I got asked this and I have not a clue or a dual trace meter at my disposal
at present. On a home power feed. One common and hot two 110 or 120 vac
feeds are the hot leads 180deg. out of phase or the same? If it's provider
specific- Com Ed, Chicago.
Thanks, I know odd question but why this is needed is beyond me, I
just figured one of you will know 12 volts is where I quit.
Fraser Competition Engines
The term phase is sometimes questioned but it is accurate for describing
what is happening as long as we remember that the supply is indeed a
single-phase supply that is provided by a center-tapped transformer. Each
end of the transformer measures 120Vrms with respect to the center-tap
(neutral) and since one end is positive when the other is negative the
voltages add to give 240Vrms across the ends (hots). The voltages are out of
phase with respect to the neutral.
This is one of the perpetual discussion and arguing points in regards to
electrical power. It has a lot to do with your understanding of phase and
polarity. Remember a voltage does not exist at any one point but only
between two points. If the reference point is the center tap or neutral, the
voltages from there to "hot" wires are 180 degrees out of phase with each
other. In this context, they can also be described as being of opposite
polarity. It may help to imagine two batteries in series, with the point
between them as the "ground", "neutral", or "reference". From that point the
voltage at one end is +1.5 volts, the voltage to the other end is -1.5
volts, and the voltage from end to end is 3.0 volts, either positive or
negative depending on which end you start at. The analogy is not perfect but
it may help understand the conditions. It is a little like two ladders put
end-to-end with one ladder in a hole in the ground and thinking about the
meanings of "up", "down", and "how long". Kinda depends on "where from" and
Putting it simply and I'll bust out the Fluke 93 when I get to the shop- if
I tap both hots will my sine wave show a 180deg deviation on 60hz trace or
would they be superimposed? Ch-a vs. Ch-b same center common obv.
Gee, 36VDC blower starters seem much easier now, I can teach a monkey
to fire up one of my engines but this makes me want to go back to school. I
feel like a dork for not knowing this.
Fraser Competition Engines
The legs are 180 degrees out of phase, else your dryer and your stove
would see 0V between "hot" and "the other hot" instead of the 240V that
they want to see. Half of the 120V plugs are connected to one leg, the
other half are connected to the other in a vain hope that they'll
balance out. The "neutral" wire is the current return, it's grounded at
the pole but the safety ground is (well, should be) run to a seperate
ground at the meter or breaker box.
At the pole At your house
---------. ,------------------------------------------- hot
---------' '--|---------------------------------------- the other hot
| .--- ground
created by Andy´s ASCII-Circuit v1.24.140803 Beta
They are in phase (that's why they call it single phase) so the voltages add
up to 240. If you add +120 to -120 you get zero. And it is not really a
parallel circuit. A parallel circuit would double your amperage not your
I guess my question is answered. I told him to contact an electrician, I
don't know what he is up to but knowing him a fire is probable. This is like
"Rocket Surgery" (At least I was right about the waveform pattern, I feel a
bit redeemed). This is the guy I loan my plasma and cheap tig to so I felt
some detective work in order. Needless to say, nothing is leaving the shop
until his permit gets a green tag.
Have a good weekend,
Fraser Competition Engines
They are in phase if you are using either end as a reference, as you would
be doing when adding or seriesing the two 120's to get 240. At an instant
when one hot leg is 100 volts negative to the neutral, the neutral is 100
volts positive to that same leg and the other hot leg is 100 volts positive
to the neutral. So the last mentioned hot leg is 200 volts positive to the
first hot leg. By the same logic and under the same conditions, but using
the center tap as the reference point, the two end legs are out of phase
since the first is negative to the neutral and the second is positive to the
neutral. At any time one hot leg is positive to the neutral the other hot
leg is negative to the neutral and vice-versa. Also at any given time that
the neutral is positive to one leg the other leg is positive to that same
leg by twice the voltage. Positive, negative, and phase in this context just
mean polarity or direction of voltage and can immediately reverse if you
change the points "from" and "to". We can not change the performance of the
circuit by discussion or definition of words but only our understanding of
it. I was trying to use the "water analogy" once to explain an electrical
circuit and one student said, "Mr. Young, I don't understand plumbing any
better than I do wiring!". Hope this helps.
thanks (still on the right track here); could you say (to reinforce my
limited-but-hopefully-correct-understanding) that with a 3 phase system
each leg would be "out-of-phase" with respect to each other by 60°.
just possibly can you introduce the term "lead" (as in this phase LEADS
by 60°), and make a final comment as to wire color scheme.
In a 3-phase system each leg is 120 degrees out of phase with each other
(3x120=360 degrees). Although if you said phase C lagged phase A by 120
degrees it could also seen as leading by 240 degrees from the previous
cycle. I can't comment on the wire color scheme. Normally for a piece of
3-phase equipment it doesn't matter which phase is connected to which
terminal. 3-phase motors' direction of rotation is dependent on the phasing
and if it turns in the wrong direction you just reverse any 2 wires
(phases). In a large factory that uses 3 phase power the phases are all well
identified such that any equipment like motors that are phase sensitive will
operate correctly the first time.
Depending on your reference you can say any phase leads (is ahead of) or
lags (follows behind) another phase.
very good; clear, plain, fully understood. definitely the right track
here: thank you
now, so as not to have to go through aprox. 60 postings (clipped & saved
in my 3phase folder) what is the "wild" leg?
In a 240 volt 3 phase delta with a center tap, there is 120 volts
between 2 legs and the center tap. The 3rd leg is the "wild leg", with
208 volts between it and the center tap. It's marked with orange tape
or an orange wire...