Subpanel Protection: Can it be in SubPanel?


  #1  
Old 07-26-01, 07:27 AM
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All of the "Doit Yourself" books I have seen portray putting the breaker protecting the subpanel in the mainpanel. My main panel has places on the bus bars to connect wires directly. Does the NEC allow me to just run wires from these lugs to a subpanel with a "main" breaker protecting ITs bus bars? I have a 1978 NEC and 384-16 (I hope they maintain consistency in numbers!) appears to say that the protection can go in either place. But then I wonder why the books don't mention this.

I don't want to consume the two circuits in the main panel for the feed.

The main panel is rated at 100A, and according the the label I could put lots of doubles in it, but it is full of singles.

Thank you. webclark@rochester.rr.com

 
  #2  
Old 07-26-01, 09:21 AM
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You can tap off the bus-bars if the tap conductors terminate on a circuit-breaker next to the existing panel- #8 wire in the minimum size conductor.You must be ABSOLUTELY sure the bus-bars on on the LOAD side of the Main circuit-breaker and are dead when the Main CB is Off.
 
  #3  
Old 07-26-01, 10:03 AM
J
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ACClark,

These are called "sub-feed lugs. I think that's a good plan. And it is somehwat commonly done. One thing to keep in mind is that you must completely separate your grounds and neutrals in any sub-panel. This includes having separate bus bars for each, and making sure the neutral bus is not bonded to the steel of the enclosure.

Hope that helps.

Juice
 
  #4  
Old 07-27-01, 02:45 AM
Wgoodrich
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Switchman you said;
You can tap off the bus-bars if the tap conductors terminate on a circuit-breaker next to the existing panel- #8 wire in the minimum size conductor.You must be ABSOLUTELY sure the bus-bars on on the LOAD side of the Main circuit-breaker and are dead when the Main CB is Off.

Reply;

I question where you came up with the 8 Awg conductor. I didn't see the subpanel size stated in the original post and the main breaker in the first panel is 100 amp rated.

I agree that you may use the 10' tap rule coming from that set of lugs on the main buss in the main panel. Yet if you use this tap rule the feeder must be sized to equal the ampacity of the main breaker in the second panel and that second panel must be grouped with the first panel because both panels would be considered as main service rated panels requiring the neutrals and grounding conductors to be married together. The original post hasn't told us the size of the main in that second panel.

Now I suspect that those lugs on the buss of the main panel is actually controlled by the 100 amp main breaker in the first panel which is the main service rated panel. If this is true then these lugs are protected by the 100 amp main breaker. If the feeder size equals the ampacity of that main breaker in the first panel sized by 310-15-B-6 being a # 4 awg., then that second panel may be located anywhere within that same building or even in a detached building. If this panel is located elsewhere within that same building and more than 10' from that main panel and the feeder is sized and protected by the main breaker in that first panel, then that second panel would be a non service rated panel [sub-panel] and the neutral bar and equipment grounding bar would then have to be separated. Also this second panel would not require a main breaker contained in that second panel because it would be a non service rated panel [sub panel].

Can you explain where you came up with the #8 wire you spoke of, I lost you on that wire sizing?

Wg
 
  #5  
Old 07-27-01, 07:33 AM
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Art.240-21 (2) Taps not over 25' long. (a)ampacity of tap conductors not less than 1/3 of the rating of the over-current device protecting the feeder- (b) the tap conductors terminate on a CB that will limit the load to the ampacity of the tap conductors.
 
  #6  
Old 07-27-01, 03:40 PM
Wgoodrich
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Switchman, I got ya now. I just didn't see you speak of the overcurrent device in the subpanel sized to protect that tap or any comment as to what size non service rated panel he planned to install.

Wg
 
  #7  
Old 07-30-01, 06:59 AM
J
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Wg,

You stated something which I have never heard before: Grouping sub-panel with main panel using 10' tap rule and both panels considered as main service equipment with neutrals & grounds bonded together in both panels. I am not familiar with this and always thought that any panel separate from the enclosure housing main service disconnect must have separated grounds & neutrals. Can you share with me what article permits this? I'm just very curious.

Thanks,

Juice
 
  #8  
Old 07-30-01, 11:33 AM
Wgoodrich
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Juice
The question is whether that set of heater lugs in the first panel is controlled by the main breaker in that first main service rated panel. The 10' or 25' tap rule may still be used but that second panel would not be a subpanel if that main breaker in that first panel does not control the heater lugs located in that first service rated panel. The person who posted the question said "My main panel has places on the bus bars to connect wires directly." The word directly through some doubts whether that main breaker actually controls that set of heater lugs located in that panel. Some panels have a direct feed through set of lugs that are not controlled by the main breaker in that first main service rated panel, designed to use the tap rules. How they get by the, you can't feed one panel through another panel rules, I don't know. I have found the direct connect heater lugs in a panel that stays hot even with the main off. A non service rated panel is only a non service rated [sub-panel] if there is an overcurrent device that is a main form of disconnect serving as the main service disconnect in that first panel that will shut off both panels when that main is turned off. If neither panel would shut off the other panel you would have two main panels. I don't think the person posting the original question has this scenerio. We just have not confirmed that the main breaker in that panel with the heater lugs on the buss actually will shut off those heater lugs too. If this panel is one of those panels that have a feed trough buss not controlled by the main breaker in that panel with the heater lugs then you would have two main service rated panels.

Wg
 
  #9  
Old 07-30-01, 12:00 PM
J
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Thanks Wg. Most of the panels I've seen with sub-feed lugs the lugs are indeed controlled by the main OCPD. Having a panel with lugs that are not is kinda scary to me. The idiot that owned my house before me hung a sub-panel, and connected it by buddying up in the main lugs with the service conductors! Sub-feeds that are not protected by your main OCPD are just about the same thing. I guess I just can't see the purpose for such a design in a residential application.

Juice
 
  #10  
Old 07-30-01, 12:39 PM
Wgoodrich
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Juice, the direct feed though lugs that I have found in the field are older style panels. I don't remember seeing a new panel that was not controlled by the main breaker in that panel containing those feed through lugs. Wonder if the older Code versions allowed to power the second panel through the first panel, just curious. Don't like the idea myself. Just wanted to make sure he did not have one of those direct feed lugs in his panel.

Wg
 
 

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