I bought this device called “Sense” which measures your electricity usage. Its a small box you install in breaker box. It requires an existing 240v breaker for power but only uses 0.1A (I believe requirement for 240v isnt it needs power but how it draws it from different legs).

It has 3 wires (2 hot 1 neutral), and they are 16ga THWN.

Question is, can i pigtail on an existing 20A 240v breaker that I use for my baseboard heater?

 

You need to follow the manufacturer's instructions TO THE LETTER. A link to the device might help.

 

Here is the installation guide:https://sense.com/help/installguide.pdf Their website says "consult an electrician". I asked an electrician and he says consult the manufacturer

 

The instructions are clear, IF you have the spaces you need to install a 2-pole 15 ampere circuit breaker. If you do NOT have the spare spaces then you can connect to an existing 2-pole circuit breaker. Note that the lower the rating on the CB the better and also that you cannot connect two wires to most CBs. Remove the existing wires from the CB you plan to use and "pigtail" the power meter wiring to the existing conductors along with short pigtails to go to the CB. I personally would seriously consider adding in-line fuses to the power meter but it is not a requirement.

Your bigger problem may be in attaching the current transformers to the incoming conductors and finding a place for the power meter package. I know that the way my panel is arranged I could never fit the current transformers.

 

I fit everything already and installed it, so all the issues with fitting etc are thankfully not a problem for me. For now I disconnected the two wires from my 240v 20A breaker completely and Sense is directly connected and I am going back and forth between buying a new breaker or using pigtails on this one.

what do you mean by in-line fuses? Also If I decide to buy a CB, I was going to go with 20A because it's only $5 more expensive than 15A and I might use it as a spare if the other one breaks. If only Sense is using a breaker, the A rating on it should be irrelevant right? 15, 20, 30, 40, shouldn't matter? Not sure why instructions say the smaller the better? Because they used 16ga wiring, even if something goes wrong, 15A breaker wouldnt trip before the wires give out.

 

Is the device Underwirters Laboratories or other agency approved?

Every device has a maximum circuit rating. If this rating is not published or obtainable and the device is UL approved then you may assume the rating is 20 amps.

The device may have leads smaller than the amperes rating for the circuit but there is a limit to how long these leads may be (the tap rule which I don;'t have in front of me). Example of the tap rule; a light fixture has 18 gauge leads from the lamp socket to be wire nutted into a circuit of no greater than 20 amps.

Devices may have built in in-line fuses or breakers to provide appropriate protection in case a malfunction in the device causes a dead short circuit. An in-line fuse is connected between the power source and the device. If you add your own then you would need two of them, one for each of the hot leads to allow connection to a circuit with more than the maximum amperes rating for the device..

Devices without UL or other agency approval might be advertised and sold for installation on "any circuit" but not stating an amperes rating. Adding, say, 1 amp or 2 amp in-line fuses would be a good idea.

 

I wouldn't worry about inline fuses. You will be fine using the 20A breaker. The manufacturer states if installing a new breaker you should use the smallest one available but doesn't mandate that if using an existing breaker it also needs to be the smallest available.

 

The device is UL listed and has been "rigorously tested" as per the manufacturer. The wires have 16 AWG THWN stamp on them. I'm just going to go with 20A AFCI 240v breaker. $5 more than 15 amp and I get the added benefit of swapping it out with others in the future in case of an emergency. 15A 240v is not a very useful/common breaker to be honest, I have never seen one before.

 

The reason your device only has 16 AWG wire is because it doesn't use much current as you already found out.
You are fine with pigtail so long as the pigtail is done with 12 AWG wire.

Basically, you are tapping off a 20A circuit instead of wiring 16AWG in series with it.

 

Well, I'm not physicist, but since the 20A 240V breaker is for baseboard heater that maxes out at 19A, couldnt you say that in the event it goes over, it can melt this 16ga pigtail? Or even under normal operation, is it safe?

 

Do what? The sensor #16 is tapped into the #12 so the heater is on #12 from the tapped point to the breaker. The pigtail needs to be #12. Even if the heater pulled 20A the #16 to the sensor will never be subject to that load.

 

Understood, let me try this. However, there is one issue which I didnt consider/forgot to mention here, my 20A breaker is AFCI. The heather has no neutral wire and doing this pigtailing might not work.

 

AFCI will not trip unless it detect an arc. Having neutral or not does not matter. AFCI works by monitoring a frequency associated with arcing.

And even if it was a GFCI breaker, it still doesn't matter because all current checks out. Current goes out from one terminal and returns to the other. If there is no difference, it will not trip.

 

The CTs do not have or need a neutral. None of the heater load is going to flow in the #16.