Originally Posted by
John K Jordan
As I mentioned, I'm not an electrician (except in Mexico) but my understanding is the wire gauge needed for any circuit depends on two things - the maximum sustained amperage and the length of the wire. Not accounting for both can be a mistake. For example, a wire correctly sized for a short run may not be enough for a very long run. Do the calculations. For these reasons and more I ran 8 and 6 gauge from 50amp breakers to several places in my shop.
You are right, John. It's called voltage drop - the longer the wire run from the panel, the more resistance the wire creates. Excessive resistance can cause dangerous heat build up. In the trade we typically begin to look at the need to calculate voltage drop when pulling wire if the run exceeds ~100'.
But when looking at loading breakers, the rule of thumb we used (I worked in the Chicago area) was not to exceed 80% of the breaker's designated ampacity. In other words, add up the amperage of all of the loads on a given circuit and make sure it doesn't exceed 80% of the breaker rating. eg: 20A breaker = 16A maximum load.
Another thing a lot of people don't understand is breakers, or fuses, are there to protect the wire connected to them, not the load. The breaker is designed to trip before the wire heats up and fries, sometimes resulting in a fire.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain