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So the question is as follows: 

"Your mobile HF transceiver draws 22 amperes on transmit. The 
manufacturer suggester limiting voltage drop to 0.5 volt and the 
vehicle battery is 3 metres(10 feet) away. Given the losses below at 
that current, which minimum wire gauge must you use?"

and the options are as follows:

A) Number 8, 0.05 V per metre (0.01 V per foot)
B) Number 12, 0.11 V per metre (0.03 V per foot)
C) Number 10, 0.07 V per metre (0.02 V per foot)
D) Number 14, 0.19 V per metre (0.06 V per foot)

I believe that the answer is option B as it is the highest voltage drop that is below .5 volts for 10 feet of cable. But the answer key states that answer C is correct.

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    \$\begingroup\$ I take it, if anyone can answer, that the stated drops in the possible answers are per metre for the given load and not just per amp per meter as they would usually be stated in reference guides? - I haven't gone and looked for my guide in the library to check. \$\endgroup\$
    – Willtech
    Commented Sep 13, 2018 at 11:10
  • \$\begingroup\$ What exam is that? You should tell the professor that voltage drop depends on amperage. The way this is stated gives the impression of voltage drop being independant from current. Especially when you throw a trick like "distance back and forth" while dropping the important part "voltage drop is linear on current" \$\endgroup\$
    – Jeffrey
    Commented Sep 13, 2018 at 17:20
  • \$\begingroup\$ @Jeffrey the question states that voltage drop is related to current "Given the losses below at that current", also the exam is for an amature radio license in my country \$\endgroup\$
    – Mohammad Ali
    Commented Sep 13, 2018 at 23:17

1 Answer 1

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There's 6 metres of wire. 3 metres to the radio and 3 metres back to the battery.

0.11V/m * 6m is 0.66V which is more than 0.5V.

This is one of those questions that's easy to get wrong if you haven't seen it before.

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    \$\begingroup\$ And in real life situations, be careful of the difference between voltage drop per meter in a wire and a cable (several wires, for example two wires in a power cable). \$\endgroup\$
    – alephzero
    Commented Sep 13, 2018 at 10:56
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    \$\begingroup\$ +1. Dave Jones would call this a "trap for young players". :) \$\endgroup\$
    – user98663
    Commented Sep 13, 2018 at 12:55
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    \$\begingroup\$ The actual test question aside, 12 gauge is pretty skinny for 22 amps, and you'd get a fair amount of heating. For household wiring, you would only ever put 20 amps or less on 12 gauge. \$\endgroup\$
    – Tristan
    Commented Sep 13, 2018 at 16:08
  • \$\begingroup\$ If it was continuous current then I'd agree with you but radio transmitters for Hams don't run at 100% duty. You only talk in short bursts therefore the same heating concerns don't apply as much. \$\endgroup\$
    – Chef Flambe
    Commented Sep 22, 2018 at 5:51

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