Amperage/Voltage through small wire? - Page 1
Forum section: electronics / beginners

author: Falcon69
author_rank: Super Contributor
timestamp: March 12, 2014, 07:48:13 pm
content: I have a project that requires 4-conductor wire.My question is, can 60mA be safely pulled through a 28awg 7-wire stranded wire at 5 volts and through 50ft?Realistically, it would be 45mA continuous with an additional 15mA every once in awhile (whenever a button is pushed).Most of everything I find says yes. 360mA, but it doesn't say for what voltage or length of the cable.The cable I am thinking of using is 28awg 4-conductor with a drain wire and foil shield AND a braided shielding (double shielded).Thanks in advance,Jason

author: c4757p
author_rank: Super Contributor
timestamp: March 12, 2014, 07:58:51 pm
content: Yes, you're fine, in terms of safety.Check theresistance. 28awg is 65 milliohm per foot. At 50 feet, that is 3.3 ohms.Power dissipation is (current^2)(resistance), or 12mW, or 240µW/foot.You'd make the wire warmer by looking at it with an angry expression.Watch out for voltage drop, though. You'll lose (current)(resistance) = 0.2V. I can't tell you whether that's acceptable for your application.

author: Falcon69
author_rank: Super Contributor
timestamp: March 12, 2014, 08:05:01 pm
content: thank you c4757p.voltage drop should be fine. After going through the protection circuit (5 volts drops to 4.6volts), then through cable of 50 feet (drops another .39volts), I estimate about a total of 4.2volts going to the electronics. That should be more than enough to activate all the logic gates and inverters. I'll need to change the resistors for the LED's, but should work fine.I was just concerned with the wire not being able to transmit that much current/power.ThanksJason

author: IanB
author_rank: Super Contributor
timestamp: March 12, 2014, 08:07:20 pm
content: Quote from: c4757p on March 12, 2014, 07:58:51 pmIt will be double this if considering a round trip circuit. 50 ft there and back is 100 ft total.

author: IanB
author_rank: Super Contributor
timestamp: March 12, 2014, 08:09:16 pm
content: Quote from: Falcon69 on March 12, 2014, 08:05:01 pmThat depends entirely on the resistance of the wire. Look up the resistance per foot and you can determine the answer for any wire, any length.

author: klr5205
author_rank: Regular Contributor
timestamp: March 12, 2014, 08:10:44 pm
content: Looks like everybody as preempted my response but I'll post anyway to feel like I contributed.Safety-wise, you should be fine.Also - and this may not be significant for you - keep the resistance of such a length of thin wire in the back of your mind if you're doing anything analog... 28 awg wire has a resistance of 64.9 ohms/1000ft so 50ft of it should have a resistance of 3.2 ohms, meaning you'd expect a drop of about 0.2V at 60 mA.

author: Falcon69
author_rank: Super Contributor
timestamp: March 12, 2014, 08:46:11 pm
content: oh, so I need to calculate a .4volt drop and not the .2volt.Should still be okay to supply the logic gates and inverter. the Hall sensors works all the way down to 3volts.

author: IanB
author_rank: Super Contributor
timestamp: March 12, 2014, 09:38:44 pm
content: Also, keep in mind that the static voltage drop is not all you need to worry about. Any sudden changes in current such as logic gates switching or buttons being pressed will cause simultaneous steps or spikes in the supply voltage. If they are too large these could easily spoil your day. You should pay careful attention to buffering and bypassing of the electronics at the remote end with large enough capacitors in the necessary places to keep voltage spikes under control.