Article 240/Tap conductors

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alixenos

alixenos

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Florida
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Electrical Design Engineer
Hello,

Article 240 says that conductors shall be protected at the point where they receive their supply of power, except in 24.21(A) through (H).


24.21(B) feeder taps not more than 10 FT....

"For field installations, if the tap conductors leave the enclosure
or vault in which the tap is made, the ampacity of the
tap conductors is not less than one-tenth of the rating of
the overcurrent device protecting the feeder conductors"

what does that mean? and why this paragraph mentions the one-tenth of the rating of the feeder conductors?

Also, in general, are tap conductors protected against overload where they terminate? and protected against fault current by the OCPD that protects the tapped feeders?

I'm just confused about this one-tenth, to me a tap conductor is a a conductor whose ampacity is exceeded by the rating of the circuit upstream of the tap!!

Thank you
 
If your feeder is 400 amps then 1/10 is 40 amps. That means the tap conductor cannot be smaller than a number 8 assuming no de-rating or other issues that may change the ampacity of #8
 
If you have a short at the device being fed by the tap or with the tap conductor itself, the supply overcurent device needs to operate,
The 1/10 minimum size gives some assurance that will take place.
Smaller than that the tap might become a primacord.
 
alixenos said:
...

what does that mean?
Click to expand...

It means exactly what it says.

and why this paragraph mentions the one-tenth of the rating of the feeder conductors?
Click to expand...

Are you asking why 1/10th for 10ft specifically? It was a pretty arbitrary pair of numbers chosen by a code-making panel decades ago. Don't overthink it.

Also, in general, are tap conductors protected against overload where they terminate? and protected against fault current by the OCPD that protects the tapped feeders?
Click to expand...

In general, *yes*, that's the idea. You'll notice that the rules get more restrictive for longer taps inside buildings. The tap conductors must be larger to go beyond 10ft and are prohibited entirely beyond 25ft in many situations.
 
Thank you, yes I was questioning the particular 1/10 for 10ft.

Another question, If I tap secondary of a transformer, and I don't want to protect the secondary of the transformer (a primary protection only is implemented) Can I run the secondary conductors as long as I was, and terminate them with an OCPDs?

Where exactly in the 240.21 I can look to address the above?
 
240.21(C) addresses transformer secondary conductors.
The requirements are very similar to the 240.21(B) tap rules.
Also, don't overlook 408.36
 
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