Another Q for CAP et al.(sparkies)
OK
Got my panel and wire.
Here’s what I got to work with so far.
Got 3-8 ga. THHN in 1″ pvc w/ 10ga. ground.
Chart says this is good for 55 amps.
now is where I get cornfused I got 8-3 NM-B for the rest of the way to the panel.
But is this rated for 40 or 50 amps????
is NM-B TW or THW??
If I use the 8-3 should I use a 40 amp breaker?
is is kosher to switch from 8 THHN to 6Ga. “Romex”? (In a Junction box)
I can probably live with a 40 amp Shop, at least for the foreseeable future.
The max load I can see is The tablesaw (13a.) the lights, a radio,the fan for the heat, and the compressor kicking on..
If the Table saw and the Comp. are on different “sides” of the panel will 40 amps be enough?
Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
Replies
Generally, you need overcurrent protection for the smallest downstream conductor. So 6 ga and 8 ga protected by a 40-amp breaker would be okay. Not the best practice though, because it is confusing in the future. If someone looks at only one end, they'd think the whole run is 6 gauge and maybe put a bigger breaker in. So an inspector might disallow it.
Different wire types do have different ampacities. All 8 gauge wires were not created equal. Insulation type, temp rating, heat disappiation, etc.
40-amps of 208/220 ought to be fine for a 5-hp tablesaw and 5-hp A/C as long as they don't start at the same time (and maybe if they do). I was starting two 4.5-hp motors at the same time on a 40-amp C/B last week okay.
Yea, suprise!
The conductors in NM and SER cables have rated ampacities equal to TW-insulated wire.
I'd run 6 AWG cable (that's 6-3 with ground, right?).
Yes, make the connections from the NM conductors to the wire in conduit in a large junction box. A 4-11 extra deep box would be sufficient (with 42 cu in volume), but if there's room for it, a big one (like 8" by 8" by 8") will make it a lot easier to tuck the wires and splices back in. For splicing, I suggest using split bolts or, better, Polaris connectors.
As far as any problem from changing gages, I wouldn't worry about it. It's a bit more of a concern when there's a branch circuit that goes from #12 in the panel to #14 somewhere. But in your case, the #8 downstream has the same ampacity as the 6-3 cable. If in the future someone does something wrong out of ignorance, well, you can't assume responsibility for that.
One more thing that I do and recommend to you--when you've got everything wired up, before you energize the feeder circuit, measure resistance to ground from each of the feeder conductors, across each hot to the neutral of the feeder, and across the two hots. If there's a ground fault or short, better to find it with an ohmmeter (at 9 volts and a few milliamps) than with 60 amps at 120 or 240 volts! I do the same quick check (with a continuity tester) before powering up a branch circuit as well.
Cliff
Edited 6/28/2004 1:18 pm ET by CAP
What about the limiation to use 60 degree rating?
In the 99 NEC 336-26 "Ampactiy. They ampacity of types NM, NMC, and NMS cable shall be that of 60 C conductors and shall compley with section 310-15.
Thge 90 C rating shall be permitted to be used for ampacity derating purposes provided the final derated ampacity does not exceed that for a 60 C rated conductor."
And for SE cable.
338-4 "installation methods for brach circuits and feeders.
(a) Interiior Installations. In additon to the provisions of this article, Type SE service enetrace cable used for insterior wiring shall comply with the installation requirements of Parts A & B of Article 336 (which include 336-26) and shall comply with the applicable provisions of Article 300.
Bill,
The 60 deg rating is in fact the TW (and UF) rating (or allowed ampacity).
I used the term "rated ampacity" which is a simplification. I try to stay away from sparky jargon as much as possible when explaining things here, as long as it doesn't interfere with clarity.
And yes, you can adjust or derate the ampacity using the 90 degree column value (e.g., THHN ampacity).
This kinda makes the point of why electricians/electrical contractors are worth what they charge. Even an apparently simple question, like "how much current can a wire carry safely?", can be answered only by knowing a bunch of complex and interlocking requirements and limits. And all the requirements have to be met to provide a safe installation. For residential work, actually doing the work is fairly simple. Knowing how to do it right is the key.
Cliff
> Even an apparently simple question, like "how much current can a wire carry safely?", can be answered only by knowing a bunch of complex and interlocking requirements and limits
Yes, I've often thought that the whole ampacity issue in the NEC is treated in an overly cumbersome way. But it's also argueably the most important part of the NEC. Any ideas on how it could be re-written to be easier to apply, and still maintain the same level of safety?
-- J.S.
The NEC is a law book.
It is written by committees of interested parties such as insurance companies, electricians, manufacturers, engineers and so on. These are paid votes -- that is, they are pretty much forced to vote the way they do by whoever selected them. The electrians [the grunts] got limits on the number of bends in conduit not because it is hard for them [us] to pull but because someone came up with the idea that a hard pull -- too many bends -- may damage the insulation, thus increasing the risk of shorts and fires.
The new requirements for AFCI is a brilliant markering ploy. What better way to sell your product than to pass a law requiring everyone building a new house to buy one?
As for the amperage tables, these are indeed important. However, there is a lot of slack and sloppiness built in. "It's OK to go to the next larger breaker..." They list wire at 90º Centigrade but since no breakers are 90º rated, you can't use that rating. But you can derate from that rating but then still can't use that derated rating because it's still too high for the breaker [which is 60º for less than 100 amps and 75º for more than 100º. Why is there a 601 amp fuse?
And then there is demand load. This is fun. Try playing around with 20 stoves in an apartment complex. Or even your own house. What do you do with a stove rated at over 20 KVA? So you do all these demand calculations and end up with a test answer of 145 amp panel so that becomes a 150 amp panel and you go to Home Depot and you buy a 200 amp panel because it is cheaper than a 150 amp panel which they don't have but will misorder for you and will be delivered two weeks after the house is occupied.
I would like to see some competition to NFPA. Perhaps states could adopt H. P. Richter's Practical Electric Wiring as their Code for Residential. But the problem is that that book in turn refers to the NEC Table 310-16. It would be nice to have a simple table like 15 amps = #14, #20 amps = #12 and so on. You could do this for something static like Romex but the other types of wires have different types of insulation melting points so you would need a diferent table for each type. And then there is a difference between free air and being hidden in a wall full of insulation.
The true answer is allowed by using the Neher-McGrath formula. But this is an engineering formula and there is always at least one unfindable element involed in an engineering formula [usually a factor].
And I don't think you are anywhere near cynical enough regarding the writing of the NEC. I maintain that the code is deliberately written in an ambiguous way. Why ? Simply because that's how the Shapiros and Mike Holt's of the world make their living. Don't understand a particularly badly written section ? Well, buy my book, order my video, attend my seminar coming to your city on......
If they were serious about wanting a clearly written document, they could just hire an English major to proofread and point out all the ambiguous or contradictory passages, and make these guys write it clearly. He/she could start with the proposed changes for the next code cycle, and eventually move on to the whole document.
And if anybody on the Code Committee is reading this, yes, I would gladly pay an extra dollar to pay this person's salary, if I get a readable document.
CAP,
Heres what I have decided for now.
I found out My panel takes Homeline or GE breakers.
Lo and behold I got a 40 amp double homeline in my collection.
So since I have the 8-3 and the breaker, I'm gonna save some $$$ for now and go with 40 amps.
I WILL test with my meter before putting the juicetricity to it!
Also gonna get som split nuts for my splices, wire nuts don't give me a good feeling on such a big wire.
Do I just wrap 'em with a heap o' tape ? or is there something better?
Thanks again for all the expert advice!!!
Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
T--
Get the split bolts good and tight first (use two wrenches). To insulate them, the old way was to use rubber electrical tape first, which is fairly thick, and to use a lot of it. It pads the sharp edges of the bolt and also provides electrical insulation. When it's done right, the splice looks really smooth on the outside--you can't see the outline of the bolt. Then cloth friction tape was put on over that to protect the rubber tape. On a big split bolt, you could use an entire roll or more of rubber tape.
For small bolts, I think you can just wrap the splice with good electrical tape (Scotch 33+, or better, the thicker Scotch 88). Make sure you get four or more layers over every part of the bolt, and overlap each wrap by half.
One technique that I've used on occassion where the splice may need to be inspected or broken is to wrap the first layer with the sticky side out. Keeping the adhesive off of the splice makes it a lot easier to cut the tape off. Just start wrapping sticky side out, when you've covered the splice, twist the roll of tape 180 degrees and continue.
I use mainly Polaris-style connectors now. So much quicker and easier.
FWIW, here are a couple other tips on using PVC electrical tape:
-- as you finish the wrap, cut the end--don't pull it to break it off. Breaking it off stretches the tape, and the free end will come loose from the wrap as the tape recovers.
-- if you want a really secure wrap, use a small dab of PVC cement on the tail end of the tape. The tape is PVC, and the cement will chemically weld the layers together. I usually only do this when the splice is going to be exposed to the weather (like split bolts on overhead lines).
Good luck on the project.
Cliff
I am sorry, I miss read your answer and did not look at the actual tables. I though that your answer refered to using the 75 degree.
This has been a confusing area for me and don't know why the limitation.
I can see it for the smaller sizes, but there is alread a limit max breaker size for #14, 12, & 10 that limits all wiring of that size to the 60 degree column.
But it does not make any sense to limit NM and SE for things like range branch circuits and sub-pannel feeders where you are talking about 60-100 or more amps.
And you can use the same SE at what is equivalent to the 90 degree rating if it is used as a service entrance cable.
And a couple of years ago a friend of mine bought a house built in 1950 and with an addition built in the 70's. It appears that the orginal electrical system was very well done, for the times. BUT over the times the orginal pannel was replaced with a 100 CB pannel, but with the orginal 60 amp cable for the service drop, 3 wire feeder to the sub-pannel for the addition, 30 amp breakers on some of the brach circuit in the sub-pannel. No NM connectors on some of the circuits, hidden splices and boxes in the wall, switched neutrals, DW tapped off the clothes dryer circuit and last (that I have time to list), but not least, a plastic coffee can lid as the cover on a j-box.
Anyway, while I added a number of circuits and fixed a lot of the problems I did not have the time that was needed to upgrade the servivce and fix some of the other major problems. Nor did I have an experience working with the service and power would have been off totally for a week or more as I figured everything out and I would not spliced the hot drop the way that the electrican did.
At the time I was wondering about the cable that he ran for the sub-pannel. I don't remember the size, but at the time I know that he had to use higher than the 60 degree column.
I ask an electrican online friend in a forum that I was active in at the time. He claimed that it was OK un the 2000 NEC, but could not quote the specifics.
One of the guys I work with found out about that the HARD way.
He lost the connectors he had for the sub panel feed.
so he sent a helper to the hardware to get 2 more.
When every thing was done he flipped the breaker and the connector coming out of the main panel exploded almost in his face!
It was a cheap connector that had a sharp edge that that cut into both hots!
he found pieces of it across the basement.
REALLY lucky!
I will mention to him about testing with a volt meter first.
thanx all
Oh yeah...
only bummer is that the chunk of 8-3 I bought was the single most expensive item!
77$ ouch!
Mr T
Happiness is a cold wet nose
Life is is never to busy to stop and pet the Doggies!!
Edited 6/28/2004 6:07 pm ET by Mr T