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Low-voltage connections to current transformer for make-up air unit

jadziedzic | Posted in General Questions on

I’ll be using an Electro Industries packaged make-up system with two current transformers to energize the unit:  one on the range hood circuit and the second on the electric dryer circuit.  Both CTs will be located in or adjacent to the breaker panel, with the output side of each CT routed to the make-up air unit control board.  How does one comply with NEC rules that low-voltage and line-voltage wiring (connections) can’t be within the same junction box (compartment)?

Is it permissible to place the CT inside its own J-box and route the CT low-voltage leads through a short nipple to a second J-box where the connection to the LV control wiring to the make-up air unit will be made?  This seems to follow the design for LED lighting transformer junction boxes where the line-voltage and low-voltage connections are made within separate compartments.

Anyone used the Electro Industries units like the EM-MC05-240-1-08 and devised a solution to this quandry?  How do folks who install current transformers to monitor branch circuits within breaker panels deal with this?

Would appreciate any info so I can have a reasonable solution in hand when I approach the electrician; who may of course already know the answer, but if not I’d like to have an option or two to present to him.  Thanks!

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  1. DC_Contrarian | | #1

    I like to use a Functional Devices Relay-in-a-Box. Like this:

    It has a relay inside an enclosure that screws into a knockout in the side of a junction box. The line voltage connections are made inside the junction box. The low voltage connections pass through the junction box and out through a clamp. In theory you could splice them out in the open but I like to run them into another box and splice there for a neater look.

    Instead of two boxes you can use a 2-gang box with a divider, like this:

    You can also buy divider plates that go into regular 2-gang boxes to separate low voltage from high voltage.

  2. Expert Member
    BILL WICHERS | | #2

    You can have control wires inside the enclosure of a powered device if those wires are there for the operation of the device. There are also some rules allowing for sharing a raceway as long as ALL of the wiring is rated for at least the voltage of the highest voltage circuit in the raceway. Most low voltage cable is only rated for 30v, which is one of the reasons it can't be run in a common raceway with power conductors.

    It's not usually a problem to wire CTs inside a breaker box, but you have to wire the control circuit with power wire (stranded THHN or similar). Breaker boxes don't usually have a lot of extra space though, so I prefer to put a large junction box next to the breaker box, then run all the monitored power circuits through that box and but the CTs in that box. Control wiring then goes from that box to wherever it needs to go in a seperate raceway. Relays and low voltage transformers can be handled the same way.

    This isn't anything unusual and is done all the time. Your electrician will be familiar with what to do. The general rule is that there is supposed to be a barrier between low voltage and line voltage devices, but when the SAME DEVICE is actually working with both voltages, that's a special case. Do try to keep the wiring neat though.


    1. jadziedzic | | #3

      Thanks Bill, appreciate the detailed explanations!

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