OK Imerc and barry and any other sparkies out there…..
I am going to run some wire out to my shop for a sub-panel.
I bought a bunch of 3/4″ pvc conduit when the corner hardware store closed.
So I am cipherin’ how much wire I can put in my tube.
According to the Code-Check guide, published by the esteemed Taunton Press, I get the following:
6 ga. THHN =0.0507sq. in. per conductor X 3 conductors (2 hots 1 neutral, [ground doesn’t count])
=0.1521sq. in.
3/4″ pvc == (.75/2) ²X 3.1415287564398374734836=0.4417 sq. in.
X40% =0.1767 > .0.1521 so I’m OK with 3-6awg THHN !!!???
This look OK to youse guys?
Also…
How come the chart in the book says that for 2 wires the fill is 31% and for more than 2 it is 40%????
seems that more wires will give off more heat!!??
I’m cornfused!
WEIN?
Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
Replies
T-
If you're going to bury the conduit, you need to run THWN-insulated wire. Most is dual rated (THHN/THWN), but if it is THHN only, don't use it underground.
You said "ground doesn't count". Wrong. A grounding conductor doesn't count for the purposes of derating for ampacity, but any wire you put in the pipe counts towards fill.
It's good that you're running a grounding conductor. You may know this, but you also need a ground rod or two at the outbuilding.
Why with two conductors you can fill to 31%, but with three the percentage fill can be slightly higher (40%)? The conductor fill allowed is based on two factors: having adequate room to pull the wire through the pipe (actually the conduit system, especially bends), and heat dissipation.
Heat dissipation is mainly dealt with by decreasing the ampacity (derating) of the conductors, based on the number of current-carrying conductors in the pipe.
The percentage fill is based primarily on how wires will line up and stack while being pulled--will the wires fit, and make the turns without binding or damaging the conduit or the wire insulation. A simple example--while being pulled through an elbow, three wires can form a triangle while two will sit side by side. So you can fill the conduit cross section fuller with three wires than two, and get away with it.
And now, the answer to the $64,000 question: yes, Code does allow four AWG 6 THWN conductors in a 3/4" schedule 40 PVC conduit. It won't be a fun pull, thats for sure. But the wires will fit and not self destruct.
Given that you're pulling a feeder for a 240/120V subpanel, you can run a grounding conductor that's one or two gages smaller than the hots or neutral. For instance, AWG 6 THWN copper is rated for 65 amps, if the breaker and lugs to which it's attached are rated for 75 degrees. Code allows you to run an AWG 8 equipment grounding conductor if the circuit protection is between 60 and 100 amps. If you're protecting the feeders at 55 amps (for 60 deg terminations, or if you're running #6 to minimize voltage drop because of distance), you could use a #8 ground. Even going down one gage will make a big difference in the ease of the pull.
So, if you're going to pull three #6 and one #8 copper into a 3/4" sched 40 conduit, I recommend--
1. no more than two 90 degree bends or equivalent, and make 'em wide sweeps (large radius). Code allows a total of 360 degrees of bend, but that's nuts if you're close to the allowed fill. Unless you have a power wire puller, and are feeling very lucky.
2. use liberal amounts of yellow 66 lube (known as "baby sh!t" in the trade).
3. if it's much of a distance, use a galv. rigid conduit (heavywall) sweep at the beginning of the pull. Use MA fittings and couplings to go from sched 40 to GRC and back again. If the top of the rigid sweep is buried at least 18" below grade, you don't have to ground it. I suggest using the GRC sweep because on a long pull, a lot of tension and heat can develop at the first sweep, with the wires cutting right through the elbow. This usually happens just before you're done, and jams the wires solid, or shaves/melts the insulation off of one or more. Or both.
One other tip--when the pipe is in place, use a mouse and a shop vac to pull a string through. Make a mouse out of a corner of a plastic grocery bag (tie it onto some string or twine), and use the shop vac to suck the mouse through the pipe. Use the twine to pull your pull rope in. Lube the pull rope with yellow 66 as you pull it in. It makes it a little harder to grab the rope during the pul, but will coat the pipe with lube before the wire is pulled in. Use lots of lube on the wire as it's fed into the pipe, as well.
Good luck.
Cliff
Edited 6/25/2004 7:21 pm ET by CAP
I want to run a phone and possibly cable (might need internet in the shop) So maybe I will splurge for some 1" and use the 3/4 for the phone and cable.
Thanks for the advice!!Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
A most excellent idea, on both counts.
Have fun!
Cliff
I thought I read here before if running a subpanel like this you should not have a ground rod at the outbuilding if running four wires. The example is coming off entrance panel with breaker, running 2 hots, neutral, and ground, direct burial (called trailer wire around here). I was told that there should not be two seperate ground rod locations. Not talking about doubled rod at entrance.
Edited 6/25/2004 11:50 pm ET by RASCONC
The NEC requires a grounding electrode system at each separate building or structure (except a building or structure served by only one circuit that has an equipment grounding conductor). This requirement has existed for some time. See 2002 NEC, 250.32(A).
The reason for a grounding electrode at each building is to provide a way to dissipate the energy from a lightning strike or a high-voltage power line cross (with the secondary distribution lines). It's always a good idea to run an equipment grounding conductor (EGC) with a feeder between buildings, and in some cases it is required. But the EGC will have a higher impedance than the wire from the subpanel to it's grounding electrode (called the grounding electrode conductor). This means that a high energy impulse won't dissipate as readily via the EGC to the service ground rod (or electrode) compared to the ground rod at the subpanel.
Putting it another way, if there's a high energy pulse (kilowatt to megawatt) heading for the subpanel, you want to make it as easy as possible for that energy to get to earth and dissipate. The local ground rods (& any other electrodes) and the wire between those ground rods and the subpanel do that. The grounding electrodes should be as close as possible to the subpanel, and the grounding electrode conductor should run via shortest path possible from subpanel to electrodes, with no sharp bends and no splices (or only bomb-proof splices).
The EGC leading back to the service panel, on the other hand, is the same length as the feeders, is (or can be) smaller gage than the grounding electrode conductor, and may have garden-variety splices or terminations in it. This is allowed because the EGC is designed to convey ground fault current to travel back to the source panel, to trip the overcurrent device.
The energy from a ground fault will be a lot less than that from a lightning strike--because the breaker (or fuse) opens and limits the ground fault current. Systems are designed so that the ovecurrent protection device can withstand the available ground fault current. You'll see this listed on circuit breakers as "ampere interrupt rating" (AIR). Residential breakers and panels usually have a 10,000 AIR. That's because the typical utility drop is fused at 10 kA, or the transformer and drop are inherently limited to supply that much current.
Commercial breakers and panels are typically 22,000 AIR, because the supply trannys are able to provide more power. AIR values for overcurrent protection devices can run a lot higher, depending on how much current the utility (or customer) transformer can supply. Those breakers get extremely expensive. Cheaper than a new building, though.
It's not practical or cost-effective to require or to use breakers (in non-utility settings) that will withstand the energy in a lightning strike or power line cross. So you provide a local grounding electrode, a good path to it, and just hope that when the pulse arcs over to the subpanel can (housing), it'll drain off to earth before it gets into the building wiring.
Cliff
Edited 6/26/2004 1:19 am ET by CAP
Edited 6/26/2004 1:23 am ET by CAP
Great advice!!!
DID you write a FHB article?
Either way, you just made my job easier!
thanx
More questions while I have your attention.....
I plan on putting in 3-4 30 amp circuits and possibly a 40 (for a Comp.)
Any reason why I shouldn't use some double pull breakers I have and run two (or 4?) circuits off them?
Here is my basic Idea:
50 amp doulbe pole breaker in existing panel
60 feet of 6-3 to J-B
3-6ga. +1 8ga. thru 1" conduit to shop (Burried)
Square D 100amp sub-panel with isolated nuetral and ground in shop with a ground rod.
1-40 amp, 1 or 2 30 amp, 2-20 amp, 1-15 amp breakers
maybe skip the 40 cause the panel is only available w/ 6 or 8 spots for breakers.
30 amp circuits will be 10 ga. 20 amp-12ga 15amp-14ga(lights only)
Am I missing anything?
Again, I really appreciate oyur time and consideration here!!!
Thanx again!
Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
Cap said in a previous post:
"if it's much of a distance, use a galv. rigid conduit (heavywall) sweep at the beginning of the pull. Use MA fittings and couplings to go from sched 40 to GRC and back again. If the top of the rigid sweep is buried at least 18" below grade, you don't have to ground it. I suggest using the GRC sweep because on a long pull, a lot of tension and heat can develop at the first sweep, with the wires cutting right through the elbow. This usually happens just before you're done, and jams the wires solid, or shaves/melts the insulation off of one or more. Or both."
I was really impressed with your solution to the wires cutting through the elbow, and wos wondering, don't the wires have a bit of trouble bouncing over all those transition fittings? Could you describe a 'MA' fitting? Is that just a Male Adapter? And if thats the case, why not just use a FA on the thread of the rigid elbow? Being the fast learner that I am, I have only cut through elbows twice. That I know of.
Another solution to the problem of the pull rope cutting into sweeps, it is most common on the 90 closest to the tugger as it is under the most tension for the most time. A solution that works is to thread the pull rope through a section of flex, as large as will fit into the conduit run and long enough to cover the sweep after going through any down section, and thread the flex down into the 90 where it is duct taped in place.
The galvanized steel flex spreads the load and is more heat resistant. When the head, much larger than the flex hits it the flex will strain at the tape joint. At that time the pull is stopped long enough to pull out the flex.
This won't help cutting in the middle of the run but cutting is much less of a problem there. Especially on larger conduits the difference in cost, time fitting MAs, potential issues with future corrosion and grounding between SCH40 PVC and galvanized steel is substantial. Sometimes to the point of justifying using a steel 90 for the pull and installing a RNC 90 afterwards. A technical violation of the NEC.
Another consideration is that cutting of the sweeps is usually, but not always, a sign that something is wrong. A poorly made head, insufficient, or improper, lubricant use or the need to break the run down into smaller pulls by installing another pull point or two are all possible reasons for excessive pull pressure that is the common cause of cutting.
Seeing as the cutting usually occurs at the 90 closest to the tugger keeping a close eye on the rope can spot trouble early. Cutting is usually accompanied by the rope coming out hairy not smooth. Also there is often melting of the hairs sticking out of the rope and sometimes flecks of grey PVC. Caught soon enough you can try the trick with the flex or, if your feeling lucky, slowing the pull.
I must say that cutting of sweeps seems to happen more often when a powered mechanical tugger is used. Pulls using a hand cranked tugger or by hand tend to be relatively immune. I suspect because they are very much slower and the operator is likely to be much more aware of poor pull conditions that cause excessive pull tension. He, they start sweating and they stop the pull, extract the head and double check for potential problems. Power tuggers invite a lackadaisical attitude. Brute force covering for poor technique.
hal,
Thanks, but it's not my idea, just one that I've used.
Ja, an MA is a male adaptor. The inside of the transitions is smooth, buy you're right, it's not like a straight run of pipe.
As for an FA on the Sched 40, yep, that'd eliminate the coupling. But I more often have an MA and a coupling on the truck.
My MAXIS 3000 is my good buddy for feeder pulls. It uses a hole-hawg to drive a capstan--small enough to be easily portable, but powerful enough for anything I need to do. But it's true, you can abuse that power.
4lorn1s flex trick sounds slick, and would simplify things considerably.
Man, the 'net is great!
Cliff
My new garage was 80 foot from my service panel (house) You might want to consider running a seperate service panel at the garage. I live in Indiana and NIPSCO came out and ran the wires to the new meter box at no charge. The only digging I had to do was to install the meter box (buried service cable). I installed a 100 amp service without much trouble. I included the electric service with my permit. I get charged at a slightly higher rate for the second meter box.
Go Cubs
T.,
Cliff gave you good advice.Doesn't it say something about this site that you can post a question,and the guy who writes the magazine articles will shoot you back the answer?
PVC conduit is cheap enough that you should just figure out how many amps you need first,size the wire to that,and oversize the conduit to that.
Barry