Help with some basics?

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nscherneck

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Portland, OR
I recently picked up a study guide for my state exam. It has some basic practice questions on ohms law and voltage drop. It provides the answers but not how to get them. I'm trying to run the calculation and am getting different answers. Help?

Q1. How far from a 12 volt source can you install a #16AWG coated conductor, if the branch circuit load is 0.75 amp?

A1. 47.2 feet



Q2. What size copper conductor is required to a 1.5 amp load, 75 feet from the 24 volt supply?

A2. #14AWG
 
Neither question provides enough information to determine a definitive single answer. IMO they should fire the writer of questions like these.
 
The part they are missing is the allowed voltage drop.

The only way to get any value out of these two questions is to work backwards from the answer.

You should be able to figure out the voltage drop for the wire sizes given. Then calculate the voltage drop for one wire size smaller. Ask yourself why they think one size works and the other doesn't. Is there a local regulation for maximum voltage drop, or are they using the fine print notes from the NEC, witch I believe states that 2% for a feeder and 3% for a branch circuit are recommended.

Also, perhaps you could post what you have tried, and how you arrived at two different answers. Then someone might be able to point out where you went wrong.
 
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