Doorbell Transformer in Main Panel

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enigma

Member
I've heard several references that doorbell transformers cannot be installed inside the main panel due to 110.3(B). Makes sense.

However, what about the fact that doorbell transformers usually have threaded collar and a nut to mount them to a spare knockout? As far as I know these are ok to affix to the OUTSIDE of the load panel. If that is the case, then how does that square with 110.3(B)? If its on the outside surface, do we feel that 110.3(B) does not apply or do the panel manufactures allow for this in their UL listing?

Thanks!
 

jusme123

Senior Member
Location
NY
I've heard several references that doorbell transformers cannot be installed inside the main panel due to 110.3(B). Makes sense.

However, what about the fact that doorbell transformers usually have threaded collar and a nut to mount them to a spare knockout? As far as I know these are ok to affix to the OUTSIDE of the load panel. If that is the case, then how does that square with 110.3(B)? If its on the outside surface, do we feel that 110.3(B) does not apply or do the panel manufactures allow for this in their UL listing?

Thanks!
read 725.133 '08 code
 
why is it so difficult to the easy thing.
For as long as I can remember all you need to do is have a single gang box blank cover with KO and ba da bing buda boom its done.

Or you can use one of those combo two gang boxeswith a lo vo divider and a cover plate if you don't want the exposed trans.

We don't need to find a way to put a trans in a panel, deal with the low vo wires in and whatever other obscure things.
 

enigma

Member
I appreciate the reply, but frankly its not very helpful. I'm asking a specific technical question about code interpretation. I'm not 5 mins away from installing a transformer inside a panel, but I want to understand what is driving the code because I am designing products that depend on getting it right and knowing what is possible from a code standpoint.



why is it so difficult to the easy thing.
For as long as I can remember all you need to do is have a single gang box blank cover with KO and ba da bing buda boom its done.

Or you can use one of those combo two gang boxeswith a lo vo divider and a cover plate if you don't want the exposed trans.

We don't need to find a way to put a trans in a panel, deal with the low vo wires in and whatever other obscure things.
 

M4gery

Senior Member
why is it so difficult to the easy thing.
For as long as I can remember all you need to do is have a single gang box blank cover with KO and ba da bing buda boom its done.
What is the difference between doing that, and putting the tranny in a KO on the panel?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
However, what about the fact that doorbell transformers usually have threaded collar and a nut to mount them to a spare knockout? As far as I know these are ok to affix to the OUTSIDE of the load panel. If that is the case, then how does that square with 110.3(B)? If its on the outside surface, do we feel that 110.3(B) does not apply or do the panel manufactures allow for this in their UL listing?
If the transformer is made to mount in a KO, and the panel has a KO, what's the issue?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
For as long as I can remember all you need to do is have a single gang box blank cover with KO and ba da bing buda boom its done.
What is the difference between doing that, and putting the tranny in a KO on the panel?
As long as the exterior of the panel is exposed, it's no different than a KO-mounted surge protector to a panel or a RIB to a box.

The cover-with-KO would be used on a flush-mounted box, typically in a closet near the front door.
 

enigma

Member
Please excuse my ignorance but what is an RIB? Also what were referring to when you mentioned a "cover with knockout"?

Thanks much

As long as the exterior of the panel is exposed, it's no different than a KO-mounted surge protector to a panel or a RIB to a box.

The cover-with-KO would be used on a flush-mounted box, typically in a closet near the front door.
 
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