3-phase AC current measurements with clamps

lazzalazza Solar Expert Posts: 330 ✭✭✭
Hi,

We're about to put in an Efergy E2 energy monitor for a house that has 3 phase electricity. The system is with a neutral, hence a Wye system, so each phase gives 230V between it and the neutral wire.. can I therefore simply put one clamp around the neutral wire to measure the total power consumption?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,883 admin
    Re: 3-phase AC current measurements with clamps

    No--You cannot.

    The neutral wire carries the difference of the A/B/C three phase legs...

    If you run 10 Amps A to Neutral => 10 amps on Neutral

    If you run 10 amps on A+B+C then neutral would have (ideally) zero amps.

    Its Vector Math, so the additions are not simple (if you have 10 amps on all three A-N with 1.0 PF and B-N with 0.5 PF and C-N with 0.67 PF, you will have some sort of non-zero current on the Neutral).

    However--I do have a question... In the US, we use Delta and Wye (or "Y" or "Star") transformers from the utility (each has their own advantages/problems).

    In the US we us Phase to Neutral (120 VAC) and Phase to Phase (240 VAC for Delta, 208 VAC for Wye).... And higher voltages too.

    Are you using both Phase to Neural and Phase to Phase for this installation--Or just Phase to Phase (and no neutral needed)? Or only Phase to Neutral? (I don't know how the standard residential grid power is wired in Europe/Spain).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: 3-phase AC current measurements with clamps
    lazza wrote: »
    Hi,

    We're about to put in an Efergy E2 energy monitor for a house that has 3 phase electricity. The system is with a neutral, hence a Wye system, so each phase gives 230V between it and the neutral wire.. can I therefore simply put one clamp around the neutral wire to measure the total power consumption?
    No. If the source/load is balanced, the current in the neutral will be zero. If not the neutral will carry only the difference between the unbalanced phases. You've got to monitor all three phases for a building that has some balanced loads and some single phase loads.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: 3-phase AC current measurements with clamps
    lazza wrote: »
    Hi,

    We're about to put in an Efergy E2 energy monitor for a house that has 3 phase electricity. The system is with a neutral, hence a Wye system, so each phase gives 230V between it and the neutral wire.. can I therefore simply put one clamp around the neutral wire to measure the total power consumption?
    As stated, you cannot even measure the total current that way, let alone the total energy. By knowing both the current in each phase and at the same time the voltage on each phase (so that you can detect the phase relationship between the current and the voltage) you can measure the power flowing in or out.

    If all of the loads are purely resistive, you can calculate the power knowing only the magnitude of the currents and voltages.
    SMA SB 3000, old BP panels.
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