Newbie Question/Clarification of US CT

I have been reading thru the forums, and I THINK i understand what is required for the US CT to be used on the mains...

Use a SCT-1250-000 with no burndend...  Then on the board of v2 solder in a 39 ohm 1% resistor...

Is this correct? Where is the burndend resistor that would need to be replaced?  I know i would need two, one for each phase at 200A...


I did read the forums, its just not in one place so I understand it.



Robert Wall's picture

Re: Newbie Question/Clarification of US CT

That's all correct. The 39 Ω resistor will give you 1.04 V rms per 200 A primary current. The burden resistor is in parallel with the CT input, it's 18 Ω (15 Ω if you have a very old V2). It's R6 for CT1, R3 for CT2 and R16 for CT3. Your current calibration constant will be 44.872.

nichols430's picture

Re: Newbie Question/Clarification of US CT

Going to be buying the stuff here in the next few days. 

What is the calibration constant... that has to be in the software, I'll read up on that.

Thank you

Bill Thomson's picture

Re: Newbie Question/Clarification of US CT

I have a question... If he has a 200A service (quite common in the US) as opposed to actually needing to measure ~200A, his max load is very likely going to be considerably less than 200A. Wouldn't he be better served by CT with a lower rating, say a 100A or a 70A unit? (I have an all electric house with a 200A service, ~52kW of large appliance type loads, yet typically see a max load of ~15kW since it's very rare that more than 2 or 3 of those loads are active at the same time)

The 70A CT would be linear to 91A (given Magnelab's 130% over rated value spec) which works out to ~20kW (assuming a mostly resistive load)

Another choice would be to specify the 1V RMS output option for the CT. (A few months ago, Magnelab started offering their CTs with 1, 2 and 5V RMS output options at a slightly higher cost per CT.)

The link says 333mV, but the 1V option is on that page, listed under "Acessories."

Robert Wall's picture

Re: Newbie Question/Clarification of US CT

That argument would be OK if we knew what the maximum demand would be, and we knew that the emonTx would survive a downstream fault. We've put series resistors in the input on the V3 so that we can be reasonably sure that CT saturation and those will prevent a damaging current into the input, but the V2 doesn't have those and I haven't seen where the saturation curve of the Magnelab CTs levels off.

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