Archived Forum

I am really surprised 200A monitoring is not supported out of the box as that is so common in North America, am I missing something obvious?

Hi Luke,

Having 200 Amp service means the load center is rated at 200 Amps. The majority of transformers feeding N. American homes are 15 or 25 kVa units, that are shared with 1, 2, or 3 other houses. Unless your house is the only one connected to the transformer your maximum available current is likely to be quite a bit less than 200 Amps. A 15kVa transformer can supply a max of 62A continuous, and a 25kVa unit a max of 104A continuous. Both can supply 150% of max for short periods, typically, 1 to 3 hours depending on ambient temp and load.

As an example, I have ~52kW of large load appliances, (all electric house) yet I'm on a 25kVa transformer. (it would be rare, and in one instance, just plain stupid) that all of them would be powered on simultaneously. (e.g. the space heaters and air conditioners!)

The real obstacle to monitoring a N. American 200A service is the size of the feeders. The CTs sold in the OEM shop wont fit the 2/0 Copper, or 4/0 Aluminum wires typically used.

Your 400A service indicates either, a large house, or an all electric house with more than the usual number of large load (240V) appliances, so you're probably connected to a 37.5 or possibly even a 50kVa transformer. Even if your house is the only one connected to a 50, your max continuous available load current is 208A.  (150% rule, same as above) You'd have to be connected to a 100 kVa transformer in order to actually have a feed capable of supplying 400A continuously. I'm not saying they don't exist in a residential scenario, but all the pots that big, that I've ever seen, have been connected to businesses, schools, etc.

Thanks for the reply Bill!

Very good points.  I agree I will not be drawing that much current with my current home setup so 100A clamps will be okay but you brought up a good point about them not fitting the 4/0 wire.  Can I just buy larger clamps with a 0-50mA output current or is the emon calibrated for the Yhdc.

The house is build with massive overkill, it is 3800 Sq. Ft., I am switching a lot of the large loads over to natural gas and switching out a lot of loads to much more energy efficient options like all LED lighting and variable speed HVAC and variable speed pool pumps.

The house has a 100KW generator which is also just wild overkill.  Here is the automatic transfer switch, 4/0 wires.
https://goo.gl/photos/96meeEe5HkmYjVwF9

This is also in a costal environment and the load centers are outdoors so would like to keep as much of the open energy monitor equipment protected, could put them in a PVC enclosure.  What setup would make the most sense to monitor the four feeder wires?

[Duplicate post deleted. (RW)]

Take a look at the Building Blocks article about use in N.America, if you haven't already done so.

One emonTx, with the burden resistors changed to suit your choice of CT, is where I'd start.

The house is build with massive overkill, it is 3800 Sq. Ft.,

Sounds like a really nice place! Which coast are you on?

The house has a 100KW generator which is also just wild overkill.

No worries about losing commercial power! Seriously though, you're right. I live in Tornado Alley and most of the EPGs here are in the 10-15kW range.

The Magnelab SCT-1250 CT would fit your 4/0 feeders. The opening is 1.25 inches. You'd want the unburdened version. (P/N SCT-1250-000)
www.magnelab.com/products/ac-split-core-current-sensor-sct-1250/

The Wattcore WC-3 would also work:
www.amazon.com/WATTCORE-WC3-Split-core-Current-Transformer/dp/B007RFKUGK/ref=sr_1_4?s=industrial&ie=UTF8&qid=1439405668&sr=1-4&keywords=wattcore+wc-3

Here's the link to the article Robert mentioned:
http://openenergymonitor.org/emon/buildingblocks/EmonTx-in-North-America

Thank Bill.  I am on the gulf coast of Alabama.

I got a kick out of magnelab fruit in all there product pics.

It would be great if I could find a large current clamp to go around both 4/0 wires on each transfer switch lug so I could only use two current clamps and the emonPi to monitor the whole house load to start out then later I can split out the two load centers with four clamps and another module like the emonTx.  Maybe this current clamp would be good, the SCT-2000-000 prices I have found online are steep, $60 for one clamp: http://www.magnelab.com/products/ac-split-core-current-sensor-sct-2000/
What specs should I be looking for in the current clamps other than no burden resistor.

From the linked article I take it if I use the SCT-2000-000 I need to solder a 5.6ohm burden resistor in on the emon board?

Cheers, Luke

I am on the gulf coast of Alabama.

The Katrinas, Ritas and Wilmas of the weather world make an EPG a necessity in my book. It's about the same here in SW Oklahoma, just a different kind of storm.

I got a kick out of magnelab fruit in all there product pics.

Yeah, kind of silly/cheesy, eh?

It would be great if I could find a large current clamp to go around both 4/0 wires

Have you measured your feeders? Most of the 4/0 stuff I've dealt with measures from ~.59 to ~.62 inches OD. I'm thinking you should be able to get two 4/0 wires in an SCT-1250. At 48 bucks a pop, that'd be a little easier on your wallet.

What specs should I be looking for in the current clamps other than no burden resistor.

Do a "load audit" to get a rough idea of your peak load, add 20-25% for headroom, and use a CT with a rating as close to, but greater than, that number.

From the linked article I take it if I use the SCT-2000-000 I need to solder a 5.6ohm burden resistor in on the emon board?

Right. There are holes in the PCB provided for just that purpose. You'll need to remove the SMT resistors.

And get a CT with the smallest secondary current available, i.e. you don't want a 200 : 5 CT because the power in the burden resistor would be prohibitive (~ 5 W each on the PCB), and the connectors couldn't handle the current. Anything up to 100 mA is fine; above that, do the sums and check.

[If you need a lower value of burden than the SMT resistor, you could add a parallel resistor of the correct value so that the parallel combination is what you need - e.g. a 7.5 Ω in parallel with the existing 22 Ω SMT will give you the 5.6 Ω that you're looking for.]

Have you measured your feeders? Most of the 4/0 stuff I've dealt with measures from ~.59 to ~.62 inches OD. I'm thinking you should be able to get two 4/0 wires in an SCT-1250. At 48 bucks a pop, that'd be a little easier on your wallet.

I am measuring 1.33in total width, the SCT-1250 is unfortunately too small.
https://goo.gl/photos/C43w3k77K6N8RsV17

​Do a "load audit" to get a rough idea of your peak load, add 20-25% for headroom, and use a CT with a rating as close to, but greater than, that number.

I am measuring about 55 amps on each split phase leg with both AC condensers on and both pool pumps on.
https://goo.gl/photos/h1aTMjjkUtY9hk2S9
The hot water heaters are electric and I am sure there is more things that could come like lighting loads, oven, refrigerators, etc.  If I add in the two hot water heaters and the cloths dryer running it would probably push split phase up to around 100 amps each.  A lot of work is going to be done soon to increase efficiently like switch water heating to condensing natural gas units.

And get a CT with the smallest secondary current available, i.e. you don't want a 200 : 5 CT because the power in the burden resistor would be prohibitive (~ 5 W each on the PCB), and the connectors couldn't handle the current. Anything up to 100 mA is fine; above that, do the sums and check.

[If you need a lower value of burden than the SMT resistor, you could add a parallel resistor of the correct value so that the parallel combination is what you need - e.g. a 7.5 Ω in parallel with the existing 22 Ω SMT will give you the 5.6 Ω that you're looking for.]

I am confused here, Is there an article describing this?  Could I get the SCT-2000-100 version and desolder the burden resistor on the emonPi?  Does the current rated versions (none 000 part numbers) have burden resistors built into the clamp or are there coil ratios different?  Why is the coil ratio not specified in the datasheet? Sorry if I am missing something obvious.

"I am confused here, Is there an article describing this?"

No, it's all basic electrical engineering - I was only pointing out the pitfalls that you need to be wary of.
The SCT-2000-000 would need a 39 Ω burden to give you a little less than the desired 1.1 V rms @ 200 A, and that would be fine - the secondary current is 26.66 mA and the burden power is negligible.

If you get the standard SCT-2000-100, there is a built-in burden resistor but it's set to give 0.333 V out - less than one third of what you want. You need to specify a 1 V output if you want to go down that route. CTs are specified by the current ratio, not the turns ratio. You'd best ask the manufacturer why they don't give that on the data sheet. We've asked and we know that the SCT-2000 has approximately 7500 secondary turns. Magnelab will vary that by a small amount depending on the properties of the steel (principally) to give the best overall accuracy, so it will vary from batch to batch.

I am measuring 1.33in total width, the SCT-1250 is unfortunately too small.

Bummer. Looks as if you're stuck with the 1250 CT, or one with an opening large enough to accommodate your feeders..

I am measuring about 55 amps on each split phase leg with both AC condensers on and both pool pumps on.
https://goo.gl/photos/h1aTMjjkUtY9hk2S9
The hot water heaters are electric and I am sure there is more things that could come like lighting loads, oven, refrigerators, etc.

If the likelihood of most or all of those loads being powered on simultaneously is more than just minimal, then you'll want a 150A CT so you'll have some headroom.

I am confused here, Is there an article describing this?  Could I get the SCT-2000-100 version and desolder the burden resistor on the emonPi?  Does the current rated versions (none 000 part numbers) have burden resistors built into the clamp or are there coil ratios different?  Why is the coil ratio not specified in the datasheet? Sorry if I am missing something obvious.

The SCT-2000-100 with the 1 Volt output option would work, but the cost is more than the SCT-2000-000.
(They charge a NFE fee of $50, and an additional $7.00 per CT for the 1V option)

The SCT-1250-100 CT output is only .333 Volts which would work, but your resolution would suffer badly, so it's definitely not recommended.

The -000 versions of Mgnelab's split-core CTs are unburdened. The SCT-0750 and the SCT-1250 series CTs are the same except for the size of the opening. (per one of the company's VPs) The turns ratio of the 2000 series CTs is the same as it is for the 750 and 1250 series CTs. There's more info in a Building Blocks article:
http://openenergymonitor.org/emon/buildingblocks

And get a CT with the smallest secondary current available, i.e. you don't want a 200 : 5 CT because the power in the burden resistor would be prohibitive (~ 5 W each on the PCB), and the connectors couldn't handle the current. Anything up to 100 mA is fine; above that, do the sums and check.

What Robert is saying here is the current that flows through the burden resistor must be kept to a level the resistor, and PCB circuit traces, can safely handle. i.e. without damage, or risk of fire.

[If you need a lower value of burden than the SMT resistor, you could add a parallel resistor of the correct value so that the parallel combination is what you need - e.g. a 7.5 Ω in parallel with the existing 22 Ω SMT will give you the 5.6 Ω that you're looking for.]

This is simply how to get a burden resistance of 5.6 Ohms. i.e. if the 22 Ohm surface mount resistor is left in place, and you install a 7.5 Ohm resistor in the holes provided, the two resistances in parallel yield 5.6 Ohms.

I found a link on the EmonTx - Use in North America page to a seller that sells the 100mA and 1V versions of the wattcore, there is a 100A input 1V output or 100mA output CT and is cheaper than the larger magnelab CT

1V output (WC4-100-RV001): http://www.byramlabs.com/store/product_info.php/products_id/20543
100mA output (WC4-100-MA100): http://www.byramlabs.com/store/product_info.php/products_id/20537

I really should have dived into how the whole setup reads the CT before asking these questions, feel a bit silly on that. Reading the building blocks article on the CT interface to see how they setup the ADC circuit to sample the CT: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface
Looks like the 5.6 Ohm in the chart is if I wanted to use the SCT-2000-000 to measure it's full range up to 1500A (with CT secondary output of 1.58V at 1500A), if I use a target output from the CT of 1.6V at 100A on the SCT-2000-000 and a ratio of 7500 I get a burden resistor value of 84 Ohm or a off the shelf 82 Ohm resistor which would give 1.55V.

If I used the 100mA wattcore clamp I am calculating 16ohm resistor for 1.6V desired output voltage.

Thanks for all the help, I will stay away from the higher current output CTs unless I get a great deal on two then I can use an external power resistor I suppose.

1.6 V input? I thought you were using an emonTx / emonPi. In that case, you should aim for 1.1 V across your burden resistor (to allow a bit for component tolerances). 1.6 V is appropriate for the Arduino with a 5 V supply.

Robert, your right, my mind was working in Vp for the magnelab SCT-2000-000 calculation as the calculations on this page converts the output of the CT from RMS to peak to use the Vref voltage.  If I convert the 100mA RMS current output of the wattcore WC4-100-MA100 to peak current I get 0.1414A, then using the 1.6V target Vp for the 3.3V ADC: 1.6V / 0.1414A = 11.3 Ohm.  So a 11 Ohm resistor instead of 16 Ohm.

You still shouldn't use the nominal peak-peak voltage, because given worst-case component tolerances, you can lose a good few percent off that, i.e. the CT tolerance, the burden, the centre bias resistors and the ADC reference voltage all contribute. Not to mention the wave shape, which doesn't have to be a true sinusoid. I have to say it again, it's all analysed in Building Blocks. 1.1 V rms is a reasonably safe number to use as your target maximum voltage.

I must rewrite all the calibration stuff one day, because people are always falling into that trap. There's a big difference between theory and what works all the time in the real world.