Does anyone know the emgineering/technical reasons for or historical evolution of the NEC 10ft, 25ft, and unlimited outdoor tap rule as applied to branch circuits, and the tranformer tap rule for 10ft commercial and 25ft industrial. i.e why the specific differences in tap conductor length?
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NEC tap rule 1
- Thread starter LenB
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The shorter the cable, the lower the liklihood of a fault on that cable. Similarly, with underground installations, the likelihood of cable damage is low, and the liklihood of cable damage causing building fire damage is also low.
Also, there's practical considerations. For example, on the secondary of a transformer, you need to have some conductor length between the transformer and the downstream overcurrent protection, but the OC protection should be relatively close. Similarly, if you have an overhead line and you want to tap power off it, the base of the pole is a reasonable location to provide OC protection rather than at the top of the pole.
The tap rules address all of these issues.
Also, there's practical considerations. For example, on the secondary of a transformer, you need to have some conductor length between the transformer and the downstream overcurrent protection, but the OC protection should be relatively close. Similarly, if you have an overhead line and you want to tap power off it, the base of the pole is a reasonable location to provide OC protection rather than at the top of the pole.
The tap rules address all of these issues.
LenB, Most of the tap rules have been in the NEC since I started in this business in 1966. So you would have to go back beyond that to get the Reports and Code Committee members lists. Then you might be able to determine when and why these rules are in the Code. As peebee points out, in most cases, it just makes very practical sense to locate the OC protection as close as possible to the transformer or tap. If I remember corrcetly, only in recent years did the Code change the tap rules so a transformer, for instance, could be mounted high on the wall of an industrial facility.
Historical note: Theres a copy of the 1899 (yes, 104 year old 1st edition) NEC available on mikeholt.com in pdf format. There's no mention of any tap rule in there that I could find. The requirement was generally that a fuse or circuit breaker "must be placed at every point where a change is made in the size of wire". There's no requirement I could find for transformer secondary overcurrent protection.
Obviously, though, if you are changing from a really big wire to a really small wire (say, from 500MCM to #14, perhaps to power a pole-mounted light from overhead conductors), you'd need some kind of tap; you couldn't get a 500MCM to fit into a 15-amp overcurrent device. But there would also be a motivation to limit the length of such taps. Such practical considerations drove the adoption of the tap rule.
Obviously, though, if you are changing from a really big wire to a really small wire (say, from 500MCM to #14, perhaps to power a pole-mounted light from overhead conductors), you'd need some kind of tap; you couldn't get a 500MCM to fit into a 15-amp overcurrent device. But there would also be a motivation to limit the length of such taps. Such practical considerations drove the adoption of the tap rule.
beghtesady
Electrical
I believe the 10 feet tap rule comes from the fact that the manufacturers in the past were only capable to extrude the present standard 10-foot length of rigid conduits.
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