Another 208v question...

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TallDark

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A 208v device is powered from a 2-pole breaker (2 ungrounded conductors) and metered from ct's on each phase.
The attached meter records 2 separate power/energy for each leg as if it were a single phase 120 volt load.
The client tells me they calculate the load of this device my adding the two readings together.
My gut tells me this is suspect, unless the math happens to work out magically.
Would the meter need to be configured to use the voltage across those 2 legs?

Thanks.
 
If the power is "calculated" in kW (or kWh), it shouldn't need configured for the voltage between the two legs. The current will typically be out-of-phase with the 120V phase.

Take a resistive load at 208V. Its current will be 30? out-of-phase.

P = E ? I ? pf
pf = cos(30?) = .866

If you have a 10A load...

P = 120V ? 10A ? .866pf = 1039.2kW... times two legs = 2078.4kW
P = 208V ? 10A ? 1pf = 2078.4kW



Welcome to the forum :thumbsup:
 
This subject was discussed in some detail in another thread on delta current calculation.
If the meter is using a CT coil for each line and the matching potential coil from that line to the grounded conductor, each of those coil pairs will record exactly 1/2 of the load power.
This happens because the meter does not just multiply the RMS voltage times the RMS current (which would give an inflated result.)
Instead it takes into account the power factor which results because the line current and the line to neutral voltage are not in phase.
For a concrete example, consider a 1A resistive load at 208V line to line.
The true power is 208W.
The two line currents are each 1A and the two voltages are 120, which equals 208 divided by sqrt3.
The power factor seen by each meter section will be (sqrt3)/2.
So each meter section will record 1*(208/sqrt3)*(sqrt3)/2 =104. And 104+104=208.
All is well with the world!
It you just read the line currents and line voltages seperately with your meter and multiply, you will get the wrong answer.

Tapatalk!
 
This is why a 5 jaw socket and corresponding meter is required for a two hot 120/208V service. The fifth jaw is the neutral so the meter can do the vector calculation.

A 120/240 meter socket is typically 4 jaw. The the meter looks at the amps on each hot and the voltage across the hots. Since the two hot wires are 180? out of phase, no vector calculation is needed.
 
fmtjfw said:
This is why a 5 jaw socket and corresponding meter is required for a two hot 120/208V service. The fifth jaw is the neutral so the meter can do the vector calculation.

A 120/240 meter socket is typically 4 jaw. The the meter looks at the amps on each hot and the voltage across the hots. Since the two hot wires are 180? out of phase, no vector calculation is needed.
Click to expand...
Technically, this is an approximation, since the meter assumes that both line to neutral voltages are exactly half of the line to line voltage.
To the extent that the line with the higher current will actually have a lower voltage because of IR drop, the small percentage error is actually in POCO's favor, so they are willing to ignore it in the interest of having a cheaper meter.
:)


Tapatalk!
 
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