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200 amp main panel, needs to feed a 100 amp sub panel in a detached garage. 100 feet total run, underground in conduit(50′ of the way). If the sub panel has a 100 amp breaker, what size breaker in main, and what wire size and type run to the sub? I’ve had to downgrade the main panels breaker feeding the sub in a few houses (50 amp in main to run a 60 amp sub, when using Al wire)Does the sub need a ground rod driven?
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Replies
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You call me names, then you want free advice?
*Huh?
*Gunner, dont be so sensitive sparkie...:)
*I'm sorry Keith I guess your new to the board,and missed the sparky debate of last month. You call me Sparky and I'll call you stupid from now on how's that? :)
*Keith,100A breakers in both main panel and sub.THWN wire, #2 AWG copper. Number 3 copper is rated at 100A (75C insulation), but tool motors will last longer with the lower voltage drop of the #2.You must install ground rods at the subpanel (you need two unless you can prove the resistance of one rod is less than 25 ohms).IF there is no metallic path (pipe, phone wire, etc)between the two buildings, you can get by with a three-wire feeder (two hots & a neutral). If there is a metallic path, you must have 4 wires, and include an equipment grounding conductor. I recommend running the EGC now; sooner or later, someone's going to run a wire between the buildings. The feeder EGC can be #8 AWG copper if you use #2 as the current-carrying conductors.Also, if you run an EGC, you must have separate busbars for the ground and the neutral (called a floating neutral buss). The ground buss is bonded to the panel, and the neutral buss is electrically isolated from the panel. IF you don't do this, the return current will divide between the neutral and the ground conductors--and the panel, any metallic conduit, and the 3rd pin of your receptacles will be energized when juice is flowing in any circuit.Good luck.Cliff
*Found and read that thread....seem a little sensitive. Sparky is the mascot of the local power supplier. Ohio Edison. Little outlet as his head.Many of the local contractors have him painted somewhere on their trucks. Locally they call us carpenters "woodies" , I guess I could take offense to that...like calling me boner, or hard on....but hey, as long as they are calling, they can call me Mary.
*Thanks Cliff...exactly what I was looking for.
*Where's the sparky thread?
*Around here "sparky" is a common term for electrician, not at all derogatory. Good friend is licensed, does a lot of commercial and some residential, and uses the term to describe himself and others in the trade. (I guess I missed the other thread, too)
*Cliff. I've been trying to get around to wiring my shed for about a year and a half. I figure out just what I need to do then I forget. I've even taken notes, and lost them. Yours is the clearest instruction I've ever seen, so I'll just copy this. Thanks.
*I guess you can never i gauge a sparky's sensitivity.oops, sorry about that :)
*Cliff,Great info and explanaition. What is the distance limitation on the #2 wire? I will be doing the same thing shortly but with a distance to the shed of 200'. Also, is the 100 amp breaker that fits into my main 200 amp service a special part; that is designed to be a feeder for a sub panel? #2 wire is so large I imagine the breaker must be special to accept it. Thanks,Dave
*http://www.mikeholt.com/free/voltagedrop.htmnear the e stream....and the reindeer's pee stream,aj
*Whoever heard of a sparkie (electrician) not being called a sparkie. It's what all sparkies are called. Joiners/carpenters are called chippies, and bricklayers are called brickies, and furniture makers like me are called wood butchers or hedge carpenters. Slainte, RJ.
*Running 100 amp subpanels to garages, who whould have thought. (my whole shop will run off of one 20amp 110v circuit.)Keith ---You say you have had to "downgrade the main panels breaker feeding the sub in a few houses". That never happens. You size the main breaker for the wire not for the panel breaker.but #3 wire seems large enough.Dave ---#2 wire will have 3% voltage drop for a 92amp load at 200 feet. #2 will work well until the breaker trips.
*George, in the past year we have had two instances of the inspector kicking the inspection where we(electrician not me) had run the sub panel on aluminum wire.One was a 60 amp subpanel, that had a 60 amp breaker in the main. He said for a 80' run, all exposed or in crawl space the main had to be downgraded to a 50 amp. The second was the same(but only a 15' run to sub) except he said running aluminum inside you have to downgrade the main panel breaker 60 to 50. Do I understand....nope. Did I have the electrician come back and follow the inspectors rules..yep.
*drptop,Thanks. You'll be OK with #2 copper for a 200' run, unless the demand is close to 100A. In that case, the Vdrop will be a little higher than the NEC recommendation of 3 percent Vdrop for a feeder.At 80 amps, which is the max continuous demand (more than 3 hours)that a 100A circuit should supply, the Vdrop will be 2.4 percent. Your big motors (tablesaw, compressor, dust collector) will live long and prosper.The breaker is just a 100 amp, two-pole unit. The two poles are what give you 240V. It needs to be UL-listed for the panel (not just one from another manufacturer that'll fit the panel). As far as the wire size fitting the breaker, that's a big enough breaker that the terminals will accomodate #2 wire.And one more thing, tighten the terminal screws to the right torque (listed on the breaker). Too little, the termination may become loose and what you've got an arcing fault. Too tight, and maybe you strip the terminal screw or crack the plastic breaker body. A small (1/4") torque wrench will do the trick.Work safe, and Merry Christmas!Cliff
*Hey Cliff -For those of us who are electrically challenged, what's "EGC"?
*equipment grounding conductor
*On people getting bothered by being called "sparkie", I used to know a guy who would go nuts if anyone ever whistled for him (like calling for your attention from a distance). He would get really mad about it, and explained that when he was young someone told him that if anyone ever whistled for you it was like being summoned like a dog - very demeaning.He came to realize that it was needless to react so, but that connection had been put in his mind and he could not totally make that response go away.I have come to believe that it is possible for people to get pissed off about just about anything, if you meet enough of them, and you might never know why they do (they might not either!).I was gonna suggest that we only refer to each other by our first names, but remembered that some people find that insultingly informal.Oh, well.
*Aluminum doesn't conduct electricicy as well as copper does. It has more resistance. For the same current and gauge, the aluminum will get hotter. Therefore, the NEC table 310-16 has separate entries for copper and aluminum. It's all different numbers, not just a derating formula. That plus the special termination goo to prevent oxidation make aluminum not such a great thing. The cost of copper for real big feeders may make the aluminum economically attractive in some cases, but I really like copper a lot better.-- j.S.
*Keith ---I missed the comment about AL wire in your post. j.S. explained it properly: aluminum wire has different properties and a differeent table and the inspector was asking for the right size breaker for the installed wire. By the way all of the comments have recommended COPPER #2 wire not #2 aluminum.
*Just to be regionally clear....we use all aluminum conductors here once larger than number 10 wire.We also use the goo....& have very few problems....I retorque connections as I come across them though...and do see a problem with lack of goo use and lack of torque....I suggest all connections be checked periodically and if I was not the electrician.near the alu stream, aj
*George,You'll have a field day with my situation then! 400 amp service up at the house to supply a 200 amp panel at the house and a second 200 amp panel at the garage.Of course, that TIG welder does pull down some amps...
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200 amp main panel, needs to feed a 100 amp sub panel in a detached garage. 100 feet total run, underground in conduit(50' of the way). If the sub panel has a 100 amp breaker, what size breaker in main, and what wire size and type run to the sub? I've had to downgrade the main panels breaker feeding the sub in a few houses (50 amp in main to run a 60 amp sub, when using Al wire)Does the sub need a ground rod driven?