Is there a way to legally run a wire through (but not terminating inside) the electrical panel carrying 9v? US Residential, 200A panel, surface mounted on block wall.

I am installing an IotaWatt power monitor, and require two for the number of inputs. One will go on each side of the panel in a separate plastic (UL listed) box. These boxes will each contain a voltage reference source which is simply a plug-in 9V transformer. The boxes are fed 110V through separate conduit into a physically separated area in which an outlet is mounted (protected by GFI).

Separate current transformers (also listed) are put on each branch circuit, and half go out a 1" conduit into the low voltage side of each box.

All this is good and legit up to this point.

I want to run a wire from each low voltage box into the other carrying that 9V reference (this gives me a read on each phase, which should be identical but.. well, I'd like to measure). Running this outside of the panel is ugly due to the physical surroundings (not hard, ugly; even worse to put it in conduit which I would prefer as this is really close to a path to get in/out of a car).

What I would like to do is come out of the 1" conduit on each side of the panel, and cross over to the other side inside the panel. Physically it is trivial, plenty of room for wire. But my understanding is that falls into the Class 2 wiring and is not conforming. I've read various opinions that it can be by maintaining a separation from each high voltage wire, but that's unlikely to work (I'm also unclear if that advice is correct).

Is there a conforming way to do this? Using regular THHN perhaps (terminated back to 9V inside each low voltage box)? Or putting it in some kind of flexible sheath that can actually run into the LV box through the 1" (there is inadequate space for regular 1/2 NM conduit to pass inside and terminate on each side and provide a path).

If relevant (I think not but) none of the low voltage wiring leaves the low voltage box to the outside world, it all stays inside (communications if via wifi).

My alternative is to put two outlets in each box, one on each phase, and four transformers, but that is not desirable due to physical space in the low voltage boxes I already mounted. Another alternative is just to assume the voltage on each phase is identical - reasonable, what the vendor says is adequate, but a big reason for doing this is to better understand what happens in my power usage, and until you measure something it's a guess, not data.

Update 3/14/2019: To some comments, a photo of the setup may help explain. The two empty boxes will hold the Iotawatt's, they also have a divider that goes at the level of the outlet box. The block wall is about 24" from a car normally parked in the garage, and tools and ladders are stored on the wall as well so I am trying to avoid any loose wires or devices (and these boxes are about 7' up). The metal cover hides wires exiting above and goes all the way to the ceiling. Both plugs on the outlet in the box are taken (one for power, one for that leg's 9V reference transformer).

iotawatt boxes beside panel

  • A barrier (as you have with the physically separated area in the boxes you describe) is required between LV & HV in the same box; keeping it in conduit probably meets that requirement, but I'm speculating so this is a comment. NM cable is NOT conduit. Hopefully you mean non-metallic flex, or ENT, but there's room for confusion there.
    – Ecnerwal
    Commented Mar 13, 2020 at 14:59
  • Sorry, not an electrician, probably have the terminology wrong. The boxes are connected to the panel with grey non-metalic hard pvc-like electrical conduit.
    – Linwood
    Commented Mar 13, 2020 at 15:26
  • Allowed separation options for class 2 are involved, you can read article 725.136 at nfpa.org/codes-and-standards/all-codes-and-standards/… Commented Mar 13, 2020 at 16:05
  • Why can't you put the two potential transformers on the same side of the panel, physically speaking? They need to be on opposite legs, but bringing both legs of power out to the same box is trivial.... Commented Mar 14, 2020 at 3:41
  • For appearances I want the transformers inside the box I am using to house the Iotawatt; that also makes it less likely they or their (relatively speaking) fragile wires get struck in the normal course of activity in a garage. I'll try to add a photo to the original message, not sure if I can.
    – Linwood
    Commented Mar 14, 2020 at 14:54

1 Answer 1


If you are in the US they are called legs as residential is normally a single phase feed that is center tapped or a split phase.

Don’t expect them to be identical they will be close usually within 1% unloaded but not identical depending on loading I have seen them +15% off with no way to balance due to a single intermittent large load.

You can split a duplex receptacle to have each on its own circuit this requires handle ties at minimum and can use a common neutral with current code a double pole GFCI would be needed to feed these receptacles in most locations.

You can run your control wire or low voltage through the panel if the wiring insulation is rated for the voltage (your example of THHN is 600v would comply with code) but the reason we usually do not is because of the electrical noise generated with the 120v circuitry and the low voltage separation that you have identified.

  • So just for clarity all I need is THHN to cross the panel, no physical barrier inside or sleave or conduit? That is easy, and in this case I do not think noise is an issue, it's not communication cable just a voltage reference it integrates to calculate power. Thank you.
    – Linwood
    Commented Mar 13, 2020 at 15:28
  • I just realized part (at least) of what you meant by one question: I was not asking about two outlets vs one outlet split phase. To put another reference transformer in I would need a 3rd plug, so absent a single plug outlet (which probably exists somewhere) I would need a whole separate outlet, and space is an issue. The thhn works nicely if that's legit.
    – Linwood
    Commented Mar 13, 2020 at 16:11
  • I would install a barrier , inside the panel you will be in close proximity to 120v circuits, this can induce a higher voltage on your reference. Many times this is called phantom voltage or ghost voltage, it has very little potential but on a measurement system with a high impedance (needed for accuracy without loading the system) the voltage may be skewed. To prove this to yourself run 2 extension cords next together for a few feet plug 1 in and measure the voltage to true ground on the other cord all 3 wires will normally show a voltage close to the supply with just a few feet of wire.
    – Ed Beal
    Commented Mar 13, 2020 at 16:31
  • it will be run across a lot of wires exiting the panel. Do you have some kind of device in mind, suitable to go there (and probably has to be some kind of flexible sheath). I should be able to tell if that's happening, as each separate measurement device will then have both legs; if the cross-fed voltage in panel A varies from/more than the non-cross-fed version of the same leg in panel B, I'll know it is happening at least).
    – Linwood
    Commented Mar 13, 2020 at 16:55
  • I use simple metal brackets in gutter (similar to the space in a panel) to isolate low voltage from power circuits , metallic that is grounded is the best by using angle brackets screwed to the box it will be grounded if metal is used for the divider.
    – Ed Beal
    Commented Mar 13, 2020 at 17:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.