Small gauge (35AWG) aluminum wire capacity - Page 1
Forum section: electronics / beginners

author: GregDunn
author_rank: Frequent Contributor
timestamp: July 18, 2019, 05:14:05 pm
content: I'm having trouble finding AWG charts with current ratings for extremely small aluminum wires. Background: My speakers are unconventional (Magneplanar Tympani 1-C), made with straight runs of very small aluminum wire of such a length as to provide 8Ω total resistance, bonded to Mylar sheets. They are sufficiently old that even the manufacturer does not maintain detailed specifications for them any longer; I have been in touch with them of course. I'm trying to determine whether I'm providing sufficient protective fusing for them, but it's been rather difficult to locate a wire capacity table for AWG35, aluminum wire.The manual I have for them only gives a vague recommendation for amplifier power and fusing. I have been using a 1A FB fuse which seems in line with their recommendations but it would be helpful to know just what the current capacity of the wire is, so I can make the best possible choice for protection. They are somewhat inefficient, and do require a significant amount of power to perform properly.Does anyone have a wire chart that might have the info I need?

author: bob91343
author_rank: Super Contributor
timestamp: July 18, 2019, 06:32:05 pm
content: I presume one could calculate this. First, look at the melting temperature of aluminum compared to copper. Then look at the resistance of the wire per unit length. With copper you can find the value from tables, I assume, and thus see how close the melting temperature is approached at maximum current. Do the same for aluminum and allow a comparable temperature gap.It's a bit tedious but if you allow a substantial safety factor you are probably okay.Another approach might be to measure the temperature of the wire as you put various currents through, assuming you have an expendable piece of it. You can do that either with a thermometer of sorts or by measuring the resistance change with current.Most pure metals have a positive temperature coefficient of resistance of about 0.4% per degree C. So if the resistance changes 20% with current flow, you can figure 50 degrees change. If that is well below melting point you are safe. You can find the correct coefficient from a materials handbook.Just put calibrated current through the wire and compare voltage drop at various currents.I also suspect there are tables of safe current for aluminum wire, although probably not for something of such light gauge. Yet you can see the comparison to copper tables and make an educated guess.

author: floobydust
author_rank: Super Contributor
timestamp: July 18, 2019, 06:41:51 pm
content: Forcopper wireit's 4A 1 sec, or 27A 32msec, from 1928Onderdonk's equations.You have to know the fuse's clearing time.You can put in aluminium's properties for melting point, density, conductance into Onderdonk's equation to get the fusing current. It's a bit of work.

author: GregDunn
author_rank: Frequent Contributor
timestamp: July 18, 2019, 07:40:19 pm
content: I just realized, it turns out not to be the wire which is the limiting case, but the Mylar it's bonded to which melts at a lower temperature. So I suppose I can assume linearity for copper at that temperature and find out what the limiting current is without the complex calculations.Now, to track down the temperature coefficient for aluminum...

author: floobydust
author_rank: Super Contributor
timestamp: July 18, 2019, 09:04:17 pm
content: The model 1-D fusing is weird, 2-1/2A for the tweeter? Nothing for the mid/bass section?

author: tooki
author_rank: Super Contributor
timestamp: July 19, 2019, 12:03:54 am
content: Good god that’s a batshit crazy jack arrangement.

author: GregDunn
author_rank: Frequent Contributor
timestamp: July 19, 2019, 05:38:33 pm
content: When you consider what it's meant to do, it is actually pretty logical. 3/4" spacing for the main jack if you run the speaker full-range, and jumpers to connect all the speaker drivers into the crossover network. Pull the appropriate jumpers, and you have 3/4" spacing jacks to plug an amplifier into the appropriate drivers directly for biamplification of the speaker. Also, the connections which should not be accidentally shorted are far enough apart that you can't plug a standard 3/4" dual banana into them. If you're really industrious, you have access to all the drivers and crossover components, in case you want to just put an attenuator into the tweeter circuit (a real option for the 1-D and IV models) or replace any of the crossover elements individually.I actually like it better than the old style (pictured; not mine but similar) which requires you to use spade connectors and/or bare wire and short the jumper terminals with fiddly little bits of resistor lead.Same functionality but messy.Back on the subject of wire gauge, I still haven't found a good reference for small aluminum wire, but a variety of sources suggest using 2 AWG sizes larger for aluminum vs. copper. This points to an approximate current rating of about 20mA for AWG35 aluminum, which I suspect is wildly conservative. Even allowing for the fact that the tweeter is 2 parallel runs of 16Ω wire, that's not a lot of current and I know I've pushed over 1A through them with no damage. That's the typical fuse value for one of these, I'm guessing. The IV and 1-D are 4Ω systems of similar design and I know they come with a 2A fuse.

author: tooki
author_rank: Super Contributor
timestamp: July 19, 2019, 06:56:59 pm
content: Quote from: GregDunn on July 19, 2019, 05:38:33 pmThere's nothing wrong with using two rows of banana jacks. What's crazy is the insane order they're numbered in, with no apparent rhyme or reason. The numbering within the schematic is random, and the numbering of the jacks is random. Why is 8 on the top row, when all the other even numbers are on the bottom? Trace a line from each jack in the order of the numbers; it's crazy.