Real World Current Capacities of AWG Gauge Copper Wire

[David Root]

I have seen at least three very different rating charts for AWG wire amps.

One is is for "Chassis, in air". One is for power transmission based on 700 circular mils/A (VERY Conservative) and the one commonly cited from Giddings, which is in between the two above.

None of them appear to be derated for ambient temperature, which is obviously pertinent to what we do.

Ga. Chassis Giddings Power Trans.

16 22 13 3.7

18 16 10 2.3

20 11 7.5 1.5

22 7 5 0.92


Anyone care to shed some light on this? EG what ga. did Leo use, etc.

FWIW my personal feeling is that Chassis ratings are unrealistic, as are Power trans. I think I would feel comfortable with about two thirds of Giddings' ratings at 50 deg C. interior of chassis temperature.

[Ken Moon]

Mil-specs are sometimes useful for worst-case 'real-world' data.

For chassis wiring in electronic equipment, using teflon-jacketed copper wire, at a length of 2 feet, and a temperature range of 0 - 35 deg C, based on MIL-STD-975:

22 guage = 4.42 amps max
20 guage = 6.15 amps max
18 guage = 8.56 amps max

Bundled 20-ga wires in aircraft applications are usually limited in practice to 5 amps, but this is at vastly longer lengths in a more demanding physical environment than guitar amps. These figures are so conservative that they could probably be doubled in guitar amps with no problems.

In a 4-foot loop, 20 guage wire carrying 5 amps will have a voltage drop at the load of about 3.5% - not a problem.

Electrical code tables that show ampacity based on the 700 circular mils per amp rule don't apply, as you noted, to electronic equipment, but to power distribution.

here is a link to a spreadsheet based on MIL-STD-975 that you can download and play with yourself (right-click and save):

http://www.geocities.com/CapeCanaveral/Lab/9643/awg.xls

[David Root]

Ken, thanx a lot, that is real-world enough for me. I never thought to look at MIL-STD! Handy calculator too!

I have been using 16 ga on large tube heaters and 20 ga on preamps, so I'm still pretty conservative.

[Phil_S]

I think this is a tough one to get your arms around, as there is so much that is misunderstood by people who are not electricians or engineers.

I agree that "open air" ratings are not relevant. Our concern is about wire enclosed in a chassis. A 2' run of wire, except maybe for filaments, would really be quite long inside an amp. I'd guess most wire runs are less than 12" and many less than 6". The short length really works in our favor, as losses due to wire length are going to be very low. In turn this allows the use of smaller wire sizes.

I'm no expert here, but from what I've read and gathered, 22AWG is good for nearly everything except heaters. How much current are we typically working with here? Maybe 1/4A or less except maybe in an amp running an octet of big bottle tubes.

On the heater debate, I suggest the gold standard is still 18AWG, but, again, assuming the 3" distance between tubes is 6" of wire (allowing for the twist), there is probably no harm going down to 20AWG for your typical 4-6 tube amp. I've seen claims of people using 22AWG and smaller for heaters -- that strikes me as too small in most cases, though I recently built an amp with a heater requirement under 1A and used 22AWG to make life easy at the 7-pin sockets.

Just my 2 cents.

[Structo]

That's weird because when I looked up 20 gauge Teflon, silver plated stranded I thought I read it was 11 amps.

That is what I have on my heaters, I couldn't fit 18 gauge on those tube sockets no way.

[Ken Moon]

Not a problem.

20 ga is fine for heaters - if 20 ga is good for 6.15 amps, then even with 4 6550s and six or seven 12AX7s, you'd still pull less than 7 amps - these ratings are VERY conservative so that wouldn't be a problem.

The heaters on a 2xEL34 / 3x12AX7 amp, like a 50 watt Marshall, would pull less than 4 amps, so even 22 ga would be fine.

[Phil_S]

This one is a pretty good discussion on the topic:
http://www.rowand.net/shop/tech/wirecapacitychart.htm

A more general reference:
http://www.powerstream.com/Wire_Size.htm

A wiki:
http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity

One says 11A for #20 in open air. The other two say 7.5A. I'm not questioning Ken for saying 6A. I don't really know.

[Ken Moon]

I'd say 6 to 10 amps is a good safe range for 20 ga - I do like 20 ga wire a lot, mostly because it fits nicely into tube sockets, switches, etc - but 18 ga is definitely better if you want to stuff the wire into the top of a hollow turret.

When I use 20 or 22 ga into the top of a hollow turret, it wants to pop out before I can get it soldered.

I'm sure most of you have seen the beautiful gut shots of Paul Ruby's amps, and he uses 18 ga exclusively.

So don't worry too much - anything 22 ga or over can handle the current.

[Bob S]

+++ what Ken Moon says.
I also remember what Merlin said when someone complained that it was difficult stuffing 2 heater wires into 1 tube socket pin "use smaller wire"

[Zippy]

There are two things that need to be considered.

1) Current handling capacity is the overarching concern - once exceeded, excessive heating leads to failure.

2) The matter of long runs is related to voltage drop. Resistance is related to cross-sectional area and is therefore related to but not dependent on current handling.

Long runs, as noted, are not an issue - and voltage drop is not a big issue here either.

[Super_Reverb]

Another aspect to this discussion is resistivity of the various wire guages. The data I found shows [22AWG - 16.1Ohms/1000' - 7Amps] [20AWG - 10.2 Ohms/1000' - 11A] and [18AWG - 6.4 Ohms/1000' - 16A]

So, a 1' run of 20ga is 10.2 milliOhms, loaded at 11A is dissipating about 1.2W. That sounds like a lot of power, but it's spread over 12 inches, right.? BTW, don't forget about the contact resistance of the wire soldered to the turret or lug

My interpretation of open air ampacity rating is a single wire, not in a bundle. It's somewhat vague, because twisted heater wires are in a small bundle. Wire getting warm is one thing, but wire getting to hot to touch is way over capacity. If it's only getting warm and the voltage drop is acceptable to you and you're adhering to the guidelines, it shouldn't be a problem.

cheers,

rob

[C Moore]

Check......
Thanks for the info.

[leadfootdriver]

I used 22g for my amp. Sounds good.