Home circuit breaker 208 volts?

[howardnj]

Hi all, I just got around to install my 14-50 outlet in my garage. When I finished installing the outlet, I used my multimeter to test the voltages. I tested from hot1 to ground and hot2 to ground both measured at exactly 120v. However, when I measured across hot1 and hot2, I get 208v. I thought that was weird, so I tested the voltages at the breaker with the same result. I measured my other 240v circuits and also got 208v. I then measured at the thick cables coming into the breaker (mains) and it also measured at 208v. Why am I getting 208v instead of 240v? Would this affect charging? Any issues I should be concerned with?

kevinf1990

I just read thru the two pages of this thread and I find your question very interesting. I measured the voltage in my garage and my multimeter across hot1 and hot 2 read 242 volts.
I would call a couple of local electricians. Perhaps there is a simple answer or perhaps there is a problem with a transformer that your local utility may want to inspect?
Good luck and let us know the outcome

 

[terranx]

If someone said they had a cough and a mild fever you wouldn’t tell them to diagnose themselves.

I'm a doctor- trust me, we don't want every single person with a cough and mild fever coming into the office or worse yet, the ER. The vast majority of these can be self diagnosed as mild viral illnesses.

 

[CaseyL]

I'm a doctor- trust me, we don't want every single person with a cough and mild fever coming into the office or worse yet, the ER. The vast majority of these can be self diagnosed as mild viral illnesses.

I think my multimeter actually has a setting to test for viral illnesses. I forget what the units are ....

 

[qdeathstar]

You are the one that needs to catch up. Read the advice you gave earlier as to whether he has 208 or not (you said no because he is residential even after posting how he took readings and what his results are).

I said it would be unusual and that he should verify…

. If you are licensed, why didn't you answer his question correctly?

I did. I said he should call the power company and verify the voltage he is suppose recieve.

 

[qdeathstar]

Since we do not know if your voltage reading is normal as a result of the 3 phase system mention above, or if it is a result of a fault due to a high resistance connection, I would contact your power company to verify that there is no issues with your electrical.

 

[UncleCreepy]

I think we've all made our points and further virtual bitchslapping is not going to resolve anything. OP appears to have the answers he was looking for, so why don't we just call it a day and move on?

 

[Mrbrock]

Last comment and is partly a question looking for advice from someone more enlightened than me. 240 single phase is the result of 2 separate 120V hot legs while 208 is same hot legs but 120 degrees phase from each other? If high resistance on 240V led to a reading of 220V then that is the sum of the two hot legs? So if one registered 120V, the other would be 100V? So if you read 208V L1 to L2 on 240V system, L1 to ground + L2 to ground=208? if you saw L1 to ground + L2 to ground = 240V and L1 +L2 = 208V wouldn’t that indicate something other than 240V single phase?

Asking for a friend…

 

[jcanoe]

If high resistance on 240V led to a reading of 220V then that is the sum of the two hot legs? So if one registered 120V, the other would be 100V?

In a home with single phase power the voltage reading you get on one side of the panel, nominally 120V (could be a few volts higher or lower depending on the power being delivered at that moment by the utility) would be the same as on other side of the panel. When you turn on an appliance that draws significant amperage such as a microwave oven or a toaster there may be a momentary drop in voltage on the side of the panel that is supplying power. This voltage imbalance is automatically balanced due to the neutral connection being shared by both sides. The voltage of the two 120V feeds remains balanced. The only time you would get voltage imbalance is if there was a bad neutral connection at the panel. Also, the imbalance would look like 140V and 100V, not 120V and 100V and would lead to issues where one side of the home's electrical panel is providing too much voltage and the other side not enough. (Ask me how I know. )

 

[qdeathstar]

Last comment and is partly a question looking for advice from someone more enlightened than me. 240 single phase is the result of 2 separate 120V hot legs while 208 is same hot legs but 120 degrees phase from each other? If high resistance on 240V led to a reading of 220V then that is the sum of the two hot legs? So if one registered 120V, the other would be 100V? So if you read 208V L1 to L2 on 240V system, L1 to ground + L2 to ground=208? if you saw L1 to ground + L2 to ground = 240V and L1 +L2 = 208V wouldn’t that indicate something other than 240V single phase?

Asking for a friend…

You do not read 208 to ground or in comparison to ground in three phase system. Each leg of a three phase system is is has a potential of 120v to referenced to ground and potential of 208v between phases.

A split phase system has two legs that read 120v potential to ground and a 240v potential between them.

All of the legs in each system are separate from one another. The amplitude of the wave form determines their potential voltage. In a three phase system, the wave forms ossillate at an amplitude of (it’s higher but for simplicity) +120v to -120v referenced to ground.

It is not the sum of the voltages.

When you read 180v one one leg and 87v on one leg what you are actually reading is the difference resistances on each phase accoss the legs of power and a open or high resistance neutral (ground) connection.

If you read 220v accross a 240v connection, it would me that the load was causing a voltage droop.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQGn42U0sm-bfPOtpfjeAkfRcTdvQJ-hvRxKQ&usqp=CAU

Essential, you would taking your reading after the resistor in this diagram.

 

[Rocky_H]

Perhaps there is a simple answer

That simple answer has already been given. If the measurements are showing 120V and 208V it does show that this is just a building that is run from a 3 phase supply instead of split phase.

 

[howardnj]

That simple answer has already been given. If the measurements are showing 120V and 208V it does show that this is just a building that is run from a 3 phase supply instead of split phase.

The simple answer is here.

Three-phase electric power - Wikipedia

en.wikipedia.org

"In North America, a high-leg delta supply is sometimes used where one winding of a delta-connected transformer feeding the load is center-tapped and that center tap is grounded and connected as a neutral as shown in the second diagram. This setup produces three different voltages: If the voltage between the center tap (neutral) and each of the top and bottom taps (phase and anti-phase) is 120 V (100%), the voltage across the phase and anti-phase lines is 240 V (200%), and the neutral to "high leg" voltage is ≈ 208 V (173%).[13]"

 

[Mrbrock]

You do not read 208 to ground or in comparison to ground in three phase system. Each leg of a three phase system is is has a potential of 120v to referenced to ground and potential of 208v between phases.

A split phase system has two legs that read 120v potential to ground and a 240v potential between them.

All of the legs in each system are separate from one another. The amplitude of the wave form determines their potential voltage. In a three phase system, the wave forms ossillate at an amplitude of (it’s higher but for simplicity) +120v to -120v referenced to ground.

It is not the sum of the voltages.

When you read 180v one one leg and 87v on one leg what you are actually reading is the difference resistances on each phase accoss the legs of power and a open or high resistance neutral (ground) connection.

If you read 220v accross a 240v connection, it would me that the load was causing a voltage droop.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQGn42U0sm-bfPOtpfjeAkfRcTdvQJ-hvRxKQ&usqp=CAU

Essential, you would taking your reading after the resistor in this diagram.

Your example makes no sense in relation to this discussion. Single phase power from two legs of 3 phase power. The two legs to ground should each be reading 120V correct? Otherwise in your example one side of the electric panel would be providing 180V to circuits and the other side would be providing 87V to circuits.

While I appreciate the knowledge you have, you are getting way to deep into this from a technical standpoint. If I read 120V on each hot leg and 240V across, I have 240V service. If I read 120V on each hot leg and 208V across I have 208V service. If you see lower readings on each leg then you'd see lower across each and that would lead you to believe you have a resistance issue. I fail to see how the resistance issue you bring up would allow you to read 120V on each leg. Resistanace would affect all voltages being read if it is an issue.

 

[qdeathstar]

If I read 120V on each hot leg and 208V across I have 208V service.

Probably but not necessarily. The exception to the rule In this case brings along with it the possibility of death, so i was suggesting the rule be verified.

Measuring between legs and measuring between one leg and a ground aren’t related the way you think they are.

In one measurement you are measuring voltage referenced to ground and the other measurement is referenced to one another, eg not ground. It is possible to have a system installed incorrectly for stance that read 208 across phases but not 120v to ground and a high voltage between Neutral and ground.

You could read 120v to ground on both legs while one of the legs was open or had a high resistance connection because you would be reading the other legs voltage through a resistive element like a hot water heating element or baseboard heater, but then when you read across the two legs you get a strange reading across the two legs.


I agree it’s unlikely that you read 208v but you are actually supposed to read 240, but because it takes almost no effort to double check (just a call to your power company) and the penalty for being wrong is a fire, I would suggest making the call, especially sense OP isn’t an electrician.

 

[ucmndd]

I said it would be unusual and that he should verify…

Which in and of itself makes no sense as there is nothing at all “unusual” about 208v service to individual units in multi-unit residential dwellings in the US derived from 3 phase utility service to the building. This is the expectation, not the exception.

 

[qdeathstar]

Which in and of itself makes no sense as there is nothing at all “unusual” about 208v service to individual units in multi-unit residential dwellings in the US derived from 3 phase utility service to the building. This is the expectation, not the exception.

It be would be unusual for a residential unit.

 

[MY-Y]

Quick answer:
1. 208 is normal for three phase, 240 for single phase.
2. Three phase is unusual for residential.
3. 208 on a single phase system indicates a real problem.
4. If you cannot determine if you have a 3 phase or single phase feed, call an electrician.

 

[SalisburySam]

OMG! After reading this (long-ish) thread on power phases, multimeter usage, and licensing, I immediately (well, after coffee) went out to my garage load center and stuck my multimeter’s nasty bits into various places and GOT THE DISTINCT IMPRESSION that I have two 120v legs with 240v for my EVSE. I’m not a licensed anything so clearly this is not definitive, and neither is my multimeter licensed. So what to do on this critical issue? I called a licensed electrician. She (yes, she) came to do some other work in the home and I asked her to use her (hopefully) licensed multimeter and licensed skills to also measure my EVSE’s voltage. After a bit of a strange look, she did so, presumably for customer satisfaction as well as additional billable time on site. Incredibly coincidentally, her multimeter was the same brand and model as mine though showed much more usage, likely something to do with the rigorous multimeter licensing protocol and thorough final exam. She poked her multimeter’s nasty bits in both similar and exactly the same places I did (she let me watch, explaining as she went). Lo and friggin’ behold, I have two 120v legs with 240v for my EVSE. I was gobsmacked! Professional verification of my clumsy, unprofessional, unlicensed nasty bit poking.

What a morning! Can’t wait to see what happens after the obligatory evening martini.

 

[jcanoe]

It be would be unusual for a residential unit.

3-phase for residential use has become more common in some areas, especially for multi-unit town homes and condominiums. Clues that the community is wired this way includes a transformer of 400A or greater and buried electrical service lines.

My previous residence, a multi-unit condominium, was built in the late 80s. There are 20 buildings of 12 units each. Each unit has 150A service. Although I never measured the voltages there were assumed 200-208V circuits for the heat pump, water heater range/oven and clothes dryer. Out front there was a ground level transformer. The electrical service in the community was buried.

The nearby public level 2 charging station that I currently use to charge my Model Y is definitely using 200 to 208V commercial power. The displayed voltage is consistently around 196V, never over 200V.

 

[qdeathstar]

3-phase for residential use has become more common in some areas, especially for multi-unit town homes and condominiums. Clues that the community is wired this way includes a transformer of 400A or greater and buried electrical service lines.

My previous residence, a multi-unit condominium, was built in the late 80s. There are 20 buildings of 12 units each. Each unit has 150A service. Although I never measured the voltages there were assumed 200-208V circuits for the heat pump, water heater range/oven and clothes dryer. Out front there was a ground level transformer. The electrical service in the community was buried.

The nearby public level 2 charging station that I currently use to charge my Model Y is definitely using 200 to 208V commercial power. The displayed voltage is consistently around 196V, never over 200V.

I know it’s not impossible. But it’s still uncommon and easy to verify with your power company.

 

[Mrbrock]

You all say it is so uncommon to have 208 single phase yet appliance manufacturers ensure all appliances work at 208-240. Why would they do this if it was so rare? Most people who buy gas ranges have natural gas but quite a few have propane. They have to buy extra parts to swap, they are usually not included for free because the majority of people don't need them.

I think it is safe to say that 208V single phase in stand alone single family residential is very uncommon. But to suggest that most MFDs (including 400 unit apartments and condos) are all fed with 240V would be insane. It is much more efficient to bring in 3 phase power and split that.