Residential Power

[SmokinJoeR]

There seems to be some controversy on the internet wherever I check on how 240 volts is brought into your house. Some sources say that the electrical output of the transformer on the power pole is a 3 wire single phase, or others say it is 2 single phase 120V that have 180 deg of phase difference. I know the transformer has a secondary that is center-tapped, but I could never get the clear answer whether or not its 240V/single phase and (2) 120V/single phase output or (2) 120V with 180 deg of difference between them.

Anyone know for sure?

Thanks!

 

[davidbeach]

There has been no prior reference to engines in this thread, so I don't know how they matter (engine and motor are not the same thing at all). Engines might be two stroke or four stroke, but none have phases that I know of.

I wouldn't worry about what you might read on the internet, present site excepted, when it comes to how 240V might be brought into a house.

 

[sslobodan]

Sorry for misspelling I am not native English speaker, and It just slipped my mind.... Thanks for correction (read motor not engine, I am overwhelmed with my own Engine projects... )

 

[sreid]

rbulsara,

I have to disagree about your point 1 but it could be semantics. The usual convention would be; take a two channel scope with the ground connected to neutral and the channels connected to Hot line 1 and Hot Line 2. Then when Hot Line 1 is at peak positive amplitude, is not Hot Line 2 at maximum negative amplitude? And is this not the basic definition of 180 degrees out of phase?

 

[rbulsara]

sreid:

The basic answer is NO. They are not two 180 degree out of phase 120V phasors. If they were, their sum would be zero. Is it? The sum of two 120V, in this case, is 240V so they are in phase with each other.

You have just shifted the reference points, as waross had mentioned earlier.

As for the scope, hook it up, check it out and post your results here.

 

[stevenal]

faq238-777

 

[BJC]

rbulsara

"There is no two phase 208V load or appliance"
Loads are treated differently on 208/120 systems even if the two phases serving a single residnece are "single phase".
Neutral currents on 240/120 volt systems add algebraically, Neutral currents in a 208/120 systems have to be added vectorially.

 

[rbulsara]

BJC:

How? except for 208 vs. 240V rating?

 

[BJC]

Draw the vecotrs, Line-1 to neutral, Line - 2 to neutral.
10 amps in each load. in a 120/240 system I-neutral = 0.
in two wires on a 120/208 system I-neutral = 10 amps.
If you wire three panels A-B,B-C and A-c with the same load the total current is 0, but not at each panel.

There is a section of the nec that talks about this, Id on't have my copy here.

 

[stevenal]

The 120/208 three wire service brought up by rbulsara does not meet the IEEE definition of single phase, although I have heard it called that. ANSI C12.1 calls it an open wye service.

 

[rbulsara]

What else is that other than a single phase serice? It is called single phase service because it can only feed single phase loads.

Do not confuse the type of circuits to type its source.

A 3 phase source still can provide single phase circuits.

BJC:

Vector calculation has has nothing to do with how a circuit is treated. Two line wires of a 208/120 service can only feed a single phase 208V load, same as two lines of a 240/120V service can only feed single phase 240V load. Other single phase loads they can feed are 120V.

Single phase load does not care where its voltage is coming from. All it looks for its rated voltage on its "two" terminals. A 120V bulb does not behave differently, just because its 120V comes from a 208/120V service or a 240/120V service or just a 120V control transformer.

 

[rbulsara]

The following is the excerpts from my local utility co.website (northeast utilities):

"Normally, one of the following will be supplied:
Nominal Voltage Phase Wires Comments
120/240 1 3 a,b,c,d
120/208 1 3 c,e
208Y/120 3 4 f,g,h
480Y/277 3 4 f,g,h

e. Three-phase supply is not normally available for single
family housing. For large residential complexes, which
may require a three-phase service to the building,
individual residential customers will be served only with
single-phase 120/208v."

 

[BJC]

rbulsara
So an electric range with a 3 wire 208/120 v 3 wire connection is single phase? Ditto a clothes dryer?
Why are the receptacles different?

 

[rbulsara]

BJC:

Yes. The range has mutiple (usually four) heating elements. They are individual heating elements and individual circuits inside. The neutral is provided only to feed 120V heaters. There no singel heating element it the range that requires three wires. Each heating element is a "two" wire circuit. It is possible that larger heating element could be a 208 or 240V single phase.

There are no 3 wire single phase loads or circuits, the 3 wire single phase services are intended and do only feed two wire circuits. Those two wire can be any two of the three wires available. But no single phase load requires more than two wires!!

 

[rbulsara]

Ditto for clothes dryer.

 

[BJC]

"But no single phase load requires more than two wires!!"

Don't disagree, don't think I ever said that.
Go to your 120/240 volt panel, measure from L1 to N and L2 to N, you get two readings of 120 volts. Measure from L1 to L2 you get 240 volts. 120 +120 = 240.

Make the corresponding measurments on a panel fed from 2 phases of a 120/208 v source.
You get 120, 120 and 208. 120 + 120 does not add up to 208. You can't explain it without resorting to vectors in one way or the other.

 

[rbulsara]

Both are single phase 3 wire "services", one originates from a 3 phase, 4W source and one from a single phase 3 wire source.

Type of source is not the same as type circuits or loads being served.

I think I have said enough. The single phase 3 wire "services" are intended only to serve two wire loads as I said before.

I am saying your vector math is wrong, it is still true but there are no 2 phase loads to serve.

I do agree that in theory, if you have a motor with two 120V windings (2-phase), it can be connected to two lines and the neutral of a 208/120 service and it will work, but then it will no longer be a single phase load and will require 3 wires, but no such equipment exist for common use.

 

[stevenal]

rbulsara,

Bolding your statements does not make them any truer. I cited sources to back up my claims; where are your's? By your argument, a three phase source feeding single phase loads cannot be described as a three phase service, and a six phase source feeding three phase loads cannot be described as three phase. And if any of them happened to be disconnected from all load they would be called zero phase?


Utilities may call the open wye service single phase for conveniance when the rate structure and service wire type is indistiguishable from 120/240 single phase. Remember that most utilities are run by the accountants.

 

[rbulsara]

stevenal:

I have no idea what you are talking about or how you reached your conclusions. Among all I thought you would be the one who would not misconstrude my statements. I am not up to playing semantics game.

 

[stevenal]

BJC,

You need to understand that rbulsara is defining "load" at the component level. At this level, I believe the only true three phase load would be a three phase motor, the rest being 2 wire single phase. Most of the rest of us define "load" as everything downstream of some convenient interconnection point such as the receptacle, weatherhead, transformer, distribution tap fuse, feeder breaker, etc.

 

[sreid]

rbulsara,

You are correct in saying that signals 180 degrees out of phase add to zero. But differential voltage is calculated by subtracting one voltage from the other voltage. So at peak voltage, 160V - (-)160V = 320V.