Electrical Forum - 100 sub panel

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Subject - 100 sub panel
m.weed	I am installing a 100 amp 240v singe phase sub panel which will be about 180' away from the 200amp load center. The sub panel will be crossing the barn then going outside to contractor portable office trailer. My questions are??? 1) Can I use a ground rod next to the trailer instead of running a fourth wire from the main load center to the sub panel as the ground. Which is better. 2)The trailer is within 6' of the barn. Can I run liquidtight to it instead of trenching to it with RMC or PVC. 3)Can I run a number 3awg wire or do I need to go to a number 2awg. Not sure if a number 3awg will be sufficient on the 180" run. Thanks, Mike
David Hyatt	Just found it 300.5(D)5, Made me look. Thanks
iwire	300.5(D)(5) To be fair this is a new rule, I think it when in for the 2002 code. Now from a practical sense, in the area of the county I work most underground raceways are not dry, almost all are partially or completely filled with water. As long as the conductors are not damaged during the installation they will last fine submerged.
David Hyatt	Cool, I didn't realize in conduit underground it would be considered at wet location, because the wire remains dry. Live and learn I guess. Thanks. Not doubting just trying to find in NEC to further explain. Do you have code ref?
David Hyatt	If you run THHN in pipe underground it will be ok, if not use direct burial cable.
iwire	David he must use one of the types cs409 mentioned. THHN underground is a violation. Of course it is kind of a moot point as most wire is dual rated as THWN/THHN.
David Hyatt	But in conduit its ok, right?
iwire	Hi David you are fast. Any raceway or enclosure underground is considered a wet location and must use conductors rated for such use like THWN or RHW etc. But almost all the wire I receive is THHN/THWN. Now when performing derating calculations on this dual rated wire you can only use the 90 C rating when installed in a dry location and if used in a wet location you must use the 75 C rating. To add more confusion types with a "-2" on the end like RHW-2 are wet location and 90 C rated. quote: 300.5(D)(5) Listing. Cables and insulated conductors installed in enclosures or raceways in underground installations shall be listed for use in wet locations.
cs409	where did u come up with the 5.12% for voltage drop on #3awg at 180ft on a 240v system?
m.weed	I'm looking at chapter 9 in the 2002 NEC. I'm pretty sure I made a mistake on the formula. I'm not even sure I have to apply it to a 180' run. I normally don't install runs this long. I'm searching like crazy for an answer with the limited free time I have. Any help is appreciated.
iwire	It is difficult to answer voltage drop questions with out knowing the load. You say a 100 amp panel, but is the load going to be 100 amps? I think not. If I figure the circuit as 180' 240 volt, 80 amps of load allowing no more than 3% drop 2 AWG copper will do it. Say the load is really 100 amps and you will need 1 AWG. Personally I would run the 2 AWG Copper and feel good about it. If you do run 3 AWG copper with an 80 amp load you will have just a small bit over 3% drop and if the load is 100 amps 3 AWG will give you 3.75% of drop even that is nothing to worry about. Remember voltage drop is not a NEC requirement only a suggestion.
David Hyatt	Question 1)You will have to install a four wire feeder. per NEC Question 2)liquid tight flex not suject to phsical damage, plus it would be a nightmare to run the wire through flex this far. I would trench it with direct burrial cable 2' deep. Question 3)If this a permanate stucture not temperary, I would adjust for Voltage drop @ 100amp, because this is the potential load. I would run #2, 4 wire direct burial.
Scott Vickrey	Mike, don't be a flextrician. And don't be afraid of a little 6' ditch. Where is the sub panel to be mounted? If it's on the barns exterior then, use EMT between panels.
m.weed	Don't get me wrong guy's. I'm not a hacker or shortcut taker. Most of the time I over do everything and have a problem with being a perfectionist. I will be doing the job this week some time. I'm going to run 1 1/4 EMT across the barn through the wall change to pvc and trench to the trailer. ( don't own any threading equipment) I will use three #2 thhn and a #8 ground. This morning the inspector told me I could use 1" emt but dosen't the ground count on the conduit fill. Thanks to everyone for your help.
David Hyatt	Yes, you do count the ground in conduit fill. You will actually have to use 1 1/4 EMT, the inspector is wrong. #8 bare or insulated will have to be stranded. Article 310.4 of course who would want to use solid.
cs409	you will have to use a THHW OR THWN THWN-2 ..ETC SENSE your going underground to the trailer...being the inspector was wrong on the size/fill on conduit am sure he wouldnt catch the thhn either!