# Detect presence of Mains voltage simple and cheaply

#### Mreeves

Joined Feb 27, 2021
13
I will give you some quick background. I live in Texas where we just lost power for about 5 days. I am buying a generator and plan to use it to back feed my electrical panel using a mechanical interlock for safety. This prevents me from back feeding the grid as it should. I have 200A 240V service that comes in on 2 live lines. I want a simple light could be neon or led or other that will indicate if power has returned to mains. I would like to use a split core current transformer so that I don’t have to disconnect these live wires. I am having difficulties finding split core current transformers rated for 200A. I see an abundance of 100A that go all the way to 300v. I am looking for a simple to install cost effective way to know if power has returned as I will have to manually switch back. At some point I may invest in an Auto Transfer Switch but these are outside my ability to afford. Any help would be greatly appreciated.

#### Mreeves

Joined Feb 27, 2021
13
I took my panel off and measured with a clamp current measure on my multimeter and it read about 22amps on both. I am unsure if they ever reach 200 amps or if each line is only supposed to get to 100amp. I have not installed the interlock or generator breaker yet but I snapped a photo of what I have. I do realize I am going to have to make some room.

#### crutschow

Joined Mar 14, 2008
26,814
I would like to use a split core current transformer so that I don’t have to disconnect these live wires
Why do you want a current transformer?
That won't detect the presence of the mains voltage, which is what you want, since there would be no current when it's not connected.

Just use two 120V LED night lights, connected from each live wire to neutral.
You could leave those permanently connected.

#### Mreeves

Joined Feb 27, 2021
13
Is that type of connection allowed per code? I was hoping to us the split core CT as I would not have to disconnect and work with the live wires directly just wrap the ct around them but I see your point. I guess that idea just won’t work. I will have to find another way.

#### Mreeves

Joined Feb 27, 2021
13
How hard would it be to build a non contact voltage detector?

#### crutschow

Joined Mar 14, 2008
26,814
There are non-contact testers that will detect the presence of line voltage when the device is brought near the line.
Perhaps you could adapt one of those.

Here's how to build one.

#### AlbertHall

Joined Jun 4, 2014
11,147
Or, maybe, two neons, each with a 100k resistor and a wire wrapped around the supply cables and the other end connected to earth.

#### Mreeves

Joined Feb 27, 2021
13
Or, maybe, two neons, each with a 100k resistor and a wire wrapped around the supply cables and the other end connected to earth.

View attachment 231563
I like this looks simple enough to execute and I have had a conversation with code enforcement and said he did not have an issue with me using a coil that just wraps around the live wires but forbid rerouting their power or making connections to them before the 200Amp breaker.

Do you have a model number for an appropriate neon lamp or what specs I should look for? I have some of this stuff on hand EL Wire I am not sure if it will work. Amazon search returned bunch of stuff like this Indicator Light. I should be able to get a 100K ohm resister easy enough. I imagine the wire would not need to be very big for this to work. I will see what semi stiff wire I have on hand.

I really appreciate the help!

Joined Jan 15, 2015
5,893
If I were you and depending on your choice of generator I would get a transfer switch which is what you sort of allure to when you mention backreading. Pretty sure you just endured a week you never want to repeat. What generator do you have in mind? Years ago I went from a small 5 KW arrangement to whole house providing 16 KVA. Either way wise move. Consider fuel type and run time.

As to neon lamps? Just about any NE2 lamps will work with 100K resistors as drawn.

Ron

#### crutschow

Joined Mar 14, 2008
26,814
If you are just wrapping the wire around the cable then you don't need the resistor (which is for a direct connection to the line), only the neon bulb.
Just about any small neon (not LED) bulb should work.
For permanent mounting, you could use a through-hole panel mount one.
Beware of lights for voltages below 110V the say they are neon (they aren't). Neon requires at least 70V to light).

You'll have to experiment to see how many turns you need for a reasonable brightness.

#### AlbertHall

Joined Jun 4, 2014
11,147
I like this looks simple enough to execute and I have had a conversation with code enforcement and said he did not have an issue with me using a coil that just wraps around the live wires but forbid rerouting their power or making connections to them before the 200Amp breaker.

Do you have a model number for an appropriate neon lamp or what specs I should look for? I have some of this stuff on hand EL Wire I am not sure if it will work. Amazon search returned bunch of stuff like this Indicator Light. I should be able to get a 100K ohm resister easy enough. I imagine the wire would not need to be very big for this to work. I will see what semi stiff wire I have on hand.

I really appreciate the help!
Such indicator lights, provided they are actually neon which I belive the one in your link is, already have the resistor built in, and should work just fine for this job. As alaready stated you will need to experiment to see how much wrapping is needed.

#### strantor

Joined Oct 3, 2010
5,508
On what principle is the neon supposed to work? AFAIK, just like a split-core CT, it will still require some current to be flowing (which it won't, because the main breaker will be off)? Don't those non-contact voltage probes need a battery-powered circuit in order to detect the presence of voltage?

#### Mreeves

Joined Feb 27, 2021
13
If I were you and depending on your choice of generator I would get a transfer switch which is what you sort of allure to when you mention backreading. Pretty sure you just endured a week you never want to repeat. What generator do you have in mind? Years ago I went from a small 5 KW arrangement to whole house providing 16 KVA. Either way wise move. Consider fuel type and run time.

As to neon lamps? Just about any NE2 lamps will work with 100K resistors as drawn.

Ron
I bought a pulsar 12kw peak duel fuel gen for $800. Auto transfer switch just not worth the price as I will have to manually start the generator. It does have a start button for electric start I am pretty sure could be automated or remoted but the choke is the problem. I did see a hack where someone used a relay and actuator and some linkage to manipulate the choke but that seems like a lot of work. Manual start and manual switching via interlock seems reasonable to me. Just trying to figure out when to switch back. I get notifications on my phone but they are not very accurate. I know I lose a little bit of power when running off of propane but it stores indefinitely so I find that attractive. Bang for the buck this generator is hard to beat. It’s a tad on the loud side but I plan to build a shed with air venting and noise deadening material. I have seen a couple of videos. #### dendad Joined Feb 20, 2016 3,795 The Neon lamp only needs a tiny current to operate, and the coil of wire around the mains cable forms a capacitor in series with the Neon lamp. It is not an inductor. A piece of foil may work ok. Just make sure you insulate in with tape so it does not even appear unsafe for when an electrician looks at the switchboard. Thread Starter #### Mreeves Joined Feb 27, 2021 13 On what principle is the neon supposed to work? AFAIK, just like a split-core CT, it will still require some current to be flowing (which it won't, because the main breaker will be off)? Don't those non-contact voltage probes need a battery-powered circuit in order to detect the presence of voltage? I will have a UPS for my internet equipment I can get at 12v DC or 24 DC or 120 V AC if I needed with or without MAINs. I thought about buying a NCV and repurposing it. Not sure if it would be easier to build my own. The auto timeout function would have to be removed or turned off. I am surprised nobody else has run into this. At night time I could see the street lamps but wouldn’t help during the day. Asking neighbors if they have power is another way but they may not be home. #### dendad Joined Feb 20, 2016 3,795 Just a thought, are you able to do this work legally? Here in Australia, one needs to be a qualifies electrician to work on the house wiring. Then it is signed off, with a paper trail. You may find it worthwhile checking your insurance policy as they will most likely refuse a payout if you do the work yourself and that is discovered. Insurance companies delight in finding such excuses! Thread Starter #### Mreeves Joined Feb 27, 2021 13 Just a thought, are you able to do this work legally? Here in Australia, one needs to be a qualifies electrician to work on the house wiring. Then it is signed off, with a paper trail. You may find it worthwhile checking your insurance policy as they will most likely refuse a payout if you do the work yourself and that is discovered. Insurance companies delight in finding such excuses! I plan to have an electrician do the work. I also plan to have a permit and have it inspected. I had a conversation with the inspector. He said UL listed parts as well as the certified electrician for the new wiring. As far as a monitoring piece he said that he had no issue with something wrapping around the main feed lines as long as their insulation was not compromised. I just cannot add any additional wiring where it connects to the breaker box. I will wrap it and insulate and if he has an issue then I will change it according to his recommendation. Yeah I don't want the insurance to blame me if the house burns down. #### Reloadron Joined Jan 15, 2015 5,893 I bought a pulsar 12kw peak duel fuel gen for$800. Auto transfer switch just not worth the price as I will have to manually start the generator. It does have a start button for electric start I am pretty sure could be automated or remoted but the choke is the problem. I did see a hack where someone used a relay and actuator and some linkage to manipulate the choke but that seems like a lot of work. Manual start and manual switching via interlock seems reasonable to me. Just trying to figure out when to switch back. I get notifications on my phone but they are not very accurate. I know I lose a little bit of power when running off of propane but it stores indefinitely so I find that attractive. Bang for the buck this generator is hard to beat. It’s a tad on the loud side but I plan to build a shed with air venting and noise deadening material. I have seen a couple of videos.
OK, with that in mind and since you only want to detect mains voltage at entry point, the bottom of your pictured breaker panel, I would just try a basic AC voltage detector circuit, the no touch flavor. These circuits are common and seen all the time in little hand held units. The circuits only require a few components. I would sense each side of your 240 VAC entry. A Google of AC voltage detector circuit will bring up a few dozen suggested designs.

Good move on dual fuel. We have natural gas but if I lose it (never have) I run with propane I have in tanks on site.

Lose power 30 seconds Generator auto starts.
Following 20 second delay transfer.
Mains power returns delay 30 seconds and transfer.
Run generator another min or so as cooldown.

The timing is all adjustable and I use what I listed.

Yeah, see you have a full panel and need to make room for a back feed breaker or add a small sub panel. Hope you guys down in Texas never have to go through this again.

Ron

#### Mreeves

Joined Feb 27, 2021
13
If you are just wrapping the wire around the cable then you don't need the resistor (which is for a direct connection to the line), only the neon bulb.
Just about any small neon (not LED) bulb should work.
For permanent mounting, you could use a through-hole panel mount one.
Beware of lights for voltages below 110V the say they are neon (they aren't). Neon requires at least 70V to light).

You'll have to experiment to see how many turns you need for a reasonable brightness.
If I buy this one with the resistor will it interfere with it working? or is just unnecessary but will still work fine?

#### crutschow

Joined Mar 14, 2008
26,814
If I buy this one with the resistor will it interfere with it working? or is just unnecessary but will still work fine?
It should work fine either way, you just may need a few more turns of wire if it has the resistor.