Iotawatt Air Conditioner Wattage

I got my Iotawatt installed yesterday and let it run overnight. I’m wondering about the measurement precision though, specifically on my air conditioner circuit. This may be more of an air conditioner-specific question than an Iotawatt question, but I figured I would start here.

I installed the Iotawatt on my outdoor breaker panel, monitoring 4 circuits – air conditioner (90A breaker), water heater (30A), electric stove (40A), and sub-panel in the garage (90A). All are 240v breakers with two 120v legs. I put the CT around one leg from the breaker, and clicked the box to double each one.

I let it run overnight and got the following usage graph:

My air conditioner is a 3 ton unit, so I was curious about whether my portable generator would be able to power it. My generator is 7500w, so according to that chart, it should be fine. I know it has a higher startup wattage though, so I’m wondering if that can be measured by Iotawatt. Does the startup wattage last for a shorter period of time than the Iotawatt can measure?

(As an aside, the 2.5 kW that the AC is using is lower than I expected, but the listed wattage on my water heater is 5500 W, so everything seems to be measuring correctly.)

The water heater should be fine as they are almost always two-wire. The stove and AC should be checked to see if they are three-wire. If so, you will need to use one of the three-wire strategies.

The sub-panel is almost certainly a three-wire load and will require two CTs.

Inrush current for the AC is usually pretty short duration and will not show on the graph. Most generators are rated for running power and have a higher starting or transient capacity. I wouldn’t try to run the AC with a 2500Watt generator, but a 3000 with a higher starting rating would probably be OK.

The way it looks to me, the garage panel and AC are two wire, and the stove is three-wire, but since you said the garage panel is almost certainly a three-wire, maybe I’m not looking at it correctly. I’ve put some pictures below; not sure if you can tell anything from them, but I’ve tried to label them.

Kind of hard to see, but there are two white wires on the neutral bar on the right side. One is a neutral that I added yesterday, to power the outlet that I have the Iotawatt plugged into (combined with the black wire from the well circuit). The other white wire looks like it is in the stove 3-conductor wire.

Also a picture of the garage subpanel, if this helps.

It looks like your sub-panel is using the ground as a neutral. That is not code at all. If there is a conduit between the two, I would look into having a neutral pulled and “unbonding” the sub-panel.

If anything happened to that ground conductor, the ground in the panel, and any appliance connected to it, could become a shock hazard. As it is, your ground is probably floating several volts away as 120V appliances become unbalanced from leg to leg.

In any event, the sub-panel is three wire and needs to be measured as such with two CTs.

Thanks. I was considering getting an electrician to come out and look into installing either a transfer switch or interlock kit, so I’ll definitely get them to look at pulling a neutral wire to the subpanel. They’re not terribly far away, so hopefully it wouldn’t be too hard.

Am I correct that the air conditioner is okay with a single CT?

Thanks for your help.

Can’t say looking at it. There is another “ground” wire there, and whoever wired the sub-panel probably wired the AC and could be using it for both a neutral and ground there as well. Might help to see where it connects to the AC. Also, you could put a CT on both for a day or so and see if they are the same. If so, doubling is OK.