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Hello Dave,

I'm in the US, and have had to deal with the CT issue too.

Here's the link  for the Magnelab 0750 CT. The 1 volt option costs an extra $7 and from what one of the other forum members tells me, they charge a one-time non-refundable engineering fee of $50.
www.aimdynamics.com/SCT-0750-Split-Core-Current-Sensor-333mV-Output-p/sct-0750.htm

The 1 volt option is a check-box located a tad more than half-way down the page.

To get an idea of which CT to use, calculate an estimate of the total load the appliances in your home represent. Keep in mind you more than likely don't run many of them at the same time. IOW, if you add the numbers on all of the breakers in your breaker panel, you'll find the total is quite a bit larger than 200. So, even though you have 200 Amp service, your typical load will be much less, especially if you have gas service, and your space/water heater, oven/stove and clothes dryer are gas-fired, vice electric. Once you have a total, allow a bit for some "headroom," say 25%, and the result should give you an idea of the CT rating you'll want.

In general terms, 100 Amps is approximately 24,000 Watts, so you'll probably find your load total considerably less than that, unless you have an all electric house, or large electric space heaters, in which case the total could easily exceed 24,000 Watts. (I have an all electric house, and my max load total is roughly 52,000 Watts.)  You may find 70, or even  50 Amps (approximately 16,800, and 12000 Watts respectively) is closer to your daily maximum electrical load.

Bill

Thanks Bill - I'll take a look at meterusa. I looked at aim dynamics and yes, apparently they charge $50 for the 1v option (the cart doesn't specify what it is, but there's a $50 charge in addition to the cost of the CTs) - that's highway robbery IMO.

As far as the amperage, I know that I'm not pulling 200A just because I have 200A service, although with how I worded it, I think I can see how it might have appeared otherwise. My question was more if I were to assume that at some point I might max out my 200A service (although HIGHLY unlikely), is that 200A 100A per 120v leg, thus a 100A CT would still be safe?

I looked at MeterUSA, and maybe they're a good company, but their order method is atrocious. E-mailing credit card info? Sorry, no thanks.

If I read right, the voltage versions (.333v, 1v, 2v, etc) have internal burden resistors - Are the 'mA' versions such as WC1-100-MA100 or WC3-200-MA100 unburdened? Would that be suitable? Or is there a way to scale the EmonTX to be able to use the much more commonly available .333v versions (or would accuracy suffer that way)?

You can use a 0.333 V version, but you lose two-thirds of the resolution, so it's not a good solution, it would mean the minimum current you could measure reliably would need to be three times larger that it would be for a 1 V version. Whichever voltage output you have, you need to remove the SMT burden anyway. If you can get a current output, i.e. no internal burden resistor, that is by far the best solution as you can scale the input to whatever you want - subject to the limitation of the VA rating of CT itself, of course. In that case, you'll almost certainly need to remove the SMT burden resistor and add a wire-ended one of the correct value.

I looked at aim dynamics ... that's highway robbery IMO

Yeah, 50 bucks was too rich for my blood too.  I went with Wattcore WC-1s. (Byram Labs)

As far as the amperage, I know that I'm not pulling 200A just because I have 200A service

You'd be surprised at how common that misconception is!

My question was is that 200A 100A per 120v leg

200 Amp service refers to what the load center and feeder cables are capable of handling.

The reality of things is often much different. PoCos usually put 2, 3 or 4 houses on one transformer. The typical transformer they use will be either a 15kVa (two houses), or a 25kVa unit. (3 or 4 houses) The 15kVa transformer has a max continuous rating of ~62 Amps and can deliver 150% of that for a short period.

The 25kVa transformer has a maximum continuous supply rating of 104 Amps, and, like the 15kVa units can supply 150% of rated max for a short period. Read "short period" as 1 to 3 hours, depending on load and ambient temperature.

Given your house is sharing the transformer with as many as three other houses, your maximum available load current isn't likely to be close to 100 Amps.

I looked at MeterUSA, and maybe they're a good company, but their order method is atrocious. E-mailing credit card info? Sorry, no thanks.

Yikes! I haven't ordered anything from them, so didn't know that.
(I bought my WC-1s from Byram Labs)
I sure as hell won't be recommending them any more. Thanks for the heads-up!

Thanks Robert - so to be clear, a WC3-200-MA100 or WC1-100-MA100 which deliver 100mA output are unburdend CTs, thus they would work provided I get and install the proper burden resistor on the EmonTX?

Edit: It does seem as if that is the case since the 'SCT-013-000' seems to be a 0-50mA CT- assuming it is, it would seem that a 100mA version would likely be the easiest to scale (1mA = 1A, 100mA = 100A?), and maybe even more accurate? I just want to be sure before I go ahead and order some.

Given Bills pointing out the power delivery limitations, would I be correct to think that using a 100A CT would give better resolution and accuracy than the 200A CT?

Dave,

Tthe only other thing I can think of regarding the WC-1 CT is the case might need some of the plastic trimmed from the ID of the shell. I had to trim mine to fit my 4/0 feeders. Other than that, they work like a champ.

For the Magnelab CTs, the SCT-0750-000 would be appropriate, however, the -000 part doesn't seem to exist.

I missed that one earlier. The -000 part number (un-burdened CT) is for the SCT-1250.

www.magnelab.com/products/ac-split-core-current-sensor-sct-1250/

The 1250 and the 0750 are identical save for the size of the opening. (according to Jeff Lawson, at Magnelab)

I went back to the Magnelab page. the unburdened 0750 (SCT-0750-000) is here:
www.magnelab.com/products/ac-split-core-current-sensor-sct-0750/
www.aimdynamics.com/SCT-0750-Split-Core-Current-Sensor-333mV-Output-p/sct-0750.htm

On the Aim Dynamics page, the unburdened 0750 is selected from the "Amp Rating" pull down menu as opposed to the "Customizations" menu.

Well, according to the specs, the WC1's opening is .62" at its smallest point and .75" at its largest - I ASSUME that's the plastic opening and not the magnet's opening. Assuming that's the case, it should fit my supply cables as they measure .59" (I measured them yesterday with a caliper).

You should be in good shape then. My feeders were 5/8 inch (.625)

Yes, that's the dimensions of the plastic shell ID. The core is square shaped ~.15 inches larger than the opening

Cool, thanks Bill - now to wait for everything to get here, then figure out what I need for burden resistors. I've read that unburdened CTs are capable of generating dangerous voltages, and I've also read that they typically have internal zener diodes to limit the voltage to something sane - Do I need to take any special precautions with these CTs?

I've read that unburdened CTs are capable of generating dangerous voltages, and I've also read that they typically have internal zener diodes to limit the voltage to something sane

Correct on both accounts. But you'd need to check the CT's datasheet to be sure about the Zeners.

Rules to follow when using unburdened CTs:

When installing the CT, always connect the CT leads before attaching the CT to the monitored circuit.

When removing the CT, always remove the CT from the monitored circuit before disconnecting the leads.

How about installing the burden resistor in the CT's plug so that it's technically internally burdened (Assuming there's enough room for it to fit)?

"How about installing the burden resistor in the CT's plug"

That would be a good way to go, if there's some space around it for cooling, as at max current it's likely to be dissipating about 150 mW. (Assuming a 100 mA output.)

(Many CT's come with a link or a plug to short the CT prior to removing the burden. It is always safe to short a CT. As Bill says, never assume a CT is safe until it's shorted or you know the main cable is not carrying current.)

Tell us which CT you have, and the maximum current you want to measure, and it's quite easy to work out the burden value.

E.g. If your CT is 100 A : 100 mA, and you require 80 A to give you maximum scale on your emonTx; then the CT gives you 80 mA max, and the burden needs to develop 1.1 V at that current (that voltage is the rms value of a 3.3 V p-p sine wave less a few percent headroom for component tolerances). The burden value is then 1.1 V / 80 mA = 13.75 Ω. You can't buy that, a 12 Ω (always go down!) will give you 91.66 A max reading so will meet your spec, a 15 Ω will give you 73.33 A. The calibration coefficient for the 12 Ω burden will be (100 A / 100 mA) / 12 Ω = 83.33

If there isn't enough room for a resistor in the plug, then since it's safe to short a CT, the other option would be to have a stub with a small connector of some sort exit the plug, so before disconnecting, connect a jumper to short it, rendering it safe.

I'll be using two 'WC1-100-MA100' CTs for monitoring my main supply, and two 'SCT-013-000' CTs to monitor the feed to my sub panel. If I were to add one or more additional EmonTX's to monitor other circuits, I'd probably use more 'SCT-013-000' CTs as they're cheap and have plenty of capacity for pretty much everything other than the main feed (and they'd probably have the CAPACITY for that, but they're not big enough to fit over the feed lines).

I don't know what scale I'd need for my main feed to get optimal scale & resolution (I'd say 75-100A would probably be the ideal area - I could always set it up for ~75, and if that's not enough, I could always change the resistor to up it to 100), but my sub panel feed probably doesn't need much more than 30A of measuring capacity. Maybe 50A to allow plenty of room.

After reading your formula again, I'll probably set the main feeds up with 12Ω resistors. Worst case, I can always change them if need be. Or maybe I'll set it up with 15Ω resistors and see what I get from there....

A thought - the V3 apparently isn't set up for it, but what about rather than using SMD or through hole resistors, setting it up (Maybe a EmonTX v4) with POTs (along with maybe a jumper to bypass the POTs if you have a burdened CT) to allow the user to fine tune the burden much more than you could with standard resistors? With a POT, if I wanted 13.75Ω, I could use a meter to dial that exact value up.

I wouldn't use a variable resistor for several reasons - you are likely never to need to change it unless your useage pattern varies markedly, the position can be changed and you lose your calibration, they cost a lot more and are less reliable. There's no need to set an exact value, you can easily tweak the calibration in the sketch. I'd use a pot in an analogue-only instrument (the sort of thing I cut my teeth on 40+ years ago, when the best resistor you could get for a reasonable price was a 10% tolerance one), but these days most people would see a pot as an indication of somebody who didn't know what they were doing, and needed a fiddle to get the result they expected.

I just got the SCT-013-000 CTs today, and hopefully will get the EmonTX tomorrow (it appears to have left Chicago and is on its way here) - Since these appear to be the exact same sensors sold in the shop and are unburdened, I'm assuming that these will be Plug n Play with the EmonTX, requiring no change to the onboard burden resistor?

Yes, the YHDC SCT-013-000 will plug straight in and give you input readings up to 100 A.
(As will any other 100 A : 50 mA CT that is capable of 1.1 V output at rated current.)