Split phase residential service

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170802-1220 EDT

The transformers at low power distribution points in an any phase system are usually single phase transformers. This is to provide simplicity in stocking transformers for both repair and new installations.

A single phase or split phase application uses one transformer. This transformer would not be described as a three phase transformer. However, a true three phase transformer could have on it a single phase secondary.

A true three phase transformer has a single core with three magnetic paths (branches).

Actually two transformers from a single phase (meaning a two wire source) can provide a split phase output.

The primary of a single phase transformer can be sourced from two wires of a single phase source, a two phase source, or a wye or delta three phase source, or a many phase source, but in any of these sources only two wires of the source are used to feed the single phase transformer.

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gar said:
170802-1220 EDT

The transformers at low power distribution points in an any phase system are usually single phase transformers. This is to provide simplicity in stocking transformers for both repair and new installations.

A single phase or split phase application uses one transformer. This transformer would not be described as a three phase transformer. However, a true three phase transformer could have on it a single phase secondary.

A true three phase transformer has a single core with three magnetic paths (branches).

Actually two transformers from a single phase (meaning a two wire source) can provide a split phase output.

The primary of a single phase transformer can be sourced from two wires of a single phase source, a two phase source, or a wye or delta three phase source, or a many phase source, but in any of these sources only two wires of the source are used to feed the single phase transformer.

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Click to expand...

Thanks for taking the time. Don't suppose you could post a link or pics to aid me in understanding?

Why do they call single phase single phase. It can be confusing to me because there are two hots and a neutral?
 
shputnik said:
Is a split phase transformer on a pole a Delta transformer?
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I misread your question the first time around. Delta and wye are terms for three phase power systems. A single split phase xfmr, like on a pole serving a residential neighborhood, is neither delta or wye, tho some can be wired series or parallel for 120 or 240 output.

"Why do they call single phase single phase. It can be confusing to me because there are two hots and a neutral?"

In single phase, you have, at the minimum, two hot wires, or hot/neutral with one waveform between them. The neutral may or not be present, or needed. Your water heater for example uses 240VAC 1 phase. You have 2 hots connected (and a ground), usually coming from a "split phase" 240V 1 phase service.

In a commercial setting, that water heater may be powered from a 208V 3phase panel. Still two hot wires, just with 208V potential instead of 240V.

Where the neutral is, or if there is even one, is irrelevant to a straight 208/240V 2 wire load.

With a neutral and two hots, now you can have 120/208V (single phase) or 120/240V (split phase).

In a split phase application (240V), the neutral is electrically in the middle of those two 120V potentials. There is 120V from neutral to hot and 240V from hot to hot.

In a single phase application (208V), the neutral is not located between those two points, but in the center of 3 points (wye connection) , one of which is not being used. There is 120V from neutral to any hot, but because the hots are only 120* apart instead of 180* as in split phase, the potential (voltage) from hot to hot is only 208V.

With a high leg 3 phase service, the neutral is located in the middle of two points (typically A and C) like split phase, but not the middle of the triangle like a wye connection. Thus with a high leg you have A and C phase 120V to neutral, and B phase is 208V.

Here's a thread with some pics for clarification/visualization:

http://forums.mikeholt.com/showthread.php?t=133976

SystemTypes-lo.jpg
 
shputnik said:
Thanks for taking the time. Don't suppose you could post a link or pics to aid me in understanding?

Why do they call single phase single phase. It can be confusing to me because there are two hots and a neutral?
Click to expand...

You don't want to go down the road of asking on this site what a "phase is" trust me, you can get hundreds of replies and be even more confused then you started out :blink: Has happened before

Your "conventional" single phase three wire source is a single coil source with access to the mid point of that coil. Voltage from mid point is equal to both outer ends and therefore is half the voltage from end to end. They could build a source with even more accessible points between the ends but is usually only done for specific applications, the three wire configuration is somewhat standard that has been accepted for general use.

What is "hot" is determined by what is grounded. For most NEC applications if there is a midpoint (neutral) that is the conductor that is required to be grounded. Regardless of how many other usable points there are in a system - all the others are "ungrounded". You can not ground more then one point or else you have fault current flowing through the bond between those points. Outside of NEC requiring the neutral to normally be the conductor that is grounded, you could ground any point of the system but as I said you can only ground one point of the system. If you had a 120/240 source coil and grounded one of the ends instead of the neutral - you would read 0 volts to ground on the grounded line, 120 volts to ground on the neutral, and 240 volts to ground on the remaining line. Still would be 120 from neutral to each outer end and 240 between each outer end.

You can get "single phase supply" (even single phase three wire) from sub parts of a poly phase source. Which is what your typical single pot POCO transformer is. It connects to just two lines of a poly phase supply which supplies it with a voltage that only has a "single phase" (going out on a limb here) property to it.
 
shputnik said:
Is a split phase transformer on a pole a Delta transformer?
Click to expand...

As others have said, a single phase transformer is neither delta nor wye, however it is common jargon to use these terms in reference to the serving system. For example, my road is fed from 4800 single phase where both conductors are hot/ungrounded. Frequently that would be referred to as a "delta" even though it is a single phase line, and some may sloppily refer the transformer as a delta, because it has two bushings which would be required for that system.
 
kwired said:
You don't want to go down the road of asking on this site what a "phase is" trust me, you can get hundreds of replies and be even more confused then you started out :blink: Has happened before

Your "conventional" single phase three wire source is a single coil source with access to the mid point of that coil. Voltage from mid point is equal to both outer ends and therefore is half the voltage from end to end. They could build a source with even more accessible points between the ends but is usually only done for specific applications, the three wire configuration is somewhat standard that has been accepted for general use.
Click to expand...
c'mon, phases are fun.
but maybe explain why a wye (pun intended) is only 208v if i cross over two 120v coils that are connected together in series but not in phase. that √3 is a mystery most of the time ;)

the 120 3ph wye always makes me think any 2phase should be ~180v :huh:.

that darn √3
part-2-img-7-22.png
 
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I recently came across this picture, I think it might be useful for you to understand the "single phase" aspect of this.

neighborhood_power_system.jpg

So looking at that from left to right going from the Utility to the house, you can see that on the utility transformer, there is a SINGLE WINDING. That's why it's called SINGLE PHASE. It's really that way on both the primary on the secondary, but on the secondary side, that SINGLE winding is provided with a "tap" in the center, and that tap is connected to Ground (although unfortunately left off of that diagram), then run into the house as a "neutral". So 240V is the voltage from one end TO the other end of that SINGLE winding, 120V is the voltage from one end OR the other end to that center tap, which is referenced to ground.

SINGLE windings, SINGLE phase...

Wye or Delta would refer to having 3 phases, so it's about how you would connect 3 of those transformers.
 
shputnik said:
Thanks for taking the time. Don't suppose you could post a link or pics to aid me in understanding?

Why do they call single phase single phase. It can be confusing to me because there are two hots and a neutral?
Click to expand...

Post #6 diagram on the left. If you look there aren't two "phases" separated by a phase angle, it's just one coil with a center tap to provide 1/2 the voltage from either end.
 
this is exactly the reason why when you put 240vct1ph on a 2ch scope with CT as the reference you get what looks like two waves 180° out of phase. in fact they are in-phase as there is only 1ph, there is only one coil that is being cut by mag field, hence 1ph. look at the pic, if you are standing on the CT, one side looks + and the other side looks minus only ever to be equal at zero volts as the potential vector changes direction (aka "AC"), and the CT sits exactly at 180°. 1ph AC is 2x180° and not really 1x360° ;)

sorry, i cant help past here :weeping:

240ct1ph.png
 
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