Sizing conduit, wire for sub-panel run


Old 11-22-06, 10:41 PM
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Sizing conduit, wire for sub-panel run

I'm new here (though I've been looking through the site long enough to know y'all are a great bunch to be helping folks like me out with expert advice!) and am hoping someone can offer me some advice/confirmation.

I'm in the process of finishing a basement and will be running feeder wires from my main panel (which is outside of the house) into the basement to feed a 60A subpanel (about a 30' run from main to subpanel). I'm planning on using individual #6 wires for two hot conductors, a neutral and an uninsulated ground. I'll be running all of that in a conduit from the main to the subpanel; I'm planning on using a flexible nonmetallic conduit (like Carflex Liquidtight or the equivalent) for the entire run from the main to the subpanel.

My questions are as follows:

1) Do my feeder wires need to be THWN even though the conduit isn't being buried or can I use THHN? The conduit will be outside and exposed to the weather for part of the run, so the generic question is whether THWN is required for *all* exterior conduit runs or just buried exterior conduit runs. For that matter, can I use regular 6/3 WG cable or is that disallowed because its an exterior application?

2) What's the permissible downsizing of the groundwire size on this kind of a feeder setup (assuming individual wires is the way to go)?

3) Is 1" NM flexible conduit sufficient for the wires I'm running?

4) Are there any issues I need to be aware of in using flexible NM conduit inside the house? It would only be run in an unfinished utility room where the subpanel is located.

5) If I ever wanted to upgrade the service to the subpanel to something greater than 60A, what's the maximum size wires I could feed through 1" NM flexible conduit? I can't imagine ever needing more than 60A given it's a 900 sqft basement with just the basic residential load--no high draw stuff, but it's always good to know...

6) Lastly, any recommendations on the best way to make the conduit penetration into the house? This will be an above-foundation penetration (i.e. through the exterior siding and rim joist). Is an Lb condulette the best way (do they even make LB fittings for NM flexible conduit?)? A exterior box? Other ideas to help prevent water penetration other than generous amounts of caulking?

Chris W.
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Old 11-23-06, 01:17 PM
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1) I think you will find that any THHN you can buy will also be listed as THWN. The difference that makes is that THWN is a 75 degree centigrade rated insulation and THHN is a 90 degree Centigrade insulation. That effects how much current the conductors can carry. Most inspectors will want you to use the THWN rating of the conductors unless the outdoor portion of the run is very short. You could use type UF cable instead of raceway as long as it is marked sunlight resistant. I've yet to see any that wasn't so marked.

2) The Equipment Grounding Conductor (EGC) is sized by the size of the Over Current Protective Device (OCPD). A sixty ampere Circuit Breaker calls for a number ten American Wire Gage (AWG) EGC. A number eight AWG EGC is good to one hundred amperes. You might want to consider using the number eight EGC from the start if there is a likelihood that you will up size the feeder later.

3) One inch liquid tight flexible nonmetallic is large enough for the wires you wish to run.

4) Just run the liquid tight flexible nonmetallic conduit so that it does not have more than 360 degrees of total bends between pull points and do not install it were it would be exposed to physical damage. You should change over to rigid non metallic conduit if you have to run it exposed on a wall or on the bottom of the joist.

5) If you do run one inch liquid tight flexible non metallic conduit you could pull in number four conductors later to go up to ninety amperes.

6) You just use a rigid PVC LB and change over to NM flex by using a slip to threaded adapter into which you screw an NM Flex connector. You may want to use two LBs back to back with one on each side of the wall.
Old 11-23-06, 09:07 PM
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Tom, thanks for the quick and thorough response (on Thanksgiving Day nonetheless!!).

Old 11-24-06, 12:19 AM
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Hi Chris,

According to the NEC Table C.5 and C.6 regarding conduit fill, the maximum number of #6 AWG type THHN/THWN/THWN-2 wires allowable in 1-inch LFNC-B type conduit is 7, and for LFNC-A is 6. Like Hornetd said, you're good with the fill.

The suggestion to use RNC if you run the conduit exposed along the bottom of the floor joists or along a wall is a good one if you believe it will be subject to physical damage, otherwise it isn't required. However, RNC generally provides better protection when exposed.

NEC 356.30 requires, with few exceptions, that LFNC be securely supported every 3ft.

Also, like most non-metallic type conduit, extreme cold may cause it to become brittle, and therefore more suceptible to physical damage.

Best wishes!
Old 11-25-06, 02:11 PM
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Thanks to rdb2113 as well--y'all are great!

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