How common is 120/208 for residential?

[K2X]

In class last night I mentioned that I had 208v in my house and the instructor was certain that I was mistaken. I just checked and it's dead on 208. Looks like my two ungrounded legs come off of one of 3 transformers set on the pole in the alley and the grounded conductor comes off that transformer too?? Sound pretty normal?? Anything I should be concerned about? I'm all natural gas right now but there is a 60's double oven range in a second kitchen that gets used Thanksgiving and Christmas . The nameplate states "for 115/230 or 120/240 systems. Thanks!!

The neighborhood was developed in the 30's and 40's but is not commercial.

 

[ultramegabob]

I have never seen it in my area.

 

[cpal]

It's not unusual in certain areas

 

[ultramegabob]

I have seen houses that only had 120 available.

 

[dana1028]

In class last night I mentioned that I had 208v in my house and the instructor was certain that I was mistaken. I just checked and it's dead on 208.

Lots of the new construction in our area is getting 208. What I have seen is large developers putting in condos and single fam res [both] in an area [condos on one side, park/rec area in the middle, SFR on the other side]....all elect. is 102/208V. We have a number of projects over the last 3 years like this.

I'm in the San Francisco bay area.

 

[iwire]

I just checked and it's dead on 208. Looks like my two ungrounded legs come off of one of 3 transformers set on the pole in the alley and the grounded conductor comes off that transformer too?? Sound pretty normal??

I figure you live in a densely populated area?

Not so unusual around here, and very common for apartment and condo buildings.

If you have an electric stove your water will boil slower in your 208 volt house compared to a 240 volt house. If you where wiring your place your single phase meter socket would have to have a 5 th jaw for a neutral connection to the meter that is not needed in 120/240 services.

 

[480sparky]

......Not so unusual around here, and very common for apartment and condo buildings.........

Not only common, but usually the only option in my area if the service is over 400 amps.

 

[K2X]

I figure you live in a densely populated area?

Single family mostly on about 6 to 7000 sq ft lots. I wouldn't consider anything here high density like in a real city.

have an electric stove your water will boil slower

208v and 6000 ft above sea level we're probably lucky to get hot water at all.:grin::grin:

have to have a 5 th jaw

I'll check into that. I've seen those meters around..

Thanks for the replies. I thought it was fairly common and was a little surprised at the instructors comments. He probably hasn't worked in my part of town yet. ....

Next question. Any advantages of 120/208 single phase over 120/240 single phase??? Thanks...

 

[480sparky]

............Next question. Any advantages of 120/208 single phase over 120/240 single phase??? Thanks...

Usually, 120/208 is just two legs of a 3-phase system. Some apartments/condos are on A & B, some on B & C, and some on A & C.

 

[broadgage]

Next question. Any advantages of 120/208 single phase over 120/240 single phase??? Thanks...

For the consumer I dont believe that there is any advantage.

240 volt appliances are probably more common than 208.
Many appliances are listed for either voltage, but would give less output on 208.
AFAIK no one makes a residential water heater for 208 volts, a 240 volt unit will work ok but will take appreciably longer to heat the tank of water.

Likewise an oven will take longer to heat up at the lower voltage.

Wireing may be slightly more expensive, since in a multi wire branch circuit, all three wires will be current carrying, rather than just two.

If all three phases of a 120/208 volt supply were available that would facilitate the use of 3 phase A/C plant or large workshop machines, however two phases does not help.

The advantage is to the serving utility, who can serve more premises from a single larger transformer.

 

[shermansfather]

I'v seen it, HO or occupant doesn't usually know it. They notice the poor electric stove and dryer performance.

 

[iwire]

The advantage is to the serving utility, who can serve more premises from a single larger transformer.

Or several transformers spread around the neighborhood but all tied together to the same 208Y/120 network. This is common in some cities with all the equipment being located underground.

Here is a pretty good shot of a Boston neighborhood ...

This entire area is likely underground 208Y/120 with some buildings using all three legs and the smaller ones only using two legs. The fact that there are a number of transformers in parallel supplying the networks results in large fault currents. It also means your very ... very unlikely to be able to get a shut down of the service lateral supplying the building.

 

[K2X]

Wireing may be slightly more expensive, since in a multi wire branch circuit, all three wires will be current carrying, rather than just two.

That's interesting.. We have a few more chapters and then we're on transformers but if you could try to give a brief explanation of that I will try to understand it. Thanks..

 

[LarryFine]

Many appliances are listed for either voltage, but would give less output on 208.

75% power for resistive loads.

 

[broadgage]

That's interesting.. We have a few more chapters and then we're on transformers but if you could try to give a brief explanation of that I will try to understand it. Thanks..

On a multi wire branch circuit from a single phase three wire 120/240 volt system, only 2 of the 3 wires are regarded as current carrying.
The voltages in the 2 hot wires are exactly out of phase, therefore if 15 amps is flowing on each hot, then no current will flow on the neutral.
If only 1 hot wire is loaded, then the neutral will carry full current, but with one hot unloaded, there are still only 2 wires out of the 3 carrying current at the same time.

However if a similar MWBC is derived from a three phase, four wire 120/208 volt system, then all 3 wires must be regarded as current carrying.
The voltages in the two hots will be 120 degrees out of phase, not 180 degrees and therefore not cancel out in the neutral.
15 amps on each hot wire will result in 15 amps on the neutral also.

This may require larger wires or larger or additional conduits, since the NEC requires that conductors are upsized if more than a certain number of current carrying conductors are grouped together.

 

[Volta]

I've seen 120/208 three phase in a single family dwelling exactly once, and it was probably sixty miles north of you.

 

[K2X]

On a multi wire branch circuit from a single phase three wire 120/240 volt system, only 2 of the 3 wires are regarded as current carrying.
The voltages in the 2 hot wires are exactly out of phase, therefore if 15 amps is flowing on each hot, then no current will flow on the neutral.
If only 1 hot wire is loaded, then the neutral will carry full current, but with one hot unloaded, there are still only 2 wires out of the 3 carrying current at the same time.

However if a similar MWBC is derived from a three phase, four wire 120/208 volt system, then all 3 wires must be regarded as current carrying.
The voltages in the two hots will be 120 degrees out of phase, not 180 degrees and therefore not cancel out in the neutral.
15 amps on each hot wire will result in 15 amps on the neutral also.

This may require larger wires or larger or additional conduits, since the NEC requires that conductors are upsized if more than a certain number of current carrying conductors are grouped together.

Thanks, that's interesting. I don't think anyone around here even thinks about that when they do upgrades etc. I was noticing that 310.15(b) (6) probably doesn't apply in my neighborhood?? That could be a a costly mistake if an inspector caught it..

 

[LarryFine]

I've even seen a high-leg Delta serve a residence. A friend of mine grew up in a house with the usual 3-wire 120/240 service drop that also includes a fourth, smaller wire (#10, I think) that supplies only the exterior AC compressor unit.