It will come from all three phases. You have voltage and current in each
phase. It doesn't come from one phase at a time and normally you measure
average power rather than instantaneous.
If you want to measure power in any one phase then your wattmeter must be
connected to phase voltage and phase current. What is called phase voltage
and phase current depends on whether the load is Y or delta.
I can only think that your question is aimed at determining, after a
spider box, which phase an outlet is running off of. If this is so,
turn off the box and unplug a single phase completely. Turn it on,
and if it no longer works, you got your phase. If that's not the
question, then I'll just give you some info on electricity to help
with whatever you are doing...
Power is sent through three conductors from the power plant and is
stepped down at a series of delta transformers until it gets to the
entrance of a building or a neighborhood. At this location, a couple
of things could happen:
1. A delta transformer steps down to 240vRMS (US) with a center tap
per phase neutral, grounded at that location. This allows for either
two 120v hots relative to neutral, or 240v relative to each other.
2. A Wye transformer steps down to 120vRMS (US) and generates a
common neutral at the center, grounded at that location.
Once this is done, an added safety ground, grounded away from the
neutral, is installed, and the system becomes either five or ten wire
(the delta has three feeds per phase plus ground). Usually the delta
method will then distribute the feeds so that they enter the home or
office as either 3 or 4 wire (depends on if the ground is local or
remote). Only one phase is usually sent to each, with a total of 240v
possible from the two hots off of one phase (they are actually phase
inverted from each other). These systems should NEVER be used with
If the wye was used, as is common in situations where 3-phase motors
are used (some central air, some industrial, all heavy rigging) or in
entertainment venues with heavy power needs, each phase, the neutral,
and the ground are sent to a disconnect where they can either be
permanently wired into a device or can be left for manual tie-in.
Regardless, all power is measured RMS. 120v AC is actually about 170v
peak. The RMS measurement between two phases is 207v. The RMS
between a phase hot and its inverse is 240v. You know you are dealing
with one phase if you get 0v between hots, or 240v between hots. When
you get 207v between hots, then they are two different phases. I hope
>I have a doubt in electrical engineering.This is regarding 3 phase.How
>can i measure from which phase power is coming in a 3 phase connection
One question - where do you live? In most parts of the world including the
US, transformers used for 3 phase are mostly star delta. There are
operational and economic advantages to this with respect to delta delta.
For example, any appreciable length of overhead line fed or connected only
to delta windings, can be trouble (this was found by experience, back in the
Don Kelly @shawcross.ca
remove the X to answer
At this location, a couple
I tried to understand you actual questions (sorry). Did you means that
what is the sources of three phase or how to measure the power between
For your information, the phase system could be in star or delta
connections depend on
user or system to use. To calculate the power from star or delta
connections, you should understand the concept and formulas to apply
Normally, a device designed to operate from 3 phase draws power from all
3 phases, and in approx. equal amounts. But if you have something like a 3
phase instant-on hot water system with 3 separate heating elements, one or
two of the elements can burn out and this unequal load makes the power per
phase unequal. To determine which element is burnt out you could remove
one fuse at a time at the switchboard and see whether there is one of those
fuses which has no effect on the heater's operation. If so, then you have
identified that phase which is delivering no power. If you can obtain an AC
ammeter of the split-core induction type which clips around any current-
carrying conductor, then taking each phase in turn it will tell you the
levels of current flowing in each of your three conductors while the
appliance is in operation.
In the above I speak of CURRENT. To measure actual POWER you would need to
connect a power measuring meter into each phase.