Wire gauge and resistance rating for small projects, a couple of questions - Page 1
Forum section: electronics / beginners

author: Decoman
author_rank: Regular Contributor
timestamp: December 05, 2017, 07:44:56 am
content: I want to order some magnet wire (they are very thinly insulated which afaik warrant caution) for a 12V/9V hobby project to be connected to my electricity grid (230V), but seeing at Wikipedia that AWG 30 is apparently rated as "338.6 milli ohm /m" and the AWG 24 as "84.22 milli ohm/m", I can't help but wonder if there is some obvious preferred choice here between AWG 30 and AWG 24.So I am wondering, is there a preferred choice in relation to the rated resistance between AWG 30 and AWG 24? Asthis isn't something obvious to me other than having less resistance in a component for electronics sounds good as a general trait. Also assuming here that we are happy with the rated "fuse current". (seen in the table linked below)https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizesI am also unsure how to interpret the "fuse current" data, which has three columns for ~10s, 1s and 32ms. Seems to me that this indicates how many seconds the wire with different wire gauges will remain intact before burning up. Am I correct in assuming that "~10s" is to mean an indefinitely long time?

author: paulca
author_rank: Super Contributor
timestamp: December 05, 2017, 08:04:42 am
content: Quote from: Decoman on December 05, 2017, 07:44:56 amBeginner to beginner.The milliOhm per meter multiplied by the number of meters will give you a resistance value for the length of wire you are using. Ohm's law will then give you the voltage drop across that, for a given current. Lets say you want to run 1Amp.Example:338.6 milli ohm /m * 5 meters = 1.7 Ohm.V=IRV=1.7Ohm * 1 AmpV=1.7V dropNow we have the voltage drop and the current we can work out the amount of power lost in the wire as heat.1.7V * 1A = 1.7WNot an awful lot of heat in a 5 meter bit of wire, but 1.7V is a significant lose that could cause your circuit to fail.Obviously if you are using shorted runs there will be less voltage drop and less heat.Be careful with wire gauge tables and charts on line, they are often "Chassis", "Automotive" or "Main standard" ratings. If you have a wire running 20 meters through a house that is meant to handle 30A and last for 50 years without getting hot it will have to be a much more sizable conductor than a 10cm lead across a desk in a hobby project which is only used for short periods under supervision. In RC Model flying we would typically drop a couple of wire sizes as we can accept a bit of heat at burst currents and the models are usually only running for 10-20mins at time.(Running 100A through 12 or 14 gauge wire would have an electrical installation engineer in tears of fear, but it's fine for short bursts over a 6 inch cable).The fuse rating with time is, I believe, as you suggest, the amount of time the wire will sustain before it burns through."~" normally means approximately. So ~10s means approximately or "circa" 10 seconds.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 05, 2017, 08:10:56 am
content: Quote from: Decoman on December 05, 2017, 07:44:56 amOne reason #24 may be preferred is that #30 is delicate while #24 is not.Quote from: Decoman on December 05, 2017, 07:44:56 amI honestly hope not!

author: Decoman
author_rank: Regular Contributor
timestamp: December 05, 2017, 08:13:13 am
content: Quote from: Paul Moir on December 05, 2017, 08:10:56 amEh, I guess I meant to write, plug a 9V or 12V power supply into the 230V electricity grid in my home.Quote from: paulca on December 05, 2017, 08:04:42 amYikes, what was I thinking? :ORegarding that table at Wikipedia I linked in my first post above,I am left with the impression that the rating table doesn't hint at what gauge size one ought to be usingand I was hoping it would, if assuming that the wiring ought to handle 10A in some catastrophic event (or, if you somehow had a need to run something at 10A persistently.

author: paulca
author_rank: Super Contributor
timestamp: December 05, 2017, 09:32:50 am
content: Quote from: Decoman on December 05, 2017, 08:13:13 amThis will give you a guide:https://www.powerstream.com/Wire_Size.htmHowever this is for installation wiring. For short <= 1 meter runs drop a size. For temporary supervised hobby projects drop another size. Dropping another size would be getting marginal and the wire might get toasty.Example: You mentioned 10Amps. The table would suggest #20 wire. If you are using 1 meter, drop that to #21. If the project is a temporary hobby project drop another size, to #22. You could possibly consider #24 gauge, but at 10 Amps it might start to get a bit hot.1 meter of #24 gauge wire has an ohmage of 84 / 1000 = 84 milli ohms. 0.084 * 10A = 0.8 volts drop and 8 watts of heat. So the wire will get fairly warm.If you wanted to enclose this in a project box then I would not drop the second gauge. So #21 (or #22 at a push). #20 wire is cheap and easy enough to work with so why not go there.