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Old 01-26-2012, 09:02 AM   #1
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Mixing 120VAC and LV 24AC circuit in a junction box


I am planning to install a current sensing relay in the wiring leading to my fireplace insert. The purpose of this will be to sense when the fireplace is on and its blower fans are running. Upon detecting current on the 120V supply line to the fireplace it will close a solid state 24 VAC relay to energize the fan blower on my furnace to allow the air in the house to circulate when the fireplace is on.

My question is regarding what are the restrictions to commingling LV wiring with 120VAC wiring. Ideally I'd like to just put a 4x4x1.5" box in the basement beside the panel and run the 120VAC to the fireplace through this box and put the little sensor/relay in that same box. I would then exit the box with LV wiring to the furnace blower circuit.

I think the NEC has restriction on commingling such applications so I am not sure the legality.

As an aside the current sensor if a magnetic field device so it isn't electrically connected to the 120VAC directly (no galvanic connection). The 120VAC wire merely needs to loop through a current transformer loop.

Can I just put the sensor in the box and run the LV out of the box via another cutout (with suitable grommet/strain relief)?


http://docs.electronicaircleaners.co...stallation.pdf

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Old 01-26-2012, 09:23 AM   #2
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Mixing 120VAC and LV 24AC circuit in a junction box


Line and low voltage must either be in a separate box or one approved for both.

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Old 01-26-2012, 09:23 AM   #3
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Mixing 120VAC and LV 24AC circuit in a junction box


I don't think it's allowed to mix wire voltages in conduit or any box.

So you are going to use the relay to make a connection on your thermostat wire? Wouldn't that turn on the furnace too.

How about a 120v relay that just overrides the switch that turns your furnace blower on and off?
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Old 01-26-2012, 09:28 AM   #4
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Mixing 120VAC and LV 24AC circuit in a junction box


http://www.homedepot.com/buy/electri...ket-48592.html
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Old 01-26-2012, 09:37 AM   #5
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Mixing 120VAC and LV 24AC circuit in a junction box


Quote:
Originally Posted by curiousB View Post
My question is regarding what are the restrictions to commingling LV wiring with 120VAC wiring.
Does Chicago require conduit for LV wire too?
I doubt it.... Pull some t-stat wire and cable tie that to the emt you'll be using for the 120V.
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Old 01-26-2012, 10:19 AM   #6
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Mixing 120VAC and LV 24AC circuit in a junction box


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Originally Posted by TarheelTerp View Post
Does Chicago require conduit for LV wire too?
I doubt it.... Pull some t-stat wire and cable tie that to the emt you'll be using for the 120V.
No need for EMT for LV but the location of the fireplace and panel are opposite ends of the house from the furnace. I was planning on putting current sensor in a junction box up by the panel and then run LV wire back to the furnace to switch the blower circuit.

Any four wire thermostat has a LV wire for cooling, one for heat, one for blower (fan), and one for 24VAC. I plan to just control the blower circuit with this relay.
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Old 01-26-2012, 10:36 AM   #7
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Mixing 120VAC and LV 24AC circuit in a junction box


Quote:
Originally Posted by curiousB View Post
Any four wire thermostat has a LV wire for cooling, one for heat, one for blower (fan), and one for 24VAC. I plan to just control the blower circuit with this relay.
Got that part...

Quote:
No need for EMT for LV but the location of the fireplace and panel are opposite ends of the house from the furnace.
ain't it always the way?

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I was planning on putting current sensor in a junction box up by the panel and then run LV wire back to the furnace to switch the blower circuit.
If the fireplace is on a dedicated circuit or close enough to the panel to "intercept" that switch leg... and run that to a box (as needed) to set the sensor device.. then you're 90% done.

In this sort of scenario I believe that any general purpose work box will meet your need with no worries about "co-mingling".

hth
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Old 01-26-2012, 12:59 PM   #8
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Mixing 120VAC and LV 24AC circuit in a junction box


Thanks TT,

That is what I'll do. The fireplace is on a dedicated branch circuit so I plan a box next to the panel and will route the neutral line only (mfg says they prefer to measure current on neutral line) through this box and pass wire through the sensor loop. Then I'll drill a 3/8" hole in side of box and put a grommet in and run the LV wire out of that hole and to the furnace.
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Old 01-26-2012, 01:21 PM   #9
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Mixing 120VAC and LV 24AC circuit in a junction box


Rather than a current sensing relay (which may be slightly tricky), why not just use a regular 120V-coil relay attached to the fireplace fan's power leads?
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Old 01-26-2012, 04:49 PM   #10
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Mixing 120VAC and LV 24AC circuit in a junction box


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Rather than a current sensing relay (which may be slightly tricky), why not just use a regular 120V-coil relay attached to the fireplace fan's power leads?

I considered that but I can't get wiring into the firebox of the fireplace without drilling a hole in masonry. It would have to be 12-18" deep of a hole so no trivial effort. Also the unit is installed and all the bezels in place so it would be quite a bit of work to get access at it.

The idea of the remote current sensing was to detect when the fans are running without wiring into the actual gas appliance. I don't know how reliable these current sense relays are but at $20 I though it was clean way to solve my problem.
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Old 01-26-2012, 07:18 PM   #11
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Mixing 120VAC and LV 24AC circuit in a junction box


You absolutely can have different voltages in the same conduit and/or box, provided that no voltage exceeds 600 (with a few exceptions that don't apply here).

I can't think of how many times I've pulled 480AC and 120 or 24 in the same conduit.

The only code issue is that all conductors must be rated for the highest voltage present.

Just use at least 125 volt wire, and you're good.
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Old 01-26-2012, 08:17 PM   #12
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Mixing 120VAC and LV 24AC circuit in a junction box


Quote:
Originally Posted by mpoulton View Post
Rather than a current sensing relay (which may be slightly tricky), why not just use a regular 120V-coil relay attached to the fireplace fan's power leads?
Suggested the same thing would be so much cheaper.
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Old 01-26-2012, 08:21 PM   #13
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Mixing 120VAC and LV 24AC circuit in a junction box


Quote:
Originally Posted by micromind View Post
You absolutely can have different voltages in the same conduit and/or box, provided that no voltage exceeds 600 (with a few exceptions that don't apply here).

I can't think of how many times I've pulled 480AC and 120 or 24 in the same conduit.

The only code issue is that all conductors must be rated for the highest voltage present.

Just use at least 125 volt wire, and you're good.
And you must maintain a 1/4" separation between class 2 wiring and line voltage wiring.
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Old 01-27-2012, 08:51 PM   #14
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Mixing 120VAC and LV 24AC circuit in a junction box


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Originally Posted by rrolleston View Post
Suggested the same thing would be so much cheaper.
Well not really. I purchased a current sense 24VAC relay for $19. I don't need to locate it in the fireplace insert unit so I avoid the masonry drilling and disassembly of the fireplace insert to install a 120VAC relay. Since the blower has speed levels the relay wouldn't like that switched 60Hz waveform anyway.

By using the current sensor relay I will install that in the basement by the panel as the fireplace has a dedicated branch circuit to it (I can sense current anywhere in the cable run to the fireplace insert). Then I can easily run the LV wiring to the furnace 70' away to activate the blower.
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Old 01-27-2012, 09:00 PM   #15
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Mixing 120VAC and LV 24AC circuit in a junction box


I've had pretty good luck with current sensors. In most industrial control panels, there are PLCs (Programmable Logic Controllers). Basically, computers that control machinery or process.

When the PLC tells a motor to start, it wants to know if it has indeed started. Most of the time, an auxiliary contact at the starter will do, but in a critical application a current sensor will tell if there's current going to the motor or not.

Rob

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