Current Vs AWG

[trem]

I did a search (I need a tutorial on how to search better) but "current"... "speaker wire" ... and "amps" brings up a lot of stuff.

Wire Gauge Guide


The above chart says that the Max Current for 14 AWG (the gauge of wire in my house) is 5.9 What does that mean exactly.?
The house breakers are 15 amps. Should 14 AWG not exceed a constant load of 5.9 amps.?
In the end, this all relates to speaker wire gauge, and how to figure what is "necessary".

This is just theoretical but:
50 watt amp
8 Ohm cab
35 feet of speaker cable. (AWG to be determined)

Just to be conservative, I would rate the amp at 100 watts and the Ohms at 10.
The square of P/R makes it 3.2 amps.....is that right.?
Assuming that math is correct... is that a reasonable way to do it, or am I being Way Too Liberal/Conservative .....leaning too far one way or the other.? I would need cable that could take a constant 3.2 Amps.
Thank You

[g1]

Check more charts. They are not specific about which conditions that current spec is for (from what I can see).
Sounds to me like it may be for bundles.
This chart has 2 current specs:
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

[R.G.]

Wire gauge and current carrying capacity are dependent on things that are not obvious, much as the power capability of a transformer does.

Wires are resistors. Current going through a wire heats it. The thing that sets the current carrying capability of wires is how efficiently they can get rid of the heat without the insulation melting or decomposing. The 14AWG in your house has specific plastic insulation that won't melt until it hits ... um, whatever that particular grade of wiring is set for. That's the current limit - when the insulation gives up and the wire shorts.

There are special cases. If the wire is "plenum rated", it is designed to carry that specified current when stuffed into a wiring raceway potentially filled with other wires similarly self-heating. The actual fusing current - where the wire melts! - is dependent on how insulated it is. There's a different answer for the same gauge wire going through a vacuum versus through air versus through insulating oil, all of which carry off different amounts of heat and affecting how hot the copper gets at X current.

For thermoplastic-insulated wires in air, for 50Wrms, I'd say that 20 gauge is plenty, 18 gauge is liberal, and 14 is massive overkill.

[Enzo]

Also your 50 watt amp does not sit there making 50 watts. If you gave it a steady signal and run it up to clip, then it might be putting a steady 50 watts into a load, but when you play, only your peaks are going to hit that, so your wire does not need to meet all the specs for whatever the current was. In other words, you never need 12ga wire to connect the speaker to that 50 watt amp.

[trem]

That chart has similar numbers.....5.9 Amps for 14 AWG.
They list current for:
Chassis and Tramsmisson.
Assuming a 35 foot speaker cable, I thought that sounded more like Transmission than Chassis.
I do realize a 50 Watt (BF Fender Bassman for example) will not be a constant 50 Watts, but just for "safety" I picked 100 Watts, because the amp is capable of peaking at about that much power.
I am not trying to calculate the exact power, just trying to get a round number and make sure there is plenty of margin for Error/Safety in choosing the AWG of the speaker cable.
.......and another chart... American Wire Gauge, AWG Cable Size Description for Copper Wire Cable

[g1]

That chart has similar numbers.....5.9 Amps for 14 AWG.

The chart I linked shows 32 amps for chassis wiring. For transmission, think of something like a generator running a constant current.

[Enzo]

The wire rating is for heat and resistive losses. Even at 100 watts peak, the peak part is operative. It only has that 100 watt peak for a brief moment, it isn't going to melt the wire, and the resistance effect will only have a slight effect on the peak of the waveform. If you look in the back of most amps, you will find that #20 or #18 wire they used more than sufficient.

[trem]

The chart I linked shows 32 amps for chassis wiring. For transmission, think of something like a generator running a constant current.

I guess, there lies my confusion. Perhaps I am taking the word "chassis" in Chassis Wiring too literally, but.....of the two options, I did not think a 35 foot speaker cable would be considered Chassis Wiring. Because of the length, I thought it would be transmission.
But I understand your point....."transmission" would be for actual supply (like from your generator) where there would be a constant load of 5.9 amps...perhaps for several hous at a time.
Thanks

Yeah Enzo...I hear you. I have never used 35 feet, but maybe 10-15 feet.? And I have never had a problem with 18 AWG running from 100 Watt Fender's and Marshall's to their respective speaker cabs.
I do not mean to dredge up the whole "Hoover Thing" .......but we ditched a couple of Hoover Wind Tunnel vacuums a few years ago, and I removed the power cords, because they have that nice (like an extension cord) Tough/Sturdy covering around the wires, so it does make for a good speaker cable.
Funny thing is (speaking of AWG) the power cord on these two Hoovers is 17 AWG. So I am a bi "better" than 18.
Thanks Again

[Gingertube]

Guys,
In the day job I do airborne survey equipment. You can apprciate that the aircraft industry is a bit paranoid about fires in the aircraft cabin and as a frequent flyer in these aircraft I share that paranoia.

For those interested the following is the method needed to choose wiring AWG in order to get certification to fly our gear.

The "Master Document" is this:
http://www.faa.gov/documentLibrary/m...AC43.13-1b.pdf
This is a huge document and Section 5 is the bit of interest for Electrical "stuff".


First you need to know the Wire Temperature rating and decide at what maximum ambient temp you want to run the gear in.
From that you do the (Wire Temp Rating - Ambient) to get "Max allowable Temperature rise" in the wire. This is the "Master Driver" for determining wire current rating.

Using the max allowable temp rise and Wire AWG - lookup the "Single Wire Free Air" current rating from tables 11-4a and 11-4b in the FAA document.

Derate according to number of wires in the bundle according to Table Fig 11-5, for WORST CASE analysis assume 100% loading of the wires.

Derate again according to maximum operating altitude according to Table Fig 11-6, I use 10,000ft as we do not operate the gear at altitudes above 10,000ft, for your tube amp you can assume 5,000ft maximum.

And that is it - the current rating of any wire is fully determined, the authorities will then check that you have a protection device (fuse or circuit breaker) rated at not more than this rating.

A VERY conservative/safe method but guaranteed to be safe enough to satisfy aircraft regulatory authorities so should certainly be good enough for your tube amp.

Cheers,
Ian

[Old Tele man]

While this paper is specifically about PCB traces, the underlying theory/mathematics likewise applies to wire:

http://www.ultracad.com/articles/fusingr.pdf

[g1]

You can get really technical, but a lot of sellers of PA gear and speaker wire just go by speaker cable gauge charts.
The primary concern is getting all the power to the cab, rather than losing it along the cable's resistance. In the chart you posted, there is a column for ohms per 1000ft, easily converted to ohms per foot. Then put that number against the speaker impedance. So you don't want something like 4 ohms total resistance for the cable if you have a 4 ohm load, you are losing half the power in the cable.
I'm not sure the particular % loss that is considered acceptable, but here is an example of one such chart.

[Ken Moon]

Here is a spreadsheet we used at work many years ago, when we were re-designing avionics wire bundles in the A-6 aircraft. Military aircraft specs are very conservative, but it's fun to play around with:

(This is the first time I've tried linking to a OneDrive doc, so let me know if the link works)

https://onedrive.live.com/redir?page...CIK5528SJvOE3M

[Leo_Gnardo]

Here is a spreadsheet we used at work many years ago, when we were re-designing avionics wire bundles in the A-6 aircraft. Military aircraft specs are very conservative, but it's fun to play around with:

(This is the first time I've tried linking to a OneDrive doc, so let me know if the link works)

https://onedrive.live.com/redir?page...CIK5528SJvOE3M

"Excel cannot open this file because it contains macros." Oh well.

FWIW I keep a roll of 16 ga speaker wire around, can't go wrong unless I'm trying to ship over 1000 watts say, or beyond 100 feet. Neither of which limits have been approached by any amp or situation I've encountered in normal life. The current limit for typical Switchraft plugs & jacks is 5 amps, same for Cannon/XLR mic type connectors. If you really have to move a lot of current there are connectors to handle that. I don't know if they still use these but up to the early 2000's Clair Bro's use "Australian" Cannon connectors rated for 30 amps to wire up big PA cabs.

[Ken Moon]

"Excel cannot open this file because it contains macros." Oh well.

Dang.

I've got to find a better way to share excel files with macros.

Here is a screenshot showing results for a 2-foot loop of 20-ga wire at a temp range of 0 to 100 degrees C:

I'll mess around later looking for a better sharing site...

[Ken Moon]

Here are Mil-STD-975 parameters for the most popular wire sizes used in guitar amps:

22 ga - Ampacity=4.42A, Ohms per foot at 25 degrees C=0.0165
20 ga - Ampacity=6.15A, Ohms pre foot at 25 degrees C=0.0104
18 ga - Ampacity=8.56A, Ohms per foot at 25 degrees C=0.0065

btw, the Navy's go-to hookup wire is Gore brand, Type E (MIL-W-16878/21, 600 Volts), which is a stranded, silver-plated copper conductor with a tape-wrapped (rather than extruded) PTFE dialectric insulator and PTFE outer jacket.