Amperage/Voltage through small wire?

[Falcon69]

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

[c4757p]

Yes, you're fine, in terms of safety.

Check the resistance. 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.

[Falcon69]

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. 

Thanks
Jason

[IanB]

Watch out for voltage drop, though. You'll lose (current)(resistance) = 0.2V

It will be double this if considering a round trip circuit. 50 ft there and back is 100 ft total.

[IanB]

I was just concerned with the wire not being able to transmit that much current/power.

That 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.

[klr5205]

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.

[Falcon69]

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.

[IanB]

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.