Why are feeder taps allowed in the first place?

[awoody19]

Thinking about feeder taps, can someone check my logic below regarding the reason why feeder taps are allowed at all? And does anyone know the origin of the specific 10' and 25' lengths as specified in the code?

I'm thinking taps are allowed given that the tap wire just can't have so much resistance (whether by length or size) that it would limit the effective ground fault path during a ground fault/short circuit event to a point that the main feeder (larger) OCP would not unlatch. Am I correct in this thinking?
Thanks,
Alex

 

[petersonra]

Thinking about feeder taps, can someone check my logic below regarding the reason why feeder taps are allowed at all? And does anyone know the origin of the specific 10' and 25' lengths as specified in the code?

My guess is someone made a decision that taps should be limited in length under certain conditions and those were the numbers someone decided on. I doubt there was any real thought beyond that. If 10 and 25 feet tap lengths are the only safe lengths, why are you allowed to have unlimited lengths if the tap conductors are outside?

I'm thinking taps are allowed given that the tap wire just can't have so much resistance (whether by length or size) that it would limit the effective ground fault path during a ground fault/short circuit event to a point that the main feeder (larger) OCP would not unlatch. Am I correct in this thinking?
Thanks,
Alex

I think as much as anything it is a matter of practicality. How else do you get from a 100 A CB to a 30 A CB? You would need some kind of special product or a rule that you couldn't tap at all.

 

[jap]

Taps are a necessity.
Without Feeder Taps we couldn't even put smaller branch breakers in a panel board with a larger OCPD.

JAP>

 

[awoody19]

My guess is someone made a decision that taps should be limited in length under certain conditions and those were the numbers someone decided on. I doubt there was any real thought beyond that. If 10 and 25 feet tap lengths are the only safe lengths, why are you allowed to have unlimited lengths if the tap conductors are outside?



I think as much as anything it is a matter of practicality. How else do you get from a 100 A CB to a 30 A CB? You would need some kind of special product or a rule that you couldn't tap at all.

Good point, didn't think about the unlimited length on outside taps. I've got to imagine there's some amount of math and reason for the lengths and percentage rules though, maybe not.. maybe outside is unlimited because it doesn't matter as much if wire burns up outside.

 

[awoody19]

Taps are a necessity.
Without Feeder Taps we couldn't even put smaller branch breakers in a panel board with a larger OCPD.

JAP>

I've never thought about it that way but I agree with that thinking. The buswork in a panelboard is just a feeder getting tapped by all the branch circuit breakers. Still, why 10' and 25' is a mystery I think about from time to time

 

[petersonra]

I've never thought about it that way but I agree with that thinking. The buswork in a panelboard is just a feeder getting tapped by all the branch circuit breakers. Still, why 10' and 25' is a mystery I think about from time to time

The breaker is connected direct to the bus bar inside a listed piece of equipment. It is not subject to general NEC requirements.

 

[LarryFine]

My guess is someone made a decision that taps should be limited in length under certain conditions and those were the numbers someone decided on. I doubt there was any real thought beyond that. If 10 and 25 feet tap lengths are the only safe lengths, why are you allowed to have unlimited lengths if the tap conductors are outside?

I think as much as anything it is a matter of practicality. How else do you get from a 100 A CB to a 30 A CB? You would need some kind of special product or a rule that you couldn't tap at all.

Remember that, especially in the early days of this unfamiliar technology, wiring was more concerned with protecting the environment from the wiring than protecting the wiring from the environment.

 

[xformer]

My guess is someone made a decision that taps should be limited in length under certain conditions and those were the numbers someone decided on. I doubt there was any real thought beyond that. If 10 and 25 feet tap lengths are the only safe lengths, why are you allowed to have unlimited lengths if the tap conductors are outside?



I think as much as anything it is a matter of practicality. How else do you get from a 100 A CB to a 30 A CB? You would need some kind of special product or a rule that you couldn't tap at all.

Your allowed to have Taps of unlimited lengths outside, due to the efforts of the NEC to keep the "Fire in the Wire" outside of the building or structure.

 

[jap]

I've never thought about it that way but I agree with that thinking. The buswork in a panelboard is just a feeder getting tapped by all the branch circuit breakers. Still, why 10' and 25' is a mystery I think about from time to time

Not a mystery, and tap rules do not allow conductors to burn up even if they are outside.

Each tap rule specifies a distance and a specific size of conductor.

Even tap conductors deserve to be protected.

Jap>

 

[petersonra]

Not so.

Outside taps require are required to have to limit the load to the ampacity of the conductors.

JAP>

Only protects against overloads, but not shorts.

however, the requirement to have an effective ground fault protection system would seem to cover it.

 

[jaggedben]

Taps are a necessity.
Without Feeder Taps we couldn't even put smaller branch breakers in a panel board with a larger OCPD.

JAP>

Um, no, that's not the same thing, because I don't think it's allowed to protect a panelboard busbar at less than it's ampacity.

They were a necessity though. I believe the tap rules go back to before panelboards became common.

 

[jap]

Um, no, that's not the same thing, because I don't think it's allowed to protect a panelboard busbar at less than it's amapcity.

What???

jap>

 

[jaggedben]

What???

jap>

See 408.36. And the definition of a tap in 240.2.
I think you have gotten turned around somehow.

 

[jap]

I was referring to your indicating that a panelboard's bussbar could not be protected at less than it's ampacity.

Not so.

JAP>

 

[jaggedben]

I was referring to your indicating that a panelboard's bussbar could not be protected at less than it's ampacity.

Not so.

JAP>

Whoops, I meant more than it's ampacity.