugged it in at home. My electrician stopped by and checked on the main panel that, indeed, the voltage off the street-side transformer is 208v. Talked to someone at the electric utility, NStar, today about it; she told me that my neighborhood and central Boston distribution circuits were built long ago as 120v/208v, rather than the current norm of 120v/240v. Does this mean that I have two-phase power at my house? Regardless, it’s not a welcome discovery, as it means that I recharge at a markedly lower rate than I would at 240v.

I'm assuming that there's nothing I can do....

 

[FlasherZ]

I was puzzled that my Model S was reporting about 204v when I plugged it in at home. Talked to someone at the electric utility, NStar, today about it; she told me that my neighborhood and central Boston distribution circuits were built long ago as 120v/208v, rather than the current norm of 120v/240v. Does this mean that I have two-phase power at my house? Regardless, it’s not a welcome discovery, as it means that I recharge at a markedly lower rate than I would at 240v.

I'm assuming that there's nothing I can do....

It's not really unusual. In the NE, many older services being provided via a special "open delta" 3-phase configuration. This system has a grounded, center neutral and 2 different legs that are 120 degrees apart in phase. You get 120V from any phase-to-neutral, and 208V measured from phase-to-phase. This design saved money because a third transformer (versus closed delta or wye) could be eliminated.

As I understand it, as neighborhoods are being updated, they are replacing the services with newer transformer banks as they can.

There are (complicated and expensive) ways of generating higher voltages from 208v if you needed it using transformers, but it would require a higher current on the input side and it'd probably be a pain, in exchange for the additional few miles per hour of charge. I'd probably recommend looking at a higher-amperage EVSE (perhaps even the OpenEVSE 75A project) or the HPWC if you needed faster charging at 208v.

 

[mknox]

I was puzzled that my Model S was reporting about 204v when I plugged it in at home. My electrician stopped by and checked on the main panel that, indeed, the voltage off the street-side transformer is 208v. Talked to someone at the electric utility, NStar, today about it; she told me that my neighborhood and central Boston distribution circuits were built long ago as 120v/208v, rather than the current norm of 120v/240v. Does this mean that I have two-phase power at my house? Regardless, it’s not a welcome discovery, as it means that I recharge at a markedly lower rate than I would at 240v.

I'm assuming that there's nothing I can do....

It is not uncommon for utilities to install 120/208 volt 3-phase distribution systems and run two phases and a neutral into the home. It is more common in underground distribution areas than in overhead areas. This effectively gives you 120/208v instead of 120/240v. This is not an "old" system issue... they're still being built like this today. (many areas in downtown Toronto are built this way) It is also common in high rise buildings.

Whenever replacing stove or water heater elements, you have to remember to specify 208v parts.

 

[FlasherZ]

It is not uncommon for utilities to install 120/208 volt 3-phase distribution systems and run two phases and a neutral into the home. It is more common in underground distribution areas than in overhead areas. This effectively gives you 120/208v instead of 120/240v. This is not an "old" system issue... they're still being built like this today. (many areas in downtown Toronto are built this way) It is also common in high rise buildings.

In the US for single family homes / duplexes / etc., it is indeed quite rare today and is an "old" system. most of these old open-delta systems are being replaced by single-phase 240V service. They're more reliable because they're half as likely to fail in case of a phase outage, they're simpler, and the power demand has risen to the point where the open-delta system doesn't give them as much of a density or economy-of-scale advantage. Only in commercial / apartment / condo type settings do you find a lot of 120V/208V service here.

Part of it is also the issue of consumers needing to deal with a special voltage.

 

[cinergi]

Bummer! I've gone from 240 to 208 and missed my 240... back to 240 now ... and it's so strong that I manage 245V at 40A so I get a really nice rate. Will be interesting to see what it's like at 80 amps when I get my HPC.

I wonder if you could get it to 240 if you asked for a service upgrade (what's your main breaker rating now)?

 

[Robert.Boston]

Bummer! I've gone from 240 to 208 and missed my 240... back to 240 now ... and it's so strong that I manage 245V at 40A so I get a really nice rate. Will be interesting to see what it's like at 80 amps when I get my HPC.

I wonder if you could get it to 240 if you asked for a service upgrade (what's your main breaker rating now)?

Bummer, indeed. I did ask, and was told politely but firmly that a service upgrade was not an option. My board is only 100A, though the other half of my house (with independent main breaker attached to a different distribution transformer) is 200A, so it's likely I could upgrade the other half to 200A, as well. Not worth it, though, given our expected time at this property.

(For those of you wondering, we first bought one half of a duplex, then the other half a few years later and combined them, loosely. Independent systems in the houses, except phone.)

 

[FlasherZ]

Bummer, indeed. I did ask, and was told politely but firmly that a service upgrade was not an option.

If you require a service upgrade to 200A, it may cause them to upgrade their transformers, in which case they might just consider putting a new single-phase transformer in for you.

 

[mknox]

In the US for single family homes / duplexes / etc., it is indeed quite rare today and is an "old" system. most of these old open-delta systems are being replaced by single-phase 240V service. They're more reliable because they're half as likely to fail in case of a phase outage, they're simpler, and the power demand has risen to the point where the open-delta system doesn't give them as much of a density or economy-of-scale advantage. Only in commercial / apartment / condo type settings do you find a lot of 120V/208V service here.

These aren't open delta systems. They are a grounded "Y" system. A three-phase transformer (or three single-phase transformers) is placed supplying 120/208 volts 3-phase, 4 wire secondary mains. From the 4-wire secondaries, 3 wires are sent to each residence (2 phases and a neutral). The load is balanced by alternating which two phases go to each residence.

It's actually quite a robust system, and more reliable than a single phase, split winding 120/240 volt system. If you lose a phase on this system, you only lose 1 leg (i.e. still have 120 volts in "half" of the house). 120/240 systems are fed from a single primary phase, and if you lose it, you lose the whole house. Back in my days of planning these types of services it was more common to see them in underground installations, although we did do a bit of overhead that way too. In addition to radial distribution, we also had a fairly extensive 120/208 volt low voltage network system in dense areas (that's a whole other story).

While 240 volts is much more common, 208 volts is common enough that there are electric driers, ranges and water heaters that can be purchased for 208 volts.

 

[mitch672]

It's likely if you live in the city you have underground service, and with only a 100A now, very likely your underground pipe is undersized for a larger service. Anything can be done, at a price... Replacing an underground pipe in downtown/backbay is an expensive undertaking though. If you are moving soon, maybe take a voltmeter to the next property and checkout the line voltage and consider that & the service size in the decision for the future property. I'm sure if you really needed to charge up quickly you could stop by Ben's place in Southie or my place in Randolph (though probably not close enough for you unless your taking a southern road trip). We both will have 75-80A EVSEs.

 

[Eberhard]

I was puzzled that my Model S was reporting about 204v when I plugged it in at home. My electrician stopped by and checked on the main panel that, indeed, the voltage off the street-side transformer is 208v. Talked to someone at the electric utility, NStar, today about it; she told me that my neighborhood and central Boston distribution circuits were built long ago as 120v/208v, rather than the current norm of 120v/240v. Does this mean that I have two-phase power at my house? Regardless, it’s not a welcome discovery, as it means that I recharge at a markedly lower rate than I would at 240v.

I'm assuming that there's nothing I can do....

You would be lucky to use 3-phase charging like the Model S europe version. 3 times as fast as now.

You can employ a 3-phase to 1-phase zick-zack transformer to improve the current by root of 3.

best

Eberhard

 

[FlasherZ]

These aren't open delta systems.

I considered they could be wye, but you rarely see this setup in the US today (as I mentioned) for single-family homes. In the Northeast US, some of the early distribution systems (especially in MA) used an open-delta configuration that provided 120V/208V, primarily to feed early 3-phase air conditioning systems. But as that need diminishes, utility companies are replacing with single-phase in residential areas.

I could certainly be wrong -- I just haven't seen a 3-phase wye distribution to a group of single-family or duplex homes in... over a decade(!) I would think that the "half-powered multi-phase house" would be rather big customer service problem compared to simply losing the entire home on a single-phase installation. I've heard the multi-phase systems are more difficult to plan (because of the balancing issues across the bank). That said, I'll take your word for it that in Canada I'd see more 120V/208V.

I guess the question to Robert would be: how old is your home? If it's an old home, can you see the transformers that serve your home? And if so, are there 2 or are there 3? That'll give us the answer.

 

[mknox]

I considered they could be wye, but you rarely see this setup in the US today (as I mentioned) for single-family homes. In the Northeast US, some of the early distribution systems (especially in MA) used an open-delta configuration that provided 120V/208V, primarily to feed early 3-phase air conditioning systems. But as that need diminishes, utility companies are replacing with single-phase in residential areas.

Unless I'm missing something, I don't see how a delta system could deliver two voltages. Delta (even open delta) does not employ a neutral and phase to phase voltage is the same across any two. Delta and open delta were common in Canada too (not so much any more) but were typically 600v systems for commercial/industrial applications. I understand 440v delta was more common in the US. I have heard of (but never seen) a 240 volt delta system with one of the windings split, but that gives 120/240 volts, not 120/208. I think these were intended to supply 3-phase delta power for large motor loads, but still provision a 120 volt circuit without the need of a step-down transformer.

Most Canadian 600v delta systems have been converted or replaced by 347/600v "Y" (4-wire) systems.

I could certainly be wrong -- I just haven't seen a 3-phase wye distribution to a group of single-family or duplex homes in... over a decade(!) I would think that the "half-powered multi-phase house" would be rather big customer service problem compared to simply losing the entire home on a single-phase installation. I've heard the multi-phase systems are more difficult to plan (because of the balancing issues across the bank). That said, I'll take your word for it that in Canada I'd see more 120V/208V.

In fairness, 120/208 is not really that common outside of hi-rise buildings and dense urban, and we still get lots of "part power" calls, but they are mainly due to a failed overhead or underground conductor or connection. Outside of hi-rise buildings, we have no residential 120/208 at the utility I currently work for.

No matter the technical details, the point is that 120/208 volt systems can and (obviously) do exist. Someone suggested that a service upgrade may solve the problem if the utility can convert to the more common split phase system. In the examples I can think of, this would be problematic for the utility. They would have to install a separate transformer for one customer as these 120/208 volt systems are more "neighborhood" based.

 

[FlasherZ]

Unless I'm missing something, I don't see how a delta system could deliver two voltages.

It's a system that is based off an open wye primary with the secondaries set up in open-delta. I think we're getting beyond the intentions of the board at this point -- it's used to deliver small 3-phase capability in rural/residential areas without needing full 3-phase wye primary and originates in the early days of 3-phase A/C motors.

No doubt that if 120/208 systems are being deployed today they're done as wye systems.

In fairness, 120/208 is not really that common outside of hi-rise buildings and dense urban, and we still get lots of "part power" calls, but they are mainly due to a failed overhead or underground conductor or connection. Outside of hi-rise buildings, we have no residential 120/208 at the utility I currently work for.

No matter the technical details, the point is that 120/208 volt systems can and (obviously) do exist. Someone suggested that a service upgrade may solve the problem if the utility can convert to the more common split phase system. In the examples I can think of, this would be problematic for the utility. They would have to install a separate transformer for one customer as these 120/208 volt systems are more "neighborhood" based.

Yup, I agree -- few houses should see 208v, mostly condos and apartments. Either way it's set up, 120/208 is done for scale - whether large-building scale or "neighborhood" scale with much larger transformers. As I understand it, though, increased loads in homes (especially those that would drive EV charging) is pushing more single-phase installation.