I have a school requiring electrical hold-open devices on doors in the corridor, controlled by the fire alarm system. The door hardware manufacturer offers a simple 120V - 24V, 400 mA (secondary) transformer mounted on a J-box cover to power the hold-open devices. My question is, do I need overcurrent protection on the primary of this transformer sized at 300%, like the NEC seems to say (transformers less than 2A, under 600V), or can I just tap off of a 20 Amp circuit to power this transformer without any protection other than the 20A breaker? I asked, and the manufacturer said there is nothing on the transformer indicating it is Class 2 or 3.
Door closer transformer OCP
- Thread starter darren71
- Start date
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: Door closer transformer OCP
seems to me there is or at least was a special rule for transformers under 25VA that were internally protected.
seems to me there is or at least was a special rule for transformers under 25VA that were internally protected.
- Location
- Massachusetts
Re: Door closer transformer OCP
If you go to article 450 you will find class 2 or 3 transformer are not covered by article 450 but are covered in article 725
If you go to article 450 you will find class 2 or 3 transformer are not covered by article 450 but are covered in article 725
Re: Door closer transformer OCP
But to comply to 725, doesn't the transformer have to be listed? (Art. 725.41(A)(1)) As I understand it, this transformer bears no such listing or marking. Is this something that is understood for a transformer under a certain size? (Such as 25 VA as mentioned earlier)
But to comply to 725, doesn't the transformer have to be listed? (Art. 725.41(A)(1)) As I understand it, this transformer bears no such listing or marking. Is this something that is understood for a transformer under a certain size? (Such as 25 VA as mentioned earlier)
hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
- Occupation
- EC
Re: Door closer transformer OCP
Should have asked the manufacturer if the transformer is thermally or fuse protected. This shouldn't be any different than a doorbell transformer where no separate OCP is normally used or required.
What bothers me is that the transformer does not appear to be listed otherwise the wiring requirement would be indicated. Rather than anguish over this it might be simpler to just get your own transformers from a reputable manufacturer. If they are not supplied with the door magnet you have to get them anyway.
-Hal
Should have asked the manufacturer if the transformer is thermally or fuse protected. This shouldn't be any different than a doorbell transformer where no separate OCP is normally used or required.
What bothers me is that the transformer does not appear to be listed otherwise the wiring requirement would be indicated. Rather than anguish over this it might be simpler to just get your own transformers from a reputable manufacturer. If they are not supplied with the door magnet you have to get them anyway.
-Hal
Re: Door closer transformer OCP
Thanks, Hal. This has occurred to me as well. I'm also beginning to wonder if the fellow I talked to with the manufacturer knew what he was talking about. This transformer should be UL listed, as it is from LCN (a reputable door closer company). Thanks everyone for the responses.
Thanks, Hal. This has occurred to me as well. I'm also beginning to wonder if the fellow I talked to with the manufacturer knew what he was talking about. This transformer should be UL listed, as it is from LCN (a reputable door closer company). Thanks everyone for the responses.
hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
- Occupation
- EC
Re: Door closer transformer OCP
My experience with LCN and most of the security/access control manufacturers is that they don't make these transformers themselves, they are more often than not offshore imports. Most security installers don't care about a UL listing anyway so that's why the cheap stuff is supplied.
-Hal
My experience with LCN and most of the security/access control manufacturers is that they don't make these transformers themselves, they are more often than not offshore imports. Most security installers don't care about a UL listing anyway so that's why the cheap stuff is supplied.
-Hal
Re: Door closer transformer OCP
darren71 - Surely it is a Class 2 xfmr.??
If it is a legitimate one, it will say Class 2 on the xfmr. If it is designed to mount outside of the J-box that should start you wondering.
I'm not going to research all the rules, but the way I remember it, a Class 2 xfmr is properly protected on the primary if it has O/C protection not exceeding 20A.
darren71 - Surely it is a Class 2 xfmr.??
If it is a legitimate one, it will say Class 2 on the xfmr. If it is designed to mount outside of the J-box that should start you wondering.
I'm not going to research all the rules, but the way I remember it, a Class 2 xfmr is properly protected on the primary if it has O/C protection not exceeding 20A.
Re: Door closer transformer OCP
I thought it is designed to be mounted on the outside of the J-box so that the 120V wiring and the low voltage wiring can be kept separate. This would be similar to a doorbell transformer, would it not? You are correct about the 20A circuit being OK for a Class 2 power source, if I'm reading the code correctly (Art. 725.51).
I thought it is designed to be mounted on the outside of the J-box so that the 120V wiring and the low voltage wiring can be kept separate. This would be similar to a doorbell transformer, would it not? You are correct about the 20A circuit being OK for a Class 2 power source, if I'm reading the code correctly (Art. 725.51).
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