Tap conductor sizing

[Mr.electric]

Hello,
just going over one of mikes dvd videos on sizing motors taps. It’s a 10’ tap rule feeding a motor and he was saying you size it 1/10th the size of the feeder ampacity. Not sure if it just got missed between all the panel member but it looks like 240.21 (b) (1) (4). Says not less than 1/10th the rating of the device protecting the feeder, not the total feeder ampacity.

Also wondering, when sizing motor branch circuit overcurrent protection if the motor type and breaker type are not specifically listed is your default a 250% multiplier.

Thanks.

 

[Carultch]

Hello,
just going over one of mikes dvd videos on sizing motors taps. It’s a 10’ tap rule feeding a motor and he was saying you size it 1/10th the size of the feeder ampacity. Not sure if it just got missed between all the panel member but it looks like 240.21 (b) (1) (4). Says not less than 1/10th the rating of the device protecting the feeder, not the total feeder ampacity.

Also wondering, when sizing motor branch circuit overcurrent protection if the motor type and breaker type are not specifically listed is your default a 250% multiplier.

Thanks.

See this sketch:


Only the red section of conductor, classifies as a tap. The black section is *the feeder* that 240.21(B) references. The blue section, load-side of Breaker B, is also a section of conductor that could classify as a feeder, but isn't what is meant by "the feeder" in 240.21(B), that governs the tap conductors. The underlying idea is to give you a length to work with to get to the first OCPD after the tap, but keep that length within reasonable limits to reduce the risk of faults.

Suppose A and B are the trip ratings of the breakers. The "one tenth" part of the tap rule, means 0.1*A. It is one third for the 25 ft tap rule, which would mean A/3 would take the place of 0.1*A for taps between 10 ft and 25 ft. The ampacity of the red section, needs to either meet or exceed B, or meet or exceed the value of 0.1*A, whichever is larger. Once no longer classifying as a tap (blue conductor), standard feeder sizing rules apply, and tap rules no longer govern its sizing.

 

[augie47]

I see what you are getting at. The wording could be improved.
The "feeder ampacity" could,be greater than the size of the OCP device protecting the feeder. The wording might be improved by using the term 1/10 of the size of the feeder overcurrent protection.

 

[infinity]

Where is the NEC is feeder ampacity defined? One would think that it is the size of the OCPD.

 

[augie47]

Where is the NEC is feeder ampacity defined? One would think that it is the size of the OCPD.

It's certainly no big deal but I think therein lays the problem... there is no "feeder ampacity" definition and the ampacity definition references the "conductor"

 

[infinity]

So the issue was Mike using the words "feeder ampacity" which is undefined? The 10' tap rule is clear that the 1/10 relates to the size of the OCPD ahead of the feeder.

 

[augie47]

So the issue was Mike using the words "feeder ampacity" which is undefined? The 10' tap rule is clear that the 1/10 relates to the size of the OCPD ahead of the feeder.

Agree. It's pretty clear in the actual NEC but I gave the OP credit for noting the discrepancy in the video.

 

[wwhitney]

Article 430 has slightly different versions of the tap rules for motors. Feeder taps are covered in 430.28 and branch circuit taps in 430.53(D). And those sections do sometimes refer to the ampacity of the conductors rather than the rating of the OCPD. Although at a quick glance, that's only for the 25' version of the rules, and the 10' version does refer to the rating of the OCPD.

Cheers, Wayne

 

[Carultch]

Article 430 has slightly different versions of the tap rules for motors. Feeder taps are covered in 430.28 and branch circuit taps in 430.53(D). And those sections do sometimes refer to the ampacity of the conductors rather than the rating of the OCPD. Although at a quick glance, that's only for the 25' version of the rules, and the 10' version does refer to the rating of the OCPD.

Cheers, Wayne

So would this mean, that if a 300A feeder is built with 600 kcmil Cu conductors, rather than the 350 kcmil Cu it'd be built with by default, that you'd need 140A worth of tap conductor ampacity for the 25 ft tap rule, rather than 100A? This sounds like it is likely an oversight in the wording, rather than the intent.

 

[wwhitney]

So would this mean, that if a 300A feeder is built with 600 kcmil Cu conductors, rather than the 350 kcmil Cu it'd be built with by default, that you'd need 140A worth of tap conductor ampacity for the 25 ft tap rule, rather than 100A? This sounds like it is likely an oversight in the wording, rather than the intent.

Indeed it sounds like the ampacity referenced should be the minimum ampacity allowed, rather than the actual ampacity of the feeder conductors.

The purpose of referencing ampacity for the 25' tap rule for motor circuits is presumably because typically the SC/GF OCPD used is much larger than the actual conductor ampacity. So the distinction is to provide an extra allowance, not an extra restriction.

Of course, you could sidestep this wording problem when a feeder is upsized by first splicing a short section of conductor of minimum allowed ampacity, and then making the tap to that short spur.

Cheers, Wayne

 

[Mr.electric]

I’m thinking if you size it to the feeder OCPD you may need a larger tap conductor because of next size up rule vs. conductor ampacity that could be quit a bit lower than OCPD. I could potentially be the difference between two different size conductors.

I’m wondering if it was purposely written 10’ taps to be 1/10th feeder OCPD and 25’ taps to be 1/3rd feeder conductor ampacity. This verbiage is written the same in 240 taps and 430 taps.

 

[wwhitney]

This verbiage is written the same in 240 taps and 430 taps.

That's not what I'm reading in the 2020 and 2023 NEC. 240.21(B)(2) (25' rule) refers to 1/3 the rating of the overcurrent device protecting the feeder being tapped, while 430.28 and 430.53(D) refer to 1/3 the ampacity of the conductors for their 25' version of the tap rule.

Cheers, Wayne

 

[Mr.electric]

Yes, you are correct on that. My confusion was if you take 1/10th breaker ampacity or feeder ampacity.