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705.12(D)(2)(2) and 240.21(B) 10-foot Tap Rule and 25-foot Tap Rule for PV Systems


Q:

I'm not super clear on the 10 ft. and 25ft. tap rules. Can you provide some concrete examples of how the conductor size is calculated for each of those?
 
A:
 
Let’s start first with the 10-foot tap rule and then infer the 20-foot tap rule from what we learned with the 20-foot rule.
 
Imagine a feeder going from a main panel to a sub panel. That feeder has a 200A breaker at the main panel.
 
Imagine that we want to connect a very small 8A inverter to the middle of that feeder.
 
The question is how large does the feeder tap conductor have to be. The feeder tap conductor is the conductor that is, in this case less than 10 feet.
 
The answer is it has to be at least 10% of the sum of the inverter feeder breaker plus 125% of the inverter current.
 
125% of the inverter current is 8A x 1.25 = 10A
 
The feeder breaker is 200A
 
200A + 10A = 210A
 
10% of 210A is 21A
 
We would need a feeder tap conductor for the 8A inverter that is at least 21A.
 
Now for the same numbers, except lets say that the feeder tap conductor between the inverter and the tap point is between 10 and 25-feet.
 
The answer is it has to be at least one-third of the sum of the inverter feeder breaker plus 125% of the inverter current.
 
125% of the inverter current is 8A x 1.25 = 10A
 
The feeder breaker is 200A
 
200A + 10A = 210A
 
One-third of 210A is 70A
 
The reason that the longer conductor has to be beefier is that there is more resistance on a longer conductor and if there is a fault, we want the breaker to open up before the wire heats up and starts a fire. A shorter wire has less resistance and a better chance at popping open the 200A breaker.
 
This is not common sense, because you would think that the conductor should be 200A to pop open the 200A breaker, but with a fault, you can have a better chance at opening a breaker than a mere overcurrent. A fault is hard-core! Like a short circuit!
 
Thanks,
Sean White


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