TOPIC: Transformers inside box

Anyone know the thinking behind not allowing transformers inside the electrical panel?  I just became aware of this and I can't find any NEC reference which would shed light on the reason.

Thanks for any help.

Are you talking doorbell type or donuts?

The typical doorbell type, not the amperage sensing donuts which feed a secondary meter. The donuts have to be inside the panel.

Jerry Peck's post from another message board.

Not allowed.

IRC. (underling is mine)
- E3303.3 Listing and labeling. Electrical materials, components, devices, fixtures and equipment shall be listed for the application, shall bear the label of an approved agency and shall be installed, and used, or both, in accordance with the manufacturer?s installation instructions.

- SECTION E4204
- - INSTALLATION REQUIREMENTS
- - - E4204.1 Separation from other conductors.
In cables, compartments, enclosures, outlet boxes, device boxes, and raceways, conductors of Class 2 circuits shall not be placed in any cable, compartment, enclosure, outlet box, device box, race way, or similar fitting with conductors of electric light, power, Class 1 and nonpower-limited fire alarm circuits.
- - - - Exceptions:
- - - - - 1. Where the conductors of the electric light, power, Class 1 and nonpower-limited fire alarm circuits are separated by a barrier from the Class 2 circuits. In enclosures, Class 2 circuits shall be permitted to be
installed in a raceway within the enclosure to separate them from Class 1, electric light, power and nonpower-limited fire alarm circuits. (Jerry's note: This exception does not apply.)
- - - - - -2. Class 2 conductors in compartments, enclosures, device boxes, outlet boxes and similar fittings where electric light, power, Class 1 or nonpower-limited fire alarm circuit conductors are introduced solely to connect to the equipment connected to the Class 2 circuits. The electric light, power, Class 1 and nonpower-limited fire alarm circuit conductors shall be routed to maintain a minimum of 1/4 inch (6.4 mm) separation from the conductors and cables of the Class 2 circuits; or the electric light power, Class 1 and nonpower-limited fire alarm circuit conductors operate at 150 volts or less to ground and the Class 2 circuits are installed using Types CL3, CL3R, or CL3P or permitted substitute cables, and provided that these Class 3 cable conductors extending beyond their jacket are separated by a minimum of 1/4 inch (6.4 mm) or by a nonconductive sleeve or nonconductive barrier from all other conductors. (Jerry's note: This exception does not apply.)


- E4204.2 Other applications.
Conductors of Class 2 circuits shall be separated by not less than 2 inches (51 mm) from conductors of any electric light, power, Class 1 or nonpower-limited fire alarm circuits except where one of the following conditions is met: (Jerry's note: This does not apply.)
- - 1. All of the electric light, power, Class 1 and nonpower-limited fire alarm circuit conductors are in raceways or in metal-sheathed, metal-clad, nonmetallic-sheathed or Type UF cables.
- - 2. All of the Class 2 circuit conductors are in raceways or in metal-sheathed, metal-clad, nonmetallic-sheathed or Type UF cables.

The NEC will say the same basic thing - do not run Class 2 circuits with power and lighting circuit conductors and not in the same enclosures ... unless the Class 2 circuit conductors are entering the enclosure to operate the equipment. An example of this is an AHU which has power circuit conductors enter the AHU as do the Class 2 circuit conductors. The Class 2 circuit conductors are allowed to enter-and-terminate within the AHU. The Class 2 circuit conductors would not, however, be allowed to enter-and-pass-through-without-terminating-within the AHU.

Perfect, thanks for the info. That's half of what I've been looking for. The other half is the concern behind the code. Any thoughts on that?

From what I have been able to read, it has more to do with the requirements for panel installation.  They don't want any items installed within the panel that are not part of it's operation.  That would include equipment and from the looks of it, low voltage wiring.

After some of the electrical modifications and repairs I have seen done, I can say it is a good thing.  Can you imagine what you might find installed inside a panel if there was no blanket "not allowed" ?  :