Electricians - Have an electrical question

I suggest posting the same question to alt.home.repair. There are pros there that definitely know the answer.

I did just what you are considering, ran 80 ft of 6 gauge wire to feed a panel on a 60A breaker. It works fine for me. Voltage drop is not an issue, practically speaking, since electricity from the utility is a little overvoltaged to begin with.

I regret not using proper wire to supply a 100A breaker. I did not realize, when I ran this circuit, that I may need to use more than 60 amps (I did it mostly for a compressor, but later bought a welder).

i

3-wire 6 gauge ampacity is 55 amps for UF and 65 for THWN. At 50 amps, and a 2% maximum voltage drop, you're limited to 105ft of #6 at 240V.

Since you're using a 60-amp breaker, I assume you shouldn't be drawing more than 50amps, continuous, no? You call out 56+ amps. That's way too much for a 60 amp magnetic breaker in continuous duty. OTOH, it is not too much for the #6 wire.

LLoyd

Ok, I just don't see 4/3 NM-B at Home Depot or Lowes. And yes I already knew I needed 4 lines, 2 hots, 1 neutral, 1 ground.

long as it is under 100 feet you are ok.

When you say 100A are you saying that is your main breaker panel that you are running this from, or that is the size of the breaker you added to your main breaker panel, to run to your new one?

Well when I calculated everything, it comes between 54 - 57 amps. That is just based on whether I get a 20amp or 27amp welder not sure yet. So I will just assume 57 amps. Now from my reading you shouldn't put more than 80% on a breaker for total wattage. So I'm guessing I should go with a 70 amp breaker instead.

Now what wire should I use, really comes into question :-) What is 4/3 NM-B rated for?

100A seems high for 4/3 NM-B, but the Electrician said that was fine? Where did you get it from?

Google "ampacity of copper wire".

LLoyd

sparty sezz:

I'm no electrician, but I just did this recently under the guidance of one. My run was similar: 230V, 70-80 ft, Mostly in basement. Differences: 100A, partially outside (detached garage). In preparation for the project, I used a couple of the calculators readily available online for calculating the voltage drop. I input a couple of different wire gauge sizes and used this data to make my wire size determination. I verified this with my electrician.

I ended up using 4/3 NM-B with Ground. According to the electrician I was working, 2 lines, 1 neutral, and ground because the sub-panel had to be grounded to the main panel by code. In addition, I used buried conduit in the stretch outside (12-15') and to get into the panel once inside the garage since it is a concrete block structure. I was told that if I used THHN that I would have to run it in conduit the entire length and I did not want the additional headache of running conduit through the mess of a basement ceiling I had to deal with.

-- Later, D-Rog

sparty sezz:

Try a real electrical supply...I would be suprised if they even have 6/3 (at least my locals). It was spendy but worth it for my situation.

Sounds right for the drop recommendation but I haven't run your numbers...

I added a 100A breaker in my main panel to supply my 100A sub-panel.

-- Later, D-Rog

If you are going to use NM (romex) wire then it must be kept dry. And NEVER run it through conduit. The emphasis never is from the book, not me. 4 gauge is good for 70 amps. ERS

Current ratings for wire is sort of voodoo-ish.

The current rating is not just based on copper gauge, but the type of insulation AND the type of raceway (conduit) used AND the number of other wires in sed raceway, as well as ambient temperature and permissible rise in temperature, etc. IOW, the concern is for *heat*. The new insulations that can tolerate more heat bump up the current rating of the same copper gauge!

Imo, these ratings are conservative. The base rating is given in the NEC (National Electric Code), w/ each municipality giving it their own spin. NYC was ridiculously strict, at one time, and then lightened up a little. For example, #8 THHN stranded wire, can *easily* handle 60-80 continuous amps over a 50 ft run--gets slightly warm. But it is "rated" at about 45.

In the very old days, the wires from the pole to the house were run *bare*, for maximum current-carrying capacity. Tough on the squirrels, tho.

OTOH, doesn't hurt to have extra capacity, esp. for things like electric heaters, lighting, the occasional microwave, fridge, etc. Thus, the "incidental load" can be pretty substantial, depending depending. On a cold day, my shop can be drawing 30 A (at 120V) before I've turned on a single machine! And, if I'm going to be using the welder at full tilt, occasionally I'll turn a few things off, or make sure the dishwasher/dryer/etc. are off in the house.

#4 Wire: Apropos of the above, the rating varies from 70 A to 278 A!! 1996 NEC. Now, the 278 amp rating was for *nickel* covered wire (apropos of the thread Surprises about electrical conductivity--nickel apparently is Da Bomb, rating-wise!), but even w/o nickel, #4 wire can be rated up to 220! Typically, tho, the rating for #4 hovers around 100, up to about 140-160. Most local codes, and the NEC, for, say, two #4/one #8-10 (neutral)/one #12 (grnd) in a 1" pipe would be 80-100 amps.

Which, again, is very conservative, imo. But, running larger wire--even considerably larger--is a cheap way to prepare for the future. You may even want to run an extra wire in the conduit for a third phase, depending on where you might place a rotary phase converter.

I don't see why not. I"ve seen HVAC peeple run romex thru conduit, lots of times, esp. where nail puncture is a possibility. Heat dissipation, mebbe?? That's all I can think of.

-- Mr. P.V.'d formerly Droll Troll

4 gauge is good for 70 amps.

I am 100% agreeing with what PV said. Do NOT do what I did (run 6 gauge wire), oversize the wire so that you have an OPTION of increasing the amperage of the circuit. Wire is cheap.

i

Ok, I just got off the phone with a local city inspector to verify. He said that between 50 - 60 amps, 6/3 NM-B is fine. For over 60 amps up to 100 amps, then they mandate you run 4/3 NM-B wire. He did say also, if running through my basement ceiling, the holes in the joists must be a mininum of 1 1/2 inches from the bottom. I guess that is incase you finish the ceiling and nail/screw drywall in.

So I will be putting in a 70 amp breaker and running 4/3 NM-B to the new breaker panel in the garage.

NEC sez that wires closer than 1-1/2'' from a member surface require nailing plates. It's just a steel guard to prevent nailing into a wire. Simpson and others make them. Most have prongs to fasten, so you don't even need fasteners.

LLoyd

As an aside, the other issue here is that the closer to the bottom of a beam or joist you put the hole, the more it is weakenned, since that's where there's the most tension. If you put the holes through the center of the joist, it has the least effect on the structure...

--Glenn Lyford

"sparty" wrote in news:1138904022.539979.6640 @z14g2000cwz.googlegroups.com:

Sparty, A branch circuit has to be rated 125% of maximum expected (calculated) load. This is 70 Amps for your 56 amp load. This gets you into the 4 gauge wire size. You can still run the 60 amp breaker in the main panel if you wish, but your wire will be rated nearer to 80 in free air, depending on the insulation type.

Here we go again - what does ampacity mean ? - amp capacity ? Current Capacity ? I can understand that in jargon and talk - but printed on boxes and breakers - that is out of science.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder

Lloyd E. Sp> 3-wire 6 gauge ampacity is 55 amps for UF and 65 for THWN. At 50 amps, and

Yep like I said, since I figure a max of 57 amps, I divided the by 0.8 to make sure I was only running 80% safe capacity. So that's why i'm chosing 70 amp breaker in my main panel. And yes, like I mentioned I will be running 4/3 NM-B Wire.

Not sure what you mean by 80 in free air.

What it means is that the wire, with insulation, is exposed to the air all around, or virtually all around. This keeps the wire cool. The wire itself can stand to be very hot but the insulation, depending on type, has varying degrees of temperature resistance. This why the Romex type of cable is not to be run inside conduit. The insulation can eventually break down and allow the wires to touch each other or the conduit and/or the insulation can catch fire. The ratings are conservative. I believe this is because people tend to push things. ERS