How to calculate the maximum cable length of fire detection loop
Your question comes down to a basic application of Ohm's Law.  It says that the voltage drop across a resistor is the current thru the resistor times the resistance.

You apparently have wire with a resistance of 3.688 m&Omega;/foot.  To get the total actual resistance, you need to know the length of the wire.  Remember that current has to run out one wire and back on another, so the wire length the current sees is twice the distance from your power supply to your device.

Since you didn't give the distance, I'll work it backwards to find the maximum distance your setup can support.

It seems the power supply puts out 24 V, and the device at the other end of the cable can operate down to 17 V.  That means the cable can drop 7 V.  You also say the current is 400 mA.  Using Ohm's law, (7 V)/(400 mA) = 17.5 &Omega;.

Now we can find the wire length that results in that much resistance.

&nbsp;&nbsp;(17.5 &Omega;)/(3.688 m&Omega;/foot) = 4745 ft

Again, it takes two wires, so the distance from the power supply to the device can be at most half that, which is 2373 feet or 723 meters.