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Measuring AC usage - aka, LOAD

Discussion in 'General Electronics Discussion' started by mjscott, Feb 2, 2017.

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  1. mjscott


    Feb 2, 2017
    I wish to build a project to measure the electricity usage of a few select items in my home: Furnace (80A @ 220vac), Hot water tank (30a @ 220vac), and Refrigerator (15A at 110vac). I live in Canada, and our residential electricity is 110/220vac at 60Hz. After much research, I've learned that the device to do the measuring at the circuit is the toroidal current transformer (CT) - much like a clamp-on ammeter. This little doughnut shaped component goes around the HOT conductor of a circuit. In the case of a 220vac circuit (such as the furnace), I will need two - one for each HOT conductor. Unfortunately, there is little information on the net about how to select and use a toroidal CT in this application - sigh.....
    Here are my unknowns:
    - what is the correct toroidal CT to buy?
    - will I need different ones for circuits of different maximum loads (15A vs. 80A)?
    - a typical toroidal CT has two leads. Across these leads do I measure voltage, or current (or both) to know the current load of the circuit being measured?

    This is PHASE one of my project! PHASE two will be interfacing output of the toroidal CTs into a computer for logging purposes. This will involve using some sort of DAC with analog inputs, which might create a new thread.

    I don't expect one person to have all the answers, so even partial experienced advice is appreciated.

  2. Gryd3


    Jun 25, 2014
    Look at some datasheets.
    They are current transformers, and will output current proportional to the current in the conductor they are mounted around.
    You connect this to a resistor, and measure the voltage across the resistor with an Analogue to Digital converter.
    Most microcontrollers have a build in DAC, but you can use external converters.
    Oh... and yes, you will need a different current transformer for heavier loads. The sheet I linked handles up to 30A.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Good advice by Gryd3.
    Probably not. To use the full 220 V, the furnace will be connected between the two 110 V lines which are in opposite phase (phase angle with repect to the sinusoidal waveform), thus creating 220 V RMS. Therefor ethe current going "in" the load from one phase will be the same as the current coming "out" from the load on the other phase. That is unless some current is diverted to neutral, e.g. for supplying an integrated control unit with 110 V only. The difference should be negligible compared to the power consumption of the furnace.

    You can use an 80 A transformer for a 15 A load, but not vice versa.
    The advantage of using transformers matched to the load is that the signal to noise ratio is optimized. This comes at the cost of having to use different transformers.
    The advantage of using a single, high current type of transformer is that you need only one type on stock. The disadvantage is that at low currents your signal to noise ratio becomes worse as you cannot use the full operating range of the transformer.
    Your choice will depend on the required accuracy of your measurements.

    A CT is a current transformer and as the name says it transforms one current (here the high primary current) into another current (here a small secondary current). You will therefore have to measure the output current of the transformer. There are several means of doing so. If you chose to convert the secondary current to a voltage using a shunt resistor, be aware that this results in a phase error of the transformer which depends on the resistance of the shunt. The transformer in Gryd3's link e.g. is tested with a shunt resistor (load) of 10 Ω only.
    You need to minimize phase error in order to correctlly calculate the power used by the furnace. If the phase error between voltage measurement and current measurement is too big, you need to compensate e.g. mathematically by applying some correction factor or by artificially phase shifting the measured dat to be in phase again.

    Note also that due to the current source nature of the current transformer's output you should never operate a current tgransformer without load. According to Ohm's law the output voltage across the load of a current transformer is Vout = Rshunt*Iout. With an open circuit on the output side of the current transformer the resistance of Rshunt is ~infinite. Even a very, very small current will produce a very high voltage (anything*inifinite = infinite) at dangerously high levels.

    Read a bit more on current transformer theory e.g. here.
    CDRIVE and Gryd3 like this.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Curiosity is killing me. I live where furnaces don't exist for heating homes. Just about all our structures are electric (forced air over hot wire) heat. That said you'd be hard pressed to find a home that draws 80Amps! What kind of furnace is this?

  5. mjscott


    Feb 2, 2017
    Wow, so much knowledge on this forum - I chose well! Thanks to all for the great start on this topic. I have much more research to do, and no doubt more questions will come from that.

    Chris: My house is about 6 years old, and has a "Heat Pump" style furnace - warm in the winter, cool in the summer. For my region, they are supposed to be very effective. However, if the temp drops TOO cold, the electric backup within the furnace kicks in (forced air over hot wire). I have no idea if this draws the full 80A, but this is the size of the breaker on that circuit.
  6. Minder


    Apr 24, 2015
    Honeywell make just the type of units in their CSDA line.
    Also for a furnace you have 120v loads (fan motor etc) as well as the 240v furnace.
    I doubt your furnace is 80amps if residential, if so you must have a large area to heat.
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