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Old 03-24-2008, 07:32 PM   #1
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New 60A or 70A service to expanded workshop


I have a workshop located at the opposite corner of the house from the electrical panel (about 100' away) that is currently supplied by a single 20A circuit installed by the previous owner. I have recently expanded my workshop and would like to add a 60 or 70A service via #6 THHN stranded copper. My options would seem to be to install a new trench and conduit or going up the wall near the electrical box, through the attic, down the opposite wall where I would only need about 15'-20' feet of trench. Option 1 would not be my first choice because of existing landscaping and the fact that Arizona 'dirt' is only slightly more workable than concrete!... digging a 100' trench 18" deep is not a pleasant thought. (even with a small trencher...)

My question is, is it code legal to go up one wall of the house, all the way through the attic (which is reasonably accessible) then down the opposite wall of the house to the ground and then up to the workshop? (PVC conduit used in the up/down/underground legs) Also, to keep the conduit close to the walls, and for aesthetic purposes, I would like to use "LB" type fittings at the point of entry and exit from the house. Can #6 (stranded) copper be used with LB type fittings or does this impose too tight a bend radius?

Any other thoughts or concerns I might have overlooked?

Thanks! Appreciate in advance any responses.

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Old 03-24-2008, 08:12 PM   #2
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New 60A or 70A service to expanded workshop


I have to assume a portion of the basement is finished. Is this why you feel like you need to trench around the house? I would try to at least pipe it inside the house to the same wall you needed the sub panel on, and pipe above ground if it wouldn't be too hideous. As long as you are using 3/4" pipe, #6 should be fine in the LB.

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Old 03-24-2008, 08:15 PM   #3
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New 60A or 70A service to expanded workshop


I like Gooses Idea if you can do that.

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My question is, is it code legal to go up one wall of the house, all the way through the attic (which is reasonably accessible) then down the opposite wall of the house to the ground and then up to the workshop?
Yes you can do this by code but you will have some issues. One... if your using conduit underground you don't want to run cable through conduit. You need to run individual Thwn wires. If you run individual wires without splicing then you will need conduit for the entire run... main breaker box to shop panel.

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Can #6 (stranded) copper be used with LB type fittings or does this impose too tight a bend radius?
#6 awg copper THWN is good for 65 amps in conduit and can be breakered at 70 amps.

#6 uf-b direct burial is good for 55 amps but is allowed to be breakered at 60 amps.

If you do not want to run conduit all the way and transition when going under ground you will need to splice. You would probably splice NM-B to THWN at the junction box before going underground. If so you will need to use the 55 amp ampacity of NM-b and 60 amp breaker. This is assuming the nm-b is #6 awg.

Now having said that I wouldn't be surprised if in Arizona attics get pretty darn hot an adjustment may be required for wire temperature corrections. So this is something to think about and if applicable it basically means you have to upsize your wire.
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Old 03-24-2008, 08:39 PM   #4
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New 60A or 70A service to expanded workshop


Basements in southern Arizona are extremely rare due to the "concrete nature" of the dirt. It is a single level house, so going through the attic is only "interior" option. I just didn't know if I was commiting some major code violation by going "all the way through" one structure (the house) to reach a second (detached) structure (the shop).

So am I to understand that if I use single conductor/jacketed, unspliced wiring all the way from panel to shop that it must be conduited even in the attic? I just assumed the wiring in the attic could be just the single (jacketed) wire. (As if I were running wire to a internal dryer outlet for example...) and that it would only need to be inside conduit for the external parts.

Yes, attics do get very hot here! But I doubt it exceeds 90c which, I believe is the rating on the wire I have... (I'll have to double check that though..)
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Old 03-24-2008, 10:23 PM   #5
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New 60A or 70A service to expanded workshop


90deg is the maximum temperature that the insulation will take before starting to degrade or fail. There is a table in the NEC that uses ambient temperature as a means to figure ampacity in a circuit based on that temperature.
You simply have to correct how many amps the cable can handle by a set number provided in the code book. This is called derating conductors. Cooler temps allow more amps. The only way to say for sure is to determine the type of wire, installation and ambient temperature.

In a single family dwelling, I can't think of any codes that would prohibit your install. I don't think you can install NM in conduit, but I may be wrong (we don't use NM in my area). I have to believe that the run would be so long you would have to upsize your wire for voltage drop AND temperature.
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Old 03-24-2008, 11:26 PM   #6
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New 60A or 70A service to expanded workshop


The wiring can be a cable like nm-b in the attic part of your run but you cannot run nm-b in conduit underground or exposed outside. It is an indoor dry location only application. Uf-b can be run underground and can be ran in conduit if you like but it will take a big conduit to accomodate # 6 UF_B. At 135F in the attic that cable be it uf-b or nm-b will only allow 53 amps using a derating factor of .71.

So your choices to avoid splicing are to run uf-b all the way inside and out or you may run thwn in conduit all the way panel to panel. To allow for your temperature in the attic and the distance you really need to consider #4 awg copper to provide a 60 amp capability of which you should not allow more than 80% continuous. Which you probably would never have.

Your other choice is to run nm-b to a junction box before going into conduit and changing over to thwn then run conduit all the way to the shop panel to avoid another splice at the shop.

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