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#202375 08/01/11 10:30 PM
Joined: Feb 2008
Posts: 193
S
Member
I got a call today regarding a rooftop breaker tripping. It is at a restaurant with a subpanel on the roof that feeds two ac units and some smaller exhaust fans. The breaker that feeds the panel is tripping every once in a while. twice yesterday, none today. I checked the amperage at about 6:30 this evening with both ac units running. It was pulling 75 amps on a 90 amp breaker and the wire was quite warm. It has #2 alum wire feeding the panel which runs for about 45ft in pvc on a white rubber room.

Could the acs running when its 90 degreees, with the increased ambient temperature cause the breaker to trip? I know that it could cause detioration of the wire. I did change the two 60 amp breakers on the ac out last year at this same time beacause they were crumbling from being to hot.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
SHort answer: yes. The ambient of the breaker does affect tripping.

The problem, in your case, is that the breaker is inside, not on the hot roof .... so the outside temps might not be the cause.

A possible cause is that yhe PoCo tran sformer is overloaded at the peak of the afternoon, and the voltage is dropping too much as a result. To confirm, you'll need to measure when it's hottest out.

Joined: Feb 2008
Posts: 193
S
Member
actually, to give more complete info. The breaker that feeds the panel is also located on the roof mounted to a parapet wall. Sorry not to disclose this sooner.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Then, yes, the breaker can be tripping early due to the higher ambient.

I believe that 20 degree marking on the sides of breakers indicates the temperature at which it is calibrated. Put it on a sunny roof, and I wouldn't be surprised if it tripped 50% dooner.

This is commonly encountered in industrial settings. One solution there is to simply open the panel and point a fan in there- not very elegant, and sure to give an inspector a case of the vapors!

A more acceptable solution is to mount a fan on the enclosure - Rittal is one maker catering to this market- and another is to mount an air conditioner on the panel- the entire business for Maclean.

Joined: Feb 2008
Posts: 193
S
Member
When I go back up there I will take a picture. Somebody from the kitchen opened up the enclosure door for air and bent an aluminum pan over it to make it raintight. The other panel opens up. They propped it up with another pan. I will look into the fans for the panel. Thanks Reno.

Joined: Apr 2002
Posts: 7,282
Likes: 3
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A solution may be ....replace said subpanel inside the structure and re-feed the loads, compliant to the current code.

Cutting a fan into a subpanel may raise some eyebrows. I know they are made, and they are for 'panels', but....?


John

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