How much current can a short run of 26AWG cable support? - Page 1
Forum section: electronics / beginners

author: Marmotta
author_rank: Contributor
timestamp: July 06, 2019, 12:38:55 pm
content: I'm trying to figure out what sort of gauge wire I need to use for power delivery in my project. I'd ideally like to use 26AWG so I can use a relative small JST connector, but I don't know if it can safely handle the current. The board I'm powering recommends a 5V 3A power supply, but the most I've managed to record it drawing is using a cheap digital multimeter is 0.85A.JST seem to have a flat recommendation of 1A for 26AWG and other online calculators have just confused me. My run of cable would be about 20cm long. Any help would be great.https://www.arcade-museum.com/manuals-videogames/N/NeoGeo%201%20Slot%20MV1C%20Users%20English.pdfhttps://www.powerstream.com/Wire_Size.htmhttp://www.jst.fr/doc/jst/pdf/current_rating.pdf

author: ArthurDent
author_rank: Super Contributor
timestamp: July 06, 2019, 02:41:58 pm
content: The concern you should have is what is going to be the voltage drop with your cable and connectors. If you need 5 volts at the device, 5 volts into the cable may be real iffy. For 20cm of cable you have 2 wires so you actually have 40cm of #26 AWG wire. Check the resistance of the wire per meter and calculate the resistance total then times the current will be the voltage drop. If this lower voltage at the end of the cable is acceptable to your circuit, go for it.The current handling capacity of wire is rather subjective and depends on open air, in a bundle of other wires, etc., so these values will vary wildly. I'd conservatively use the lowest current value listed as the limit then calculate the maximum drop for the total length of the wire I want to use in the application. I've had precision voltage sources where millivolt drops are unacceptable but just connecting an LED at the end of a long cable wouldn't be noticed.

author: rstofer
author_rank: Super Contributor
timestamp: July 06, 2019, 02:55:09 pm
content: The 3A thing reminds me of a Raspberry Pi requirement for the 3B+. The thing about the Pi is that they are pretty serious about the ampacity and attempts to use smaller wall warts have been met with failure. Some people get lucky and their system works with a lesser supply for a while.The only way I would undersize the supply is if I knew for a fact that under any operating condition, the current was less than specified. I'm not sure how I would determine that. A DMM averages things so I would have to come up with a way to capture the peak current.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 06, 2019, 03:04:40 pm
content: https://www.powerstream.com/Wire_Size.htm

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2019, 03:04:47 pm
content: According to Wikipedia, the resistance of #26 is 133.9mΩ/m. You have s total run of 40cm or 0.4m, so that's a resistance o 0.4*0.1334 = 0.05336. If the current is 0.85A, the voltage drop = V IR = 0.05336*0.85 = 0.045356V, so with 5V at one end, the voltage at the other end will be 5 - 0.045356 = 4.954644V, which will be no problem.https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizesThe only other thing you need to consider is over-current protection. The cable should be protected by a fuse or the power supply's current limit, which should be below the cable's rating. If the power supply can provide a much higher current, than the cable's rating and there's a short circuit, the cable insulation can melt, short circuit and present a fire risk.As mentioned above, the current rating depends on the temperature rating of the insulation, whether the cable is bundled and the ambient temperature. Wikipedia says 1.3A, for #26 assuming the insulation is rated to 60°C, the ambient temperature is 20°C and it's not bundled with other conductors. You'll probably want it to run at higher ambient temperatures than 20°C, but the cable insulation will also be rated to over 60°C. According to the article below, PVC cable insulation (the most common type) is commonly ratted to 105°C. The next column in the Wikipedia table says 2.2A, with 70°C rated insulation, so it's probably safe to protect the circuit with a 2A or even 3A fuse, as cable insulation can probably take that current without damage.https://www.performancewire.com/insulated-wire-protection/

author: Marmotta
author_rank: Contributor
timestamp: July 06, 2019, 03:39:08 pm
content: Thanks for the advice. The power supply I'm using is rated for 5V 4A, but it does have short circuit protection and over-current protection.https://www.mouser.co.uk/datasheet/2/670/sdi24-ud-1500492.pdfLooking at everything, it seems that in theory it should be safe enough to use 26AWG. I think I'll just run it for a while and see if there are any repercussions.

author: janoc
author_rank: Super Contributor
timestamp: July 06, 2019, 04:07:55 pm
content: The JST table is conservative but then you need to also take into account contact resistance of any connectors - you don't want the connector melting off the board neither. You can always use a 4 pin instead of 2 pin JST and 2 wires in parallel, doubling your current carrying capacity. 26 gauge wire is a bit too thin for a 3A circuit.The issue mentioned with RPi and similar boards is that on average the current draw is low - but as soon as the processor is loaded or extra HW is connected to the USB ports the current draw goes up. That's why these have so high current requirements - they may work with less but the rating has to reflect the worst case situation (e.g. the 4 USB ports on RPi 3 -> 4x500mA that's theoretically 2A current just for those peripherals).Otherwise you are going to have random crashes and problems if the current isn't available to cover a sudden current spike or the wire has so much resistance that the voltage drops below the safe limits. Multimeter is useless to measure something like that - most meters don't have means to catch those short current peaks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2019, 11:29:01 pm
content: Yeah, for how long?For continuous use, temp rise and voltage drop matter. Obviously there is a critical point where the insulation melts or the wire fuses.Also a caution about cheap connectors, if you're getting premade cables that are suspiciously cheap they're probably copper plated aluminum or steel. Double or triple the voltage drop and temp rise.If you have the choice of selecting connector and pinout, you can always use more wires (and pins) in parallel.For ordinary 75°C PVC insulation and 26AWG solid/stranded copper, I would think an ampere is pushing it. Those little JST connectors don't have much ampacity either. If you're doing like three amperes, I would use at least as many contacts in parallel (so, a 6 or preferably 10 pin connector, just for power).Tim

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2019, 01:29:54 am
content: Quote from: Marmotta on July 06, 2019, 12:38:55 pmRemember that what you can measure with a simple multimeter is some kind of average current. However, digital circuits may draw short pulses of current far higher than the average. If the wire is too long or too thin (in other words, if you don't have a "stiff" power supply) then the voltage drops caused by the pulses may cause the circuit to malfunction. A good rule of thumb is to have the power supply close to the load, and to it never hurts to have thicker wires for power delivery.

author: DBecker
author_rank: Frequent Contributor
timestamp: July 07, 2019, 02:25:53 am
content: The ampacity of the wire is rarely the limiting factory. The voltage drop may be, but it's usually the connector.The connector often has higher contact resistance, and higher thermal resistance. It usually relies on the conductors (e.g. circuit board and wire) to cool the contact. Complicating this, the circuit board sometimes relies on the attached wires for cooling. So using the connector anywhere close to its rating, especially adjacent contacts, requires at least superficial analysis and perhaps substantial de-rating over an optimistic spec sheet.For small connectors I doubt that the suppliers are economizing by using aluminum or steel. But they might not be using optimum alloys balancing conductivity, spring force and corrosion resistance. Expect tin plated copper, rather than e.g. nickel or gold over beryllium copper

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2019, 09:50:17 pm
content: Quote from: Marmotta on July 06, 2019, 03:39:08 pmNo, not that safe. The problem is, if there's a short circuit in the device at the end of the cable, the cable will go up in smoke. You need to put a fuse, preferably at the start of the cable, but at least before the device, so it blows, before the cable gets too hot.

author: L_Euler
author_rank: Regular Contributor
timestamp: July 07, 2019, 09:56:42 pm
content: The JST connectors can be made up with 24AWG wire--I do it all the time--and you can also use parallel wires.

author: janoc
author_rank: Super Contributor
timestamp: July 08, 2019, 09:18:43 am
content: Quote from: L_Euler on July 07, 2019, 09:56:42 pmThat the wire can fit there doesn't mean that this is safe. If the connector is rated for 1A then trying to push 3A through it will not end well, regardless of the wire you use. The common JST PH/XH series aren't designed for high currents.Better choice is something like Molex KK 396 / KK .156 (see:http://tech.mattmillman.com/info/crimpconnectors/#kk156)

author: 6PTsocket
author_rank: Regular Contributor
timestamp: July 08, 2019, 11:59:44 am
content: Quote from: Zero999 on July 06, 2019, 03:04:47 pmIf we are measuring small drops, fuses are not resistance free either. Check a few out.Sent from my SM-G900V using Tapatalk

author: L_Euler
author_rank: Regular Contributor
timestamp: July 08, 2019, 01:56:46 pm
content: Quote from: janoc on July 08, 2019, 09:18:43 amThat's why I also said use parallel wires (ie. multiple pins for the same power source.) And just to see how it would end, I'm going to set up a JST with 26AWG pushing 3A followed by dual 24 AWG. Results to follow.So I ran the test this evening, but only on 26 AWG. After testing with 26, the expected results for 24 were obvious.Completely unscientific, non peer reviewed test:Wire - 20 CM, 26 AWG solid, properly crimped to male and female JST pins and inserted in single carriersCurrent - 3 Amps AC sourced by variacBallast Resistor - 0.3 Ohm, 10 WTemperature of female connector at start - 25.6 CTest Time - 15 minTemperature of female connector at Stop - 35.5 C (However, I attribute the temp rise to heat conduction through the resistor lead. The resistor got pretty hot. There was no detectable temp rise at the male end.)Wire Voltage Drop - 138 mVACCalculated Wire Resistance - 45.9 mOhmConclusion: 3A on a JST pin with 26 AWG wire is completely doable. Most standards are incredibly conservative and can generally be safely bent if you understand what you're doing.

author: Zero999
author_rank: Super Contributor
timestamp: July 08, 2019, 02:14:38 pm
content: Quote from: 6PTsocket on July 08, 2019, 11:59:44 amI just have. A simple 2A, glass cartridge fuse has a resistance of just under 35mΩ and a time delay fuse is just over double that, which is negligible, if the nominal current is 0.85A. It's better to be safe, than sorry.https://uk.rs-online.com/web/p/cartridge-fuses/0563722A/https://uk.rs-online.com/web/p/cartridge-fuses/6686035/

author: janoc
author_rank: Super Contributor
timestamp: July 08, 2019, 07:16:54 pm
content: Quote from: L_Euler on July 08, 2019, 01:56:46 pmAh ok. I have misunderstood that bit.Yes, that is possible to do (and has been mentioned above already). However, if that connector is only for power, then it would make more sense to use a larger connector in the first place and avoid this hassle (and possibly save space as well).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 09, 2019, 03:36:15 am
content: Mind, for freshly mated connectors. Get some vibration and oxidation on that, some dust or what have you, and that rating may come down a lot.Tim