wire ratings

[allsixofus]

i understand what 12/2, 14/3 etc. in home wiring but not sure about #3/0, #4(bare?) etc.? What does AWG stand for? Trying to brush up for a cabin I hope to build. What conductor size is the wire from the breaker box thru the meter socket to the service entrance at the weatherhead for a temp pole at the site? Thanx for any assistance.

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] AWG is American Wire Gauge. (#10 AWG is about 1/8") The problem they ran into was that the gauge sizing system went to zero before the wire sizes quit getting bigger. SOooo.... when they got to zero, they called it Ott. (Pronounced ought) so the wire was #0 or one-ott. Then the next biggest size is #2/0 (two-ott) then #3/0 and then #4/0. THEN they went to a system that uses Circular Mils as a way of describing the wire, size-wise. So the next biggest wire from #4/0 would be called 250 MCM. That means that the total distance around the outside of all of the conductors in the stranded wire is TWO-HUNDRED-FIFTY-THOUSAND CIRCULAR MILS. Since current is carried on the outside surface of the wire, that is a way of measuring the current-carrying capability of that wire. So it goes 250 MCM, 300 MCM, 350 MCM, 400 MCM, 500 MCM, 600 MCM, 700 MCM, 750 MCM......well, you get the idea. JUST THINKING about wire sizes above 300 MCM makes my arms tired. It is so hard to bend that stuff that you almost need a conduit bender for the wire.**** Another note.....since the wire carries the current on the outer surface, the more surface that you have the better. That's why welding cable looks like HAIR....there is a lot more surface in stranded wire than in solid. [img]/forums/images/icons/tongue.gif[/img] [img]/forums/images/icons/crazy.gif[/img] [img]/forums/images/icons/smile.gif[/img]

[GaryM]

And it's flexable too! [img]/forums/images/icons/smile.gif[/img]

[Pat]

Dave, Can you give me a ref to where there is some written stuff suitable for 6th grade reading ability that explains the "skin effect" of conductors operating at a 60 Hz frequency being of interest in house wiring? I don't mean to sound upity I just never encountered that concept applied to low frequencies and low voltages.

I previously only encountered commentaries regarding the phenomenon of current density being concentrated at the surface of a metallic conductor in higher voltages and or higher frequencies. It is certainly the case with RF and also the higher voltages encountered with static electricity. We used to use copper foil of multiple inches in width for ground connections with low impedance so as to increase the surface area without wasting materials and $.

Remember when I started in science there were only 4 elements so some new stuff may have escaped me.

Litz wire was some German's contribution to practical transmission of power through a wire at higher freqs without so much loss. It is woven of very fine wires.

Here is a ref to skin effect:

http://en.wikipedia.org/wiki/Skin_effect

I borrow this chart from them:

frequency----------depth of skin effect
60 Hz--------------8.57 mm
10 kHz-------------0.66 mm
100 kHz------------0.21 mm
1 MHz--------------66 µm
10 MHz-------------21 µm

1 µm is 1/1000 of a mm.

What this shows is that the depth of the skin effect at our power freq (60 Hz) is 8.5 mm. A circular cross section with a depth of 8.5 mm is a circle twice that in diameter or 17 mm or a bit over 2/3 of an inch.

So, at 60 Hz, unless or until your current carrying requirements exceed a copper wire 2/3 of an inch in diameter the skin effect is still working for you. At amperages exceeding the capacity of a 2/3 inch diameter copper wire you need to divde the copper into multiple conductors along the lines of what Dave mentioned.

If for example, we were sizing a conductor to cary the output of a high freq inverter ballast to a fluorescent lamp then we would need to carefully consider the skin effect or else lose a lot of energy transfer efficiency. Likewise when Gary and I (and other Ham radio types) are considering RF projects this stuff becomes very meaningful as operating frequencies go up. (See chart above) At microwave/radar/satcom freqs the skin effect is extremely important (freqs of interest go up above 20-30 GHz)

At Ac power freqs (60 Hz in US, 50 Hz in most of Europe and 25 Hz in some backward third world countries) the skin effect is essentially NOT of interest in house wiring since the depth of the skin effect exceeds the wire size needed if there were no such thing as skin effect.

Give me a moment to get into my NOMEX undergarments before flaming me.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] Pat, I JUST went through this concept teaching High School Physics last Spring. The strange thing is that the Physics text DID NOT take into account the voltage; only that the skin effect did exist. There was never any mention of voltages so I just assumed it was level across the board. [img]/forums/images/icons/crazy.gif[/img] In Medium voltage systems, designers often use a stack of thin bus bar rather than one solid piece. I always assumed that it was to increase the skin rather than just making it easier to assemble, but maybe it was just the latter. [img]/forums/images/icons/tongue.gif[/img] I have done oodles of transformer tests where we used 12,000V on the primary side and attached the temporary feeders by clamping bus-to-bus. It was easy to see the efficiency of different styles of lash-ups by using the Thermographic Imaging Camera. On our primary side we were usually able to keep temperatures within limits, but the secondary side was a different matter. Once the Amperage got above 10,000, it was difficult to get sufficient "footprint" bus-to-bus to transmit the current and still keep temps low enough for the equipment to survive. We didn't want to send a new XF to a customer with blackened load side bushings. Over time I learned how to best stack bus to increase the footprint and transmitted that info to the test crew. [img]/forums/images/icons/tongue.gif[/img]

[Pat]

Dave, As you can see in the chart the skin effect has a pretty well expressed coeficient of extinction that is freq (or wavelength) sensitive. Like light penetration into an opaque solid or metal, the depth of penetration is not too great but it is finite and calculable. I typically never deal with low freq currents requiring special consideration of skin effect on wire selection.

I did build a pulse laser once and the special low inductance fast discharge capacitor (15 KV) would source some amps. I used to short it our for safety reasons lest electrolytic hysteresis cause embarrassment. I used a piece of copper tubing taped to the end of a dry wood yardstick. It blew holes in the copper tubing and nearly deafened me. I made other arrangements.

Much/most of my electronics work was at the other end of the current spectrum and wire sizes. I haven't been into wire above the AWG sizes but once. This new house's 240VAC single phase runs from the transformer to the two each 200 amp boxes is "M" or something strange I didn't recognize and didn't have to deal with. The utility got it from the pole mounted transformer underground to the meter box on the wall and my hired electricians got it from the meter to the breakers. I did not envy them having to pull that stiff stuff. I wouldn't have believed it possible if they hadn't done it.

Anyway, in non-coiled up applications at 60 Hz inductance is not such an issue. At radio freqs it is and grounding runs are often made with wide foil to take advantage of the skin effect and keep inductance low. I have worked with Collins trailing wire antenna tuners removed from aircraft. The antenna tuning coils were wound with bronze bands that were silver plated. Since the RF was restricted to a very shallow depth of penetration into a conductor, the thin silver plate on the surface did the conducting and the bronze was just for mechanical strength. There was one cylinder made of anodized aluminum and another of ceramic with a spiral groove to match the width of the conductive band. To change the inductance a servo motor rolled up or unrolled the band which was wound around the cylinders. The more you wrapped onto the aluminum cylinder (where it was shorted out) the less was left on the ceramic form and incudctance was lowered and vice versa when most of the band was wrapped around the ceramic, inductance was higher. This was an HF application. Higher freqs such as in the VHF, UHF, SHF, and on up into the "magic land" of radar and microwave things get almost too crazy to understand. Although I have a radar endorsement on my commercial lisc I can count on the fingers of my hands the number of times I worked on radar. I don't even go beyond changing a fuse if a microwave oven dies.

When I was working in electronics, wire heavier than 18-20 ga was heavy. 22-28 ga was common and sometimes finer wire was used. Power supplies were the exception. TTL digital logic was power hungry at 5 volts so the power supplies got pretty hefty and had BIG wires.

I think the skin effect as regards high voltages is a separate phenomenon from the skin effect that is freq dependent. The listed skin depths of the chart I posted is the depth in that particular conductor at the freq of interest where the field strength is attenuated to 1/e of the original value. There isn't any actual cutoff point it just keeps on getting weaker and weaker and of course would eventually get so small in amplitude as to be masked by random noise. Like so many other things in physics, lower limits of interest in exponential decay is often 1/e of the initial value. Remember all that time constant nonsense.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[Pat]

Gary, What did it have to do with a rock band or a glucosamine medicine?

[img]/forums/images/icons/smile.gif[/img] [img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img] [img]/forums/images/icons/smile.gif[/img]

[Adron]

Some people get carried away from the original question occasionally.

2-conductor #14 (15 Amp) plugs, switches, lights (70% of your house wiring)

3-conductor #14 3-way switches

2-conductor #12 (20 Amp) selected heaters, motors, air compressors, etc. (anything that uses a maximum of 20 amps),
kitchen receptacles circuits

3-conductor #10 (30 Amp) any device that requires a maximum of 30 amps (the most common application would be your electric dryer)

3-conductor #8 (40 Amps) maximum of 40 amps (the most common application would be your electric range)

#4 Bare Wire is usually used as a ground wire from the main breaker box to the ground rod. #6 in the case of some of the smaller breaker box sizes. i.e 100 amp service or less.

#3/0 is some HEAVY stuff for a cabin. As I recall, #2/0 is enough for a 200 amp service and #1 for 100 amp.

All of the wiring from the meter socket through the breaker box is your responsibility and is sized according to the anticipated load and the size of the breaker box.

You really need to check the regulations in your area. I have been known to make mistakes. [img]/forums/images/icons/shocked.gif[/img]

[RaT]

It should be noted that all those sizes mentioned for that amperage were for copper conductors, not aluminum.