# Output math - amps, watts, or something else?

Finally got the IoTaWatt and I’m trying to set it up optimizing my inputs. Most have been straightforward 120-only or 240-only devices easy to monitor with a single CT.

The one thing I’d like to do is break down my “MISC” (mains - known = leftover) into L1 and L2 to get better granularity. For most things this is straightforward bu the clothes dryer runs on 240V heating but 120V motor. To get power I’ve put the wires thru the CT in opposite directions but I want to do some math with the motor (known 5.39A on L1 only)

I know that its guaranteed if its using any power it will be running the motor (which is not variable speed) so that is a constant.

I don’t know if my “constant” should be amps or watts or ???

When you use that technique for measuring a 240V circuit, the IoTaWatt doesn’t know if its one or more independent 120V circuits or two legs of a single 240V circuit. So it is treated as a 120V circuit and the Amps are the total Amps at 120V which is double the amps at 240V. If you think about it, with the heater at 240V and the motor at 120V, there is no unqualified Amps metric.

I’d recommend you use Watts as your primary metric, as that is accurate in all circumstances. My dryer uses about 200 Watts for the 120V motor.So take a closer look at that.

I still need to know which to use for entering the formulas. All my clamp meters, generator ammeter, etc. gives me readings in amps so ultimately I’m interested in knowing that to help me understand “normal” loads and what I could run off backup power.

I suppose the question I’m trying to understand if I need to put a correction in the formula, what are the “input” units and “output” units. I know CTs measure in amps not watts (since it has to also combine with voltage to get watts).

I see a few ways it could be programmed “under the hood”
-Everything comes in as raw amps, math is done, then multiplied by the voltage-detector to the desired units
-Everything comes in converted to watts, math is done, then converted to the desired units
-Converted to desired units as it comes in, before any math is applied

This would be a lot easier if my usecase only cared about total power, but I want to (as best I can) handle per-leg power. I know I could do it with a 2nd CT on the other leg of the dryer but I’d prefer to have more things I can measure if possible and handle it with the math and known constant load if possible.

The watts reading is going to be skewed anyway since the L1/L2 voltages are not equal and also the voltage transformer isn’t on its own dedicated circuit so it sags down as the circuit has more load on it. The CT amps output won’t be affected that same way which also simplifies things for my interest.

You specify the units to be used in the calculator. Watts and Amps are both possible, however it’s one or the other. It wasn’t designed to do what you are trying to do. You can come close using the min and max functions very creatively, but it’s very convoluted and not something I would recommend doing.

I think you’re overthinking this. With respect to generator sizing, any L1/L2 voltage difference isn’t going to make or break a generator capacity calculation.

Because the IoTaWatt is near the panel, the wire from the VT plug to the panel is usually pretty short. Voltage drop at the VT plug is a function of the current in the wire times the resistance from the plug to the panel, which should be very small. So any voltage drop (or increase) that you see due to a high-load 120V appliance is probably not unique to the VT circuit, but a function of the resistance of your mains feeds and neutral as well as the internal resistance of the pole transformer.

At some point it has to do some math though to get the units sorted out…still don’t understand where that comes into play.

Maybe I’ll give up and take away the sensor I was going to put on the kitchen and use it to split the dryer…that seems to be the only appliance that is half-120 and half-240 volt.