Tap Rules

It has to do with the locations of the disconnect mechanisms for feeder lines....what part don't you understand?

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/FeederSecondaryTapRules~20020326.htm

Making Sense of the Tap Rules | EC Mag

Understanding the Rules for Feeder Taps | Code Basics content ...

You are getting into the rationale--the code's purposes are protection of property against fire and protection of people from injury. Outside taps are (by the rules in the code that define "outside") very unlikely to do either of these. The wire in your question can easily have a failure, but is very unlikely to cause either a fire or an injury. Thus the very permissive nature of this rule.

--JMM

I realize that you are discussing US practice here, but it may help to describe the rules which have applied in UK since 1981[and rest of EU].

These are "principal" based, rather a practical "rulebook" and the following apply to a tap or tee of a low rated conductor direct onto a higher rated conductor. The following is my own summation of the rules.

The overload protection of the low rated conductor may be placed at its remote end provided this conductor is adequately protected against short-circuit by the short-circuit protection of the higher rated conductor.

This requires a calculation of the actual short circuit current - it seems contrary but [in most cases] the smaller conductor becomes vulnerable to excessive heating if the short-circuit current is not enough to open the fuse or breaker quickly!

I illustrate this by a graph from the Commentary to the 15th Edition of the IEE Wiring Regulations ISBN 0 86341 040 5 below...

I have "blacked" the curves for the 100 amp fuse and the 6 square millimetre conductor. A 6 sq.mm copper conductor would require overload protection by a 40amp fuse or MCB.

Note that currents to the right of the conductor line will overheat it (and that the fuse curve is the maximum not the average rupture current).

So for short circuits over ~900 amps the conductor is protected, but less than that overheats the conductor before the fuse blows.

This means that a short run of conductor is protected, but a longer run with higher resistance does not ensure a high enough short circuit current to blow the fuse quick enough.

I agree with the posts which regard the basis of the size rules described as suspect.

67model