What is the difference between 208V vs 240V single phase ?

[jaehlee99]

When I read some specification in the equipment schedule, some equipment was supplied by 208V single phase, some equipment supplied by 208V three phase.

As far as I know, we can get 240 voltage from line to line or 120V voltage from line to neutral in the three phase system.

How can we get 208V in the single phase system?

Many thanks

 

[ActionDave]

. .....How can we get 208V in the single phase system?

Many thanks

When it is supplied from two hots of a three phase system.

 

[jaehlee99]

Thanks for your your quick reply.

We can get 208V between any two hots in the three phase, and we can get 240V between two hot in the single phase system. But how can we get 208V in the single phase?

Thanks

 

[ActionDave]

.... But how can we get 208V in the single phase?

Thanks

You can't. Not unless there is some three phase somewhere upstream.

What kind of problem are you trying to tackle?

 

[ggunn]

Thanks for your your quick reply.

We can get 208V between any two hots in the three phase, and we can get 240V between two hot in the single phase system. But how can we get 208V in the single phase?

Thanks

I'm not sure what you are asking. If you want 208V single phase you'll need a transformer to get it from 240V single phase. If you want 208V three phase from 240V single phase, you can't.

 

[electrofelon]

Thanks for your your quick reply.

We can get 208V between any two hots in the three phase, and we can get 240V between two hot in the single phase system. But how can we get 208V in the single phase?

Thanks

Use a buck transformer

 

[jaehlee99]

When I post this thread at first, some equipment requires 208V single phase system.
As far as I know, we can get only 208V in the three phase system. I can not understand that point. Is it possible for us to get 208V in the single system?

Many thanks.

 

[GoldDigger]

When I post this thread at first, some equipment requires 208V single phase system.
As far as I know, we can get only 208V in the three phase system. I can not understand that point. Is it possible for us to get 208V in the single system?

Many thanks.

Quite simple when you understand how the terminology is commonly used.

If the equipment has only two input terminals, then it is using a single phase source, whether that source comes from the two hot wires of a single phase system or from two out of three wires of a three phase system.

If the equipment required three phase it would have three ungrounded input terminals. (Leaving corner grounded delta out of consideration.)

The common (in the US) three phase wye service notated as "208Y/120 three phase four wire" will only allow you to get 208V from line to line.

 

[ActionDave]

When I post this thread at first, some equipment requires 208V single phase system......

What equipment?

 

[jaehlee99]

I appreciate your clear explanation to me .

Some condensing units for air condition system.

Many thank

 

[ActionDave]

.....Some condensing units for air condition system.

You don't have anything to worry about.

 

[kwired]

When I post this thread at first, some equipment requires 208V single phase system.
As far as I know, we can get only 208V in the three phase system. I can not understand that point. Is it possible for us to get 208V in the single system?

Many thanks.

It is possible to design a single phase transformer secondary with only 208 volts across it, but if you center tapped it there would only be 104 volts from the neutral to the other two lines, so you won't have both 208 and 120 volts available like you do with a 208/120 three phase wye system. It is not that 208 volt single phase transformers can't or don't exist - there just isn't much demand for them making them sort of rare.

 

[gar]

151121-2409 EST

jaehlee99:

Your question was:

What is the difference between 208V vs 240V single phase?

Both are a two wire source and load. Does not imply that either wire is near ground potential. A single phase induction motor is not self starting, and it requires something to get it started. Usually something to produce a phase shifted magnetic field.

The big factor in your question is that a constant power load will have a higher input current when the supply voltage is lower. Whereas, a constant resistance load will draw less current at reduced voltage and produce much less output power.

I can have single phase at any voltage I want. In a vacuum tube radio there was typically a 2.5 V or 6.3 V filament source, and possibly more than one high voltage soruce like 300 V, likely 600 V center tapped.

A more than single phase system will have more than two wires for the source and load.

A three phase wye system with a nominal 208 V line to line voltage has a nominal 120 V line to neutral voltage (208/1.732). You can power either a 208 single phase load from two ungrounded lines, or a 120 V single phase load from any ungrounded line to neutral.

If you had a 240 V wye system, then line to neutral would be 139 V.

If you have a 240 V delta with one secondary center tapped, then you can have the following single phase voltages 120, 240, and 208. You are not advised to use the 208 output. And you can have 240 three phase.

In previous posts you have been provided with some real world possibilities.

.

 

[Carultch]

When I read some specification in the equipment schedule, some equipment was supplied by 208V single phase, some equipment supplied by 208V three phase.

As far as I know, we can get 240 voltage from line to line or 120V voltage from line to neutral in the three phase system.

How can we get 208V in the single phase system?

Many thanks

A 208V single phase system usually starts from a 208V three phase WYE system, when two of the three phases and the neutral are used for a feeder to your system. This is common in apartment residential applications, because it is more efficient to supply the service as a whole with 3-phase, and the feeder for each dwelling unit is connected to 2 of the 3 phases, to make it function as close as possible to the standard 120/240V single phase service that you find in a single family dwelling.

The difference is that the two ungrounded lines of the 208V system are not exactly opposite in phase, like the two lines of the 240V system are. Instead, they are 120 degrees out of phase. If you draw a triangle with 12 inches on one leg, 12 inches on the other leg, and 120 degrees between the legs, the remaining side will be 20.8 inches long. And this is how 120 is related to 208, for this grid topology.

Most equipment that is built for 208V single phase, is dual rated to work at 240V single phase as well. It is rare that you'll have equipment that is only capable of working at 208V single phase, because the application is likely intended for dwellings, where it would need to work in an apartment dwelling just as often as it would need to work in a single family dwelling.

If you did have equipment that was sensitive to 208V specifically, you'd need a buck-boost transformer to convert 240V to 208V. Note that it will no longer be 120V to ground as the 120/240V system originally was. It will become 104 volts to ground and 208V line to line.

 

[kwired]

If you did have equipment that was sensitive to 208V specifically, you'd need a buck-boost transformer to convert 240V to 208V. Note that it will no longer be 120V to ground as the 120/240V system originally was. It will become 104 volts to ground and 208V line to line.

A separately derived 208 volt source with center tap will have 104 volts from each end to the center tap.

A buck boost transformer will be 120 volts to ground on one leg and the voltage to ground will be 120 plus or minus however much change in voltage was made (exception would be if one input was the high leg of a high leg delta system).

 

[Mark-in-Orlando]

Just some information on the subject.
I am moving from a home with Standard "240VAC Single Phase/120VAC Split Phase". I am moving into a high-rise apartment/condo building that uses "208VAC Single Phase/120VAC Split Phase". I figured the Hot to neutral would be like 104V but nope it was 120V. I don't understand the physics of how 120/208VAC split/single phase works, I only know the readings on my meter and the fact that the electrician expected this. I am buying Bosch appliances, both the Bosch Washer, Bosch Dryer and the Bosch Range can use either 208VAC or 240VAC single phase. (Yes the washer runs on 208V/240V) They do mention that the Range takes longer to heat up the oven with 208V. (Actually I am switching to the Miele Washer and Dryer because of required clearance issues and they both run on 120VAC)

I am also installing a steam shower and Steamist offers their products in a 240V version and a 208V version. Many others do not and the performance suffers, if you use a 240V appliance with 208V(note: you should not do this if you do not have written information from the manufacturer that it will not create a problem). If you have a product that requires either 240VAC or 208VAC you can get a transformer to change the current. If your panel is 208V and you have a 240V appliance that requires A 30 AMP Breaker. the Transformer will need at least a 40 AMP Breaker at 208VAC.

In the past, I have installed expensive imaging systems that ran on 208VAC 3 phase in a commercial environment. I think you are only likely to see 208VAC in areas that are commercial. Certain types of equipment need 3 phase power, usually high amperage motors. Or sometimes perhaps motors that need to operate more smoothly. In my residential building each apartment has Single Phase 208VAC Split Phase 120VAC. If one needed some type of equipment that required 3 phase power they would have to run an additional Hot wire and use a new 3-phase panel. A 3-phase panel would have 3 hot wires, a neutral wire and a ground wire. In my unit the Ground bar is separate from the Neutral bar, isolated Ground. In most residential homes they are connected(the neutral Bar is connected to the Ground bar).

 

[Frank DuVal]

Mark, you are confusing single phase with three phase systems.

The term ""208VAC Single Phase/120VAC Split Phase". does not exist. To get 120 vac and 208 vac from the same system requires a 120 degree phase shift, which you get from three phase system. It is just using three of the 4 wires off a three phase transformer connected in Y (wye) configuration.

Draw the Y, mark each of the three ends of the Y 208,, and mark the center of the Y Neutral. You have 120 from any phase to neutral and 208 from any phase to any other phase. Your new abode just uses two phases and the neutral. i.e. 120/208 single phase, even though it is actually two of the three phases..... That's how the Physics works....

 

[jim dungar]

It is just using three of the 4 wires off a three phase transformer connected in Y (wye) configuration.

Much of our industry does refer to residential services as being single phase even though the voltage is 208 L-L and 120 L-N.
Some people refer to this as an open-wye service.

 

[jaggedben]

(Five year old thread.)

 

[sw_ross]

(Five year old thread.)

It seems that Mark-in-Orlando found this older thread through a search query and responded to it with questions about how a 208/120 single phase service works.