400A service, 3 panels, and lots of unused circuits

I’ve got a pretty complicated electric setup at my house and I’m trying to figure out if buying a bunch of IoTaWatts might solve my needs. The root problem is that my house is using far, far too much power and I don’t know which devices are responsible. My plan is to brute-force it by trying to monitor everything.

The problem starts with the sheer number of panels I’ve got. I’ve got a residential 400 amp service panel that’s fed by a total of 6 aluminum wires that look like 2- or 4-AWG each. There are two wires for each of the input phases plus neutral. It looks like this under the hood:

From that panel, there’s a 200 amp circuit going to a neighboring sub-panel, and a 100 amp circuit for another sub-panel in the garage. I might (for now) just monitor that garage sub-panel without bothering to put another device there to monitor the circuits individually, but I do want to measure most of the circuits in both of those two side-by-side main panels. Each of those panels is using roughly 22 positions each.

The good news about those two panels for me is that there are a bunch of the circuits I don’t necessarily care about monitoring individually because I expect them to have no draw ever. My plan is to use some headphone splitters to capture two (or more?) circuits on one channel.

First question: Has anyone every used a many-way splitter like this one to join several clamps onto one channel? I’d probably be looking to monitor several 15- or 20-amp circuits where I only really care to know if there’s any current draw at all. If there is I can investigate manually. Are there any electric loss implications for splitting the signal so many ways?

Second question: I read in this topic that there were some 400 amp clamps available by special order. What sort of wire(s) will those clamps fit around? I don’t know if it’s normal for 400 amp main breakers to be fed by as many wires as mine is, and it’s not exactly an easy sort of scenario to describe into the forum’s search engine…

Two-point-five question: Where/how would special order those 400 amp clamps?

Third question: Are there any clamps that are rated for less than 50 amps? If not, is there any problem putting a 50 amp clamp on a circuit that’s never going to draw that much? I’ve mostly got 15- and 20-amp circuits I want to monitor.

There has been increasing interest in larger CTs in industrial and commercial installations. They are pricey, but I can get them within a few weeks and will soon begin stocking the more common models.

Your cables are right next to each other, so the question is whether you can put a single CT around each pair. You would need to carefully measure the diameter of the insulated cables. I can get CTs with 24mm and 36mm openings. If your individual cables are less than 18mm, you could probably use 36mm CTs. Otherwise, you could use 24mm CTs around each cable. You can use standard (in stock) 200A units into two inputs - 4 total for two mains, or you can use 24mm 400A CTs combined with headphone splitters into 2 total inputs for the mains.

The rest can be done with standard CTs. I have not tried a four way splitter, but I have tried 3 and it’s fine. I see no reason it should not work. Also, don’t underestimate the potential to pass multiple conductors through one CT.

There should be no losses due to using splitters. These are current loops, and so resistance, within reason, has no effect.

I will be doing a special order soon. Tell me what you want.

I don’t sell any. Echun has a 20A CT but I have tested them and they do not handle 20A when used
with IoTaWatt. 50A CTs are fine to use on 15 and 20 circuits.

Since you have 400A service, those should be minimum doubled 4/0 (four-aught). That is assuming 90C insulation. Those wires are very stiff, but much less than a single 700 mcm one would be. It will be hard to separate them enough to put a CT on each, but not clear it will be easier to put one around both cables.

You don’t really need to monitor all your circuits, especially if you can measure the mains. Even if you can’t get a CT around your mains, you can still learn alot by measuring a few circuits at a time.

Given your service, you must have a very large house and/or electric resistance heat (possibly back up for heat pump). Start measuring the big circuits. The ones that are used a lot will drive your bill. But, some might be used with a low enough percentage that they don’t have much of an impact. I have two pumps that draw a lot power, but are only on for a couple of min one to a few times a day. My base load, which is on 24 hours a day every day, has a bigger impact. Since it is always on, finding it is easier. I move my channels around every once in a while. But, it would be easier to just monitor everything. I already have two IoTaWatt devices and would need to get a couple more to monitor every circuit. But, I don’t believe I would really learn anything more (certainly not enough to pay for the additional hardware). It would be interesting though :wink:

I’m a bit confused by the second option here. It sounds like the same thing as the first option, only with 400A CTs instead of 200A CTs? I think my highest priority is to be able to maximize the number of monitored circuits, so I’d prefer the option that only uses 2 inputs.

It seems like since it’s 400A total service that each of the two phases shouldn’t exceed 200A individually (tell me if this assumption is wrong please), and further my two phases are split into two wires each to carry their 200A, so each wire individually shouldn’t carry more than 100A. At any rate, it seems like I can just buy 4 x 200A 24mm CTs and use two main inputs. If I could combine 4 x 400A 24mm into one input instead I’d prefer to pay extra to get the 400A CTs.

I measured the individual strands at 16mm. I don’t fancy trying to wrap one large 36mm CT around two wires, so I think I’d like to get 4 of the 24mm CTs. If the answer to the question above about 4 x 400A 24mm going into one input is true then I’d like to sign up for those. How should I handle the order? Want me to order the rest of the stuff I need and you can adjust it afterward?

I love this idea. Are there any things to be aware of if I do this? F.e. should I avoid mixing the AC phases?

If you use 200A CTs, they must be individually connected to different inputs because they will each output at the capacity of an input at 200A. The capacity of a single input is 50mA.

When you combine multiple CTs with a splitter, their outputs are added together. So if you combine two 200A CTs the maximum combined output would be 100mA, which overloads the IoTaWatt input.

When you use 400A CTs, they output 50mA at 400A primary, so using them on a 200A conductor means each will output 25mA at 200A primary, or 50mA combined which is in range.

So bottom line, if you really want to save two inputs, you must use ECS24400 (400A) CTs combined with a splitter. These don’t cost a lot more than the ECS24200 but I do need to special order them. It will take 2-4 weeks.

If you haven’t read the documentation about CTs, you should do that now. It doesn’t covert this concept specifically, but will give you a little more understanding about how they work. In particular, pay attention to the method used to measure both legs of a 240V circuit by passing the conductors in opposite directions. The concept is the same if you are measuring two 120V circuits that are on opposite phase (that’s actually what the 240V circuit is). You can pass multiple conductors through one CT provided you adhere to these rules:

  • They must all fit easily through the opening without forcing the CT closed.
  • The aggregate current from all circuits must not exceed the rated capacity of the CT.
  • Conductors on opposite phase must pass through in the opposite direction.

One simple way to get the direction correct is to clamp the CT around each of the conductors individually to be sure the reading is not reversed (little ↺ in the status display). If you get one wrong, it will subtract from the total rather than add, so you may notice strange behavior if one is oriented incorrectly.

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I do want to go with the 400A CTs and I don’t mind waiting for them to arrive. I’ve been wanting to do this sort of monitoring for past two years since I moved in, a couple more weeks delay won’t hurt anything.

Do you want to order them and met me know when they arrive, or can I put in an order now in the store and find a magic hidden product page for the 400A CTs?

I will be ordering CTs later this week, and I have intended to stock these going forward. So I will get a case of them and add to the store. I’ll send you a PM when it’s done.


Any chance there is still time to add 2 more 400A CTs to your order? If so, please add me to the order. Thank you.

I am ordering enough to stock a small quantity.

Can you post to this tread when the item is available for purchase in the store? FYI the iottawatt homepage appears to have a duplicate ‘How it Works’ entry at the moment. Perhaps this is intentional or is a glitch with my browser.

Nothing makes you feel more secure than a little redundancy :grin:

Thanks, I’ll clean that up.

400A x 24mm CT is now available. Also have some 400A x 36mm and 800A x 55mm that will be added in the next couple of days. Get ready for sticker shock. Not only are these items pricey, but were shipped air-freight and have the 25% Trump small business tax on top of the standard 6.1% charged for these commodities.