Minimum size of secondary conductor

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KSue

KSue

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Page 138- Understanding NEC V1

Question: What is the minimum size secondary conductor required for a 2-wire 480V to 120 transformer rated 1.5KV?

It gives the following answer:

Primary Current = 1500VA/480V= 3.13A
Primary Protection 3.13A x 1.67= 5.22A = 5A

The final answer comes to 12.5A secondary current, thus 14AWG, rated 20A at 60C.

Where did the 1.67 come from?
 
KSue

KSue

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Thanx!!!

Thanx!!!

I knew one of guys knew... THANX!!! How about one more question..

Figure 240-19 defines feeder taps not over 10'

What does it mean "not less than 1/10th the ampacity of the device protecting it." :-?

Is it referring to the protection devices below the raceway, or the protection device of the feeder in the raceway? I don't know if you have online access to the picture itself, but maybe I said it clear enough here.. let me know.

Okay how about two quetions.

The 10' feeders are not < 1/10th the ampacity of the device protecting it.
The 25' feeders are not < 1/3rd the ampacity of the device protecting it.
The 100' feeders are not < 1/3rd the ampacity of the device protecting it.

What is the sigificance of these fractional values with respect to the circuit?
 
infinity

infinity

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A feeder can have a 10' tap which would require a tap conductor at least 1/10 of the ampacity of the feeder.

So a 200 amp feeder can be tapped up to 10' with a conductor that has an ampacity of at least 20 amps.

The 25' tap rule increases the tap conductors to 25' but now requires a minimum ampacity of the tap conductors to be 1/3 or greater than the ampacity of the feeder.

So a 300 amp feeder can be tapped up to 25' with a conductor that has an ampacity of at least 100 amps.


The 100' rule is for he special situation of high bay manufacturing buildings.
 
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