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Sanity Check - Load Center/Breaker Panel access using Conduit

Sanity Check - Load Center/Breaker Panel access using Conduit

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  #1  
Old 08-24-14, 09:48 PM
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Sanity Check - Load Center/Breaker Panel access using Conduit

I have an old (circa 1900) single story house that has had generations of electrical work, but is mostly updated at this point. I would like to add one additional 120V15A outdoor circuit presently, and may like to add additional 120V20A circuits and a 240V20A circuit in the future.

The load center/breaker panel (200A) is surface mounted on the exterior of the house. The existing wiring (I assume 12/2 NM-B [white jacket]) enters the load center from the rear through a stud cavity in the exterior wall. This cable path is not accessible without removing finished interior surfaces, so that option is not preferred. An image of this existing configuration is attached:
Name:  Existing Wiring.jpg
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I would like to gain access to the breaker panel through a new 2" conduit that would be routed into the crawl space. In the crawl space, the conduit will terminate at a junction box. My understanding (aided by this forum) is that this configuration would allow for THHN-2 wiring from the breaker panel to the Junction Box, where it can be spliced into NM-B for connection to sockets/switches as desired. The overall conduit length will be ~40". An image of this proposed extension is attached:
Name:  Proposed Extension.jpg
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Although a 2" conduit may be overkill for this application, my thought process was to maximize the conductor space available (as conduit fill percentage), which is a direct function of conduit diameter. This would allow for larger THHN-2 conductors in the event they need to be derated due to the number of Current Carrying Conductors (CCCs) in the conduit. Although the planned number of CCCs is presently 2 (1 hot and 1 return/grounded leg), this may be increased in the future. This is due to future expansion, and possible rework/rewire of existing circuits that may not be presently acceptable.

My questions are:
-Is this a proper approach (i.e., is it safe, professional and within code)?

Assuming yes:
-What type of conduit would be used in this configuration? I would prefer metal over PVC for aesthetic reasons and perception of quality finish. Safety/Quality is more important than cost in this application.

-Given the current and planned branch circuits, and type of conduit (from previous question) what gauge THHN-2 should be used for:
1x 120V15A
1x 120V20A
1x 240V20A
(6 CCCs)
I have found raceway fill calculators online, but not for 2" conduit...

I realize I am being lazy in this question, as I should calculate wire gauge(s) myself; a better question would be "based on experience, what gauge THHN-2 would you recommend?"

-What type of Junction Box would be appropriate for this configuration? I would assume larger is better. I will not include separate "neutral" and "ground" bus bars in the Junction Box as the workspace (crawlspace) would not be conducive to "precision" work

-Are there any other questions I should be asking?

Additional information:
-The house has an attic, which is far more accessible than the crawlspace. However, there are likely fireblocks in the study cavities, so it is unlikely placing the Junction Box in the attic would reduce the overall impact of adding circuits (I would like to avoid impacting interior finish as much as possible). If there are contrasting opinions/experiences, please share.

-The house is technically located in a flood zone (i.e., requires FEMA flood insurance), but has a certificate indicating it is above grade, which mitigates the risk/insurance premium. I don't know whether/how this would impact the classification of the crawlspace in terms of electrical wiring.

-Location is San Jose, CA

Thanks in advance for any response. This forum and its moderators and participants do a great deal to empower users to become more self-sufficient. I will likely hire this job out to a professional, but want to make sure it is done correctly.
 
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Old 08-24-14, 10:13 PM
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Welcome to the forums.

I'd use 2" PVC with the little bit that is exposed outside and in the crawlspace the looks are less important.

1x 120V15A --- > 2x #14 THWN/THHN
1x 120V20A --- > 2x #12 THWN/THHN
1x 240V20A --- > 2x #12 THWN/THHN
(6 CCCs)
I'd run one piece of green #8 THWN/THHN to a terminal block inside the splice box for grounds only and no neutral bar.

With 6 CCC's you won't have any issue in a 2" conduit.
 
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Old 08-25-14, 06:01 PM
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Thanks for the Welcome and the advice, PJmax.

Do you have an opinion on what type of Junction Box/Splice Box would work best in this application?
 
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Old 08-25-14, 06:06 PM
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A metal or PVC junction box would work. IMO using 2" is a waste. You will still have the same derating issues as when a smaller conduit is used.
 
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Old 08-25-14, 06:11 PM
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You can use any splice box. A 6x6x4 or 8x8x4. A metal box could come with knockouts making the cable installation easier. Most plastic junction boxes would have to be drilled for every cable connection.
 
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Old 08-25-14, 06:14 PM
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The prepunched knockout are a plus for using a metal box.
 
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Old 08-25-14, 09:43 PM
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Thanks for the responses. I will look at using a more reasonably sized conduit.
 
  #8  
Old 08-26-14, 05:02 AM
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use the 2 in pipe...................you may want to add more wires later
 
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