240V breakers: number of CTs and amperage headroom

[peppersass]

I've read all the guidelines on selecting CTs for 240V circuits, and have read as many posts on the subject as I could find here, but I'm still a bit uncertain about proper configuration, and I think others here might be too (e..g., a recent post asked whether the current for the two poles should be added to determine the CT amperage rating.)

Most of the circuits in my panel use single-pole 15A and 20A breakers, so CT-40s have plenty of amperage headroom for those circuits. But I have four two-pole 240V breakers, with each pole labeled as follows:

30A - Radio amplifiers and tower raising/lowering motor
40A - Range (gas burners with electric oven)
20A - Well pump
40A - Air conditioning compressor

My assumption is that I should not add the current marked on the poles to determine the appropriate CT rating. For example, if the two poles are each labeled 20A, the maximum current that can flow through either leg is 20A, and the total current draw of the device cannot exceed 20A (not 40A). Therefore, a CT-40 should be fine for monitoring either or both legs. Correct?

But how much amperage headroom is enough? It would appear that CT-40s should be fine for the radio equipment and the well pump, but I'm not so sure about the Range and Air Conditioning compressor, both of which contain motors. Large motors sometimes have momentary current spikes at startup. Breakers tend to be relative slow to react, so they won't necessarily trip due to such a spike. It's not clear to me whether such a momentary spike is figured into the device's maximum current rating and whether I need to account for it when sizing CTs. (NOTE: sometimes the 40A breaker for the A/C trips when it first starts up at the beginning of the cooling season, probably because there's some gunk that's accumulated around the shaft that has to be broken for the motor to get going. That tells me that the A/C unit can momentarily draw more than 40A.)

Given that CT-40s exactly match the current ratings of the two breakers in question, is it OK to use them, or should I configure more headroom by selecting CT-80s for those circuits? What's the danger of under-sizing a CT for a load that can momentarily exceed the CT's rating? Can the CT be damaged? Can the GEM be damaged?

Even though the subject of 240V circuits that use the neutral has been discussed here a lot, I'm still not certain about how the GEM handles the case of devices that use the neutral. My understanding is that some 240V devices use the neutral in order to create an internal 120V circuits for certain components. It's also my understanding that the current for drawn by the 240V components flows through both legs, while the current for the 120V circuit flows through only one leg and the neutral. So, when you put a CT on each 240V leg, how does the GEM compute the total current draw? I'm guessing that since the CTs have to be wired for opposite polarity, the currents add together and the GEM divides by two. But how does that when additional current flows through one leg into the neutral?

Also, if you aren't certain whether a device uses the neutral or not, can you put a CT on both legs, as long as you follow the polarity guidelines in the manual?

[Teken]

I've read all the guidelines on selecting CTs for 240V circuits, and have read as many posts on the subject as I could find here, but I'm still a bit uncertain about proper configuration, and I think others here might be too (e..g., a recent post asked whether the current for the two poles should be added to determine the CT amperage rating.)

Most of the circuits in my panel use single-pole 15A and 20A breakers, so CT-40s have plenty of amperage headroom for those circuits. But I have four two-pole 240V breakers, with each pole labeled as follows:

30A - Radio amplifiers and tower raising/lowering motor
40A - Range (gas burners with electric oven)
20A - Well pump
40A - Air conditioning compressor

My assumption is that I should not add the current marked on the poles to determine the appropriate CT rating. For example, if the two poles are each labeled 20A, the maximum current that can flow through either leg is 20A, and the total current draw of the device cannot exceed 20A (not 40A). Therefore, a CT-40 should be fine for monitoring either or both legs. Correct?

But how much amperage headroom is enough? It would appear that CT-40s should be fine for the radio equipment and the well pump, but I'm not so sure about the Range and Air Conditioning compressor, both of which contain motors. Large motors sometimes have momentary current spikes at startup. Breakers tend to be relative slow to react, so they won't necessarily trip due to such a spike. It's not clear to me whether such a momentary spike is figured into the device's maximum current rating and whether I need to account for it when sizing CTs. (NOTE: sometimes the 40A breaker for the A/C trips when it first starts up at the beginning of the cooling season, probably because there's some gunk that's accumulated around the shaft that has to be broken for the motor to get going. That tells me that the A/C unit can momentarily draw more than 40A.)

Given that CT-40s exactly match the current ratings of the two breakers in question, is it OK to use them, or should I configure more headroom by selecting CT-80s for those circuits? What's the danger of under-sizing a CT for a load that can momentarily exceed the CT's rating? Can the CT be damaged? Can the GEM be damaged?

Even though the subject of 240V circuits that use the neutral has been discussed here a lot, I'm still not certain about how the GEM handles the case of devices that use the neutral. My understanding is that some 240V devices use the neutral in order to create an internal 120V circuits for certain components. It's also my understanding that the current for drawn by the 240V components flows through both legs, while the current for the 120V circuit flows through only one leg and the neutral. So, when you put a CT on each 240V leg, how does the GEM compute the total current draw? I'm guessing that since the CTs have to be wired for opposite polarity, the currents add together and the GEM divides by two. But how does that when additional current flows through one leg into the neutral?

Also, if you aren't certain whether a device uses the neutral or not, can you put a CT on both legs, as long as you follow the polarity guidelines in the manual?

You can never go wrong with using a larger amperage CT. Regardless of the amperage of the CT the physical diameter of the cable is what will ultimately limit you from using one of the smaller CT's.

My humble suggestion for the following circuits would be these:

30A - Radio amplifiers and tower raising/lowering motor <- Micro 40
40A - Range (gas burners with electric oven) <- Micro 50
20A - Well pump <- Micro 40
40A - Air conditioning compressor <- Mircro 50

To keep things simple the above circuits could use all micro 50 CT's which offer 1% accuracy and will be in stock in the next 3-6 weeks. If you can't wait then you can use the Micro 80's in the place of the 50's.

With a 240 line with out a neutral you can use one or two CT's it comes down to your budget and level of accuracy required. The GEM uses software to combine the two sides of the balanced 240 volt electrical feed.

In my initial testing I did not see a difference in using two CT's vs one CT and emulating the second electrical feed for my HWT. Your stove will require two CT's to monitor the 240 load as their is a neutral in place. Typically there is a convenience outlet on the top for the user which we use for our crock pot etc.

[Vanhalo]

I've read all the guidelines on selecting CTs for 240V circuits, and have read as many posts on the subject as I could find here, but I'm still a bit uncertain about proper configuration, and I think others here might be too (e..g., a recent post asked whether the current for the two poles should be added to determine the CT amperage rating.)

Most of the circuits in my panel use single-pole 15A and 20A breakers, so CT-40s have plenty of amperage headroom for those circuits. But I have four two-pole 240V breakers, with each pole labeled as follows:

30A - Radio amplifiers and tower raising/lowering motor
40A - Range (gas burners with electric oven)
20A - Well pump
40A - Air conditioning compressor

My assumption is that I should not add the current marked on the poles to determine the appropriate CT rating. For example, if the two poles are each labeled 20A, the maximum current that can flow through either leg is 20A, and the total current draw of the device cannot exceed 20A (not 40A). Therefore, a CT-40 should be fine for monitoring either or both legs. Correct?

But how much amperage headroom is enough? It would appear that CT-40s should be fine for the radio equipment and the well pump, but I'm not so sure about the Range and Air Conditioning compressor, both of which contain motors. Large motors sometimes have momentary current spikes at startup. Breakers tend to be relative slow to react, so they won't necessarily trip due to such a spike. It's not clear to me whether such a momentary spike is figured into the device's maximum current rating and whether I need to account for it when sizing CTs. (NOTE: sometimes the 40A breaker for the A/C trips when it first starts up at the beginning of the cooling season, probably because there's some gunk that's accumulated around the shaft that has to be broken for the motor to get going. That tells me that the A/C unit can momentarily draw more than 40A.)

Given that CT-40s exactly match the current ratings of the two breakers in question, is it OK to use them, or should I configure more headroom by selecting CT-80s for those circuits? What's the danger of under-sizing a CT for a load that can momentarily exceed the CT's rating? Can the CT be damaged? Can the GEM be damaged?

Even though the subject of 240V circuits that use the neutral has been discussed here a lot, I'm still not certain about how the GEM handles the case of devices that use the neutral. My understanding is that some 240V devices use the neutral in order to create an internal 120V circuits for certain components. It's also my understanding that the current for drawn by the 240V components flows through both legs, while the current for the 120V circuit flows through only one leg and the neutral. So, when you put a CT on each 240V leg, how does the GEM compute the total current draw? I'm guessing that since the CTs have to be wired for opposite polarity, the currents add together and the GEM divides by two. But how does that when additional current flows through one leg into the neutral?

Also, if you aren't certain whether a device uses the neutral or not, can you put a CT on both legs, as long as you follow the polarity guidelines in the manual?

You can never go wrong with using a larger amperage CT. Regardless of the amperage of the CT the physical diameter of the cable is what will ultimately limit you from using one of the smaller CT's.

My humble suggestion for the following circuits would be these:

30A - Radio amplifiers and tower raising/lowering motor <- Micro 40
40A - Range (gas burners with electric oven) <- Micro 50
20A - Well pump <- Micro 40
40A - Air conditioning compressor <- Mircro 50

To keep things simple the above circuits could use all micro 50 CT's which offer 1% accuracy and will be in stock in the next 3-6 weeks. If you can't wait then you can use the Micro 80's in the place of the 50's.

With a 240 line with out a neutral you can use one or two CT's it comes down to your budget and level of accuracy required. The GEM uses software to combine the two sides of the balanced 240 volt electrical feed.

In my initial testing I did not see a difference in using two CT's vs one CT and emulating the second electrical feed for my HWT. Your stove will require two CT's to monitor the 240 load as their is a neutral in place. Typically there is a convenience outlet on the top for the user which we use for our crock pot etc.

I am using the 80s on all of my breakers greater than 20a. I monitor both sides. I initially set the multiplier on the gem to simulate one side but decided against it.

[peppersass]

To keep things simple the above circuits could use all micro 50 CT's which offer 1% accuracy and will be in stock in the next 3-6 weeks. If you can't wait then you can use the Micro 80's in the place of the 50's....Your stove will require two CT's to monitor the 240 load as their is a neutral in place. Typically there is a convenience outlet on the top for the user which we use for our crock pot etc.

Thanks. Good suggestions. I think using micro-50 CTs on all four circuits is the way to go. While I'm waiting for them, I'd like to monitor the 240V circuits with micro-40 CTs that I can move from less important circuits. Is there any danger to the CT or GEM if the A/C or range current spikes a little above 40A? If so, I can forego monitoring the A/C -- I don't think it'll switch on until next Spring anyway. I think monitoring the range with a pair of micro-40 CTs is probably safer because the breaker has never tripped, but if you think that's a bad idea I'll wait for the micro-50s.

Makes sense to simplify and just monitor both legs of all the 240V circuits.

There's no convenience outlet on the range, but there's a control board which probably has a low voltage supply stepping down from 120V. I believe the igniters for the gas burners run off 120V as well. Finally, there are fans used for convection and exhaust that are most likely 120V models. So my assumption has been that the range definitely uses the neutral.

I know the tower/amp system uses the neutral because the tower motor runs off 120V provided by a subpanel, which also feeds a 120V convenience outlet. I ran 240V to the subpanel because it's 265 feet from the house. One of the amps uses the neutral for a 120V circuit that feeds the low voltage supply for the CPU. The other amp doesn't use the neutral -- it uses a step-down transformer instead. Generally that's a more expensive way to do it.

Not sure about the Air Conditioning compressor. I'd have to open it to find out. Might be that the compressor motor runs off 220V and the big fan runs off 120V, or the other way around. In any case, I'm pretty sure the outdoor convenience outlet next to the unit is run off the neutral and one leg of the 240V, so the odds are high that I'll need two CTs for that circuit.

The well pump definitely doesn't use the neutral, but in the course of checking that out I discovered that the subpanel it's fed from connects the neutral to ground! That's a dangerous code violation that I'll be fixing as part of the project (neutral must be connected to ground only at the main panel.)

[Teken]

I believe a combination of micro 40 / 50's are perfectly fine for your use case. If you believe any large circuit load may present an issue down the line simply purchase a few micro 80's. This is what I did long ago to hedge my bets and reduce any unwanted downtime.

Most AC units (fan) are 120 VAC, where as the compressor is 240 VAC. Hope very much you document and track your efforts in the Projects Forum here. I am always very interested to see what others are doing, tracking, and monitoring in their homes.

If you're not aware I have an ongoing projects thread which has been documenting my endeavors and the endless development process of my entire Home Automation (HA) network.

Its a long read but does provide a lot of detail and insight as to things that you encounter: http://www.brultech.com/community/viewt ... ?f=2&t=929

Should you have any further questions please do ask them here.

[peppersass]

OK. But still wondering what happens if a CT's maximum current rating is exceeded. I can live with inaccuracy, and I can live with damage to the CT, but I can't live with damage to the GEM. So, let me ask again: can the GEM be damaged if a CT's maximum current rating is exceeded?

As for documenting my GEM adventure, that's a good idea. I'll try to remember to take photos at each step.

I read the first part of your saga, but I think it was on another website? Anyway, I'll keep reading -- it's incredibly helpful stuff!

[Teken]

As far as I am aware the GEM is protected against a CT being over loaded. As I recall the GEM will simply display garbage data until the current has been reduced and is in its working range.




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[ben]

As far as I am aware the GEM is protected against a CT being over loaded. As I recall the GEM will simply display garbage data until the current has been reduced and is in its working range.

Yeah, Teken basically covered it, it'll display garbage on all channels.