Wire gauge ratings and use - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 18, 2020, 11:53:08 pm
content: Not certain about a cable product description. It involves the wire gauge AWG and if it can carry the power intended. Currently I'm using 12 AWG wound on a transformer secondary, I can see it can carry 20 Amps at 12 Volts easily. From the same supplier he has many wire gauges and cable types. The uncertainty is with a cable with silicone sheath that is 14 AWG and rated for 32 Amps. I know ratings are different for chassis wiring, and transmission line use. I would want to wind a transformer secondary with this 14 AWG silicone wire, but my confusion is if that would be considered chassis or transmission power use. As it's short length and wound on an inductor, could that wire handle a maximum current of 32 Amps at 12 Volts. Below is part of the description, the 14 AWG is where I'm not certain that could deliver that current rating on a transformer secondary.Thanks for reading, any help appreciated.

author: tkamiya
author_rank: Super Contributor
timestamp: September 19, 2020, 02:37:45 am
content: There are two factors that dictates current rating on wires. Copper wire heating up from ohmic loss and insulator's ability to withstand the temperature. Teflon being very heat tolerant material, higher current rating is understandable. Please note, this rating has nothing to do with voltage drop due to ohm's law!Also, when the wire is used by itself or as a pair, there is one rating, and when there are bundles, there is a derating curve. This also has to do with temperature rise and tolerance to it. In addition, some environment allows higher temperature raise than others. So it's not that straight forward.Probably, the best thing is to contact the manufacturer and discuss your particular case. Short of that, I'd stay conservative.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 19, 2020, 10:14:15 am
content: Thank you for replying, I understand what your saying, I've found literature that explains exactly what you have mentioned. My maths isn't fantastic, although I can manage ohm's law, the supplier is an eBay seller. I doubt they will want to comment on individual uses of there product, people today are aware there, or would be mindful there suggestions could be used against them for problems arising from there suggestions. It's sad really, any info I get is not any reflection on the data and person giving to me, I know it's not set in stone, and there are always variables. From one arrival I read, the problems of 14 wire gauge might not trip a breaker. As it might not carry the current to trip said breaker, and possibly over heat or smolder instead, but went on to say 20 gauge wire could carry the current, and would trip the breaker in an over current scenario.I think from that arrival I read, it's possible to carry the current, but not best practice to push the wire gauge to its maximum limit or even slightly beyond. I'm going to erge on the cautious side and stick with the 12 gauge wire I've already wound on the transformer. It performs ok, and I need 15 Amps to 20 Amps, but not the higher of the two continuously. I was just looking for a few extra volts AC, but I might just have enough voltage one it's rectified and filtered. Derating the primary input gives rise to a lower secondary output voltage. But it might be doable just with the 12 gauge wire.

author: Jwillis
author_rank: Super Contributor
timestamp: September 19, 2020, 01:57:32 pm
content: Typically you want to keep your wire between 25 to 30 degrees centigrade for any kind of transmission . Heat is not your friend in electronics . You can push wire to higher temperatures as long as the insulation can handle those temperatures before softening . Excessive heat degrades the insulation over time regardless of the type . Excessive heat also creates more resistance as well which will reduce the efficiency of a transformer. Transformers current capability is not rated by the gauge of the wire alone . The maximum flux density of the core must be able to handle the current being drawn from the secondary. If the current exceeds what the core can handle the Magnetic field will begin to collapse which in turn creates more heat.Another thing that needs to be understood is that multi core wire can have a reduction in current capability from as much as 40% with up to 3 cores (strands) to as much as a 70% reduction in current capability for 40 or more cores (strands). For this reason transformers are generally wound with solid core wire .Solid core magnet wire would be far more reliable and would also require less over all volume on the bobbin.Another thing to remember is the current capability of the secondary is proportional to the current capability on the primary . So the primary wire must be able to handle the proportional current as well.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 19, 2020, 07:51:09 pm
content: Yes I knew a bit of what you've mentioned, taking winding resistance with a meter. It was definitely higher when hot than cold. This application is for a MOT transformer, microwave oven transformer. I know there considered poor in terms of efficiency, It's just to see if I can get a psu in a finished unit that runs on a MOT. I've come to realise a single transformer won't do this, the primary winding is just to fewer turns. And I have built one unit a long time ago now, the the temperature was just ridiculous at 95C and that's with a fan. I happened upon a second transformer, rewound the secondary windings with 12 AWG. I'm not getting quite enough voltage for the current I was drawing from the series wired transformers. I managed 13.85 Volts at 16 Amps, this voltage unloaded was the high side of 18.00 Volts with no load. So the voltage drop was considerably high, the output AC secondary was 14.00 Volts from the pair of transformers. What I'm trying to achieve is using a LM2940-12 to get a 12 Volt power supply. The above voltages where after going through a 50 Amp full wave bridge rectifier, and 90000uf of filter capacitance. The transformers temperature with in the core (thermocouple slid down the side of the primary winding) reached around 51C with 4 X halogen lamps, running for 90 minutes. A bit of forced air cooling might bring the temperature down, and give back some of that voltage drop. I'm not sure if that's realistic, I can see these MOT transformers are poor in terms of performance, but have you ever gone so far with something, you just feel compelled to finish it ? As daft as that sound, that's it really. Another thing I noted was when a single transformer was drawing heavy current, the primary winding was drawing less current than when there was no load attached to it. Subsequently the temperature shut up very fast when there was no load on a single transformers secondary winding. Two transformers in series and temperature is much better, and that's with no added cooling. I would add cooling to a finished PSU. The unloaded transformer temperature reached 100C the wire I am using is silicone insulated, I added a single additional layer of heat shrink tube. I know it's maximum temperature rating is 125C, but the silicone cables insulation looked thin in some places looking at the cross section of the wire. I wanted to be certain it's insulation is as good as possible. Two transformers in series, no load on the secondaries, the primary current measurement was 1.25 Amps at 240 Volts AC. I'm hopeful I can come up with something, I've ordered some 14 gauge wire, just to see if I can push up the number of secondary turns count. I know it means a come down in current, but I can still use the finished power supply, but the application would mean I'm running less power from my application. It's intended use is driving HF linear amplifiers, so that's the need to keep the current reasonably high. 20 Amps plus would be great, but the MOT transformers core just doesn't accommodate the larger turns gauge wire.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 21, 2020, 06:10:30 am
content: Quote from: Jwillis on September 19, 2020, 01:57:32 pmI just want to add that the flux density in a core depends mostly on a voltage (primary voltage) but not on a current (with a reasonable secondary currents range).so transformer current capability - really depends on a wire gauge and ability to emit heat.

author: mzzj
author_rank: Super Contributor
timestamp: September 21, 2020, 07:12:11 am
content: You can't use open-air current rating for a wire that is packed to huge bundle(transformer coil) with little change of cooling.OLD rule of thumb is 3A/mm2 for transformer windings. 12AWG(4mm2) is good for 12AmpsHere is one table for transformer wiringhttps://www.electrical4u.net/transformer/swg-to-current-amperage-chart-transformer-wire-amperage-table/Note the table is in SWG and its not same as AWGWire gauges and kilo-circular-mills FFS

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 21, 2020, 03:00:33 pm
content: Thanks for your replys, that makes sense to me. It looks like the numbers are lower than I expected, 12AWG coming in at not much over 12 Amps, and 14 AWG in at around 8.5 Amps. Looking at the 12AWG I'm surprised at that, the 12AWG seems to handle around 16 Amps easily, no excessive heat generated. But it would be safer to stick to the numbers for the wire diameter mm2 etc. 12 Amps will give 100 watts of HF RF power from a linear amplifier easily. I was hoping for 200 watts to 250 watts really, but the transformer core just can not accommodate that many turns of a larger AWG wire gauge. Thank you for the data, and link.

author: ResistanCecalculator
author_rank: Newbie
timestamp: November 02, 2021, 05:53:14 pm
content: Hi!In simple words I can add that;If wire is thick i.e diameter of wire is more, the resistance will be less and vice versa.Resistance of wire I use to calculate from below link;https://www.resistancecalculator.com/2020/11/wire-resistance-calculator.html.