240.21(B)(5)(2) - 2011 NEC

[charlie b]

In a medium voltage application, a set of 500 MCM conductors that are protected at 430 amps will be tapped using 4/0 conductors. The tap conductors will be run in and underground ductback, entirely outside of any building. The tap terminates on an S&C enclosure that has both a disconnect switch and a fuse. The termination of the tap is on the disconnect, so that the fuse is downstream of the disconnect switch. Does this violate the tap rule?

 

[Dennis Alwon]

I don't think 240.21 pertains to over 600V in 2011. What voltage is it and what code cycle. I am guessing if it is the 2014 and your voltage is 1000v or less then it would apply. I see that charlie has the 2011 nec cycle in his title

 

[rookie4now]

In a medium voltage application, a set of 500 MCM conductors that are protected at 430 amps will be tapped using 4/0 conductors. The tap conductors will be run in and underground ductback, entirely outside of any building. The tap terminates on an S&C enclosure that has both a disconnect switch and a fuse. The termination of the tap is on the disconnect, so that the fuse is downstream of the disconnect switch. Does this violate the tap rule?

What is the distance? I don't remember the exact wording but tap rules may specify RMC instead of duct.

 

[Dennis Alwon]

Would this be the requirement you need. It does seem that taps are not allowed over 600v as they are with 600v and under.

240.101 Additional Requirements for Feeders.
(A) Rating or Setting of Overcurrent Protective Devices.
The continuous ampere rating of a fuse shall not
exceed three times the ampacity of the conductors. The
long-time trip element setting of a breaker or the minimum
trip setting of an electronically actuated fuse shall not exceed
six times the ampacity of the conductor. For fire
pumps, conductors shall be permitted to be protected for
overcurrent in accordance with 695.4(B)(2).
(B) Feeder Taps. Conductors tapped to a feeder shall be
permitted to be protected by the feeder overcurrent device
where that overcurrent device also protects the tap conductor.

 

[charlie b]

The applicable code cycle is 2011. The system voltage is 12,470. The distance is 90 feet, but I am contemplating the article that deals with outside taps of unlimited length.

I don't think 240.21 distinguishes any voltage rating from any other (i.e., it would apply equally at 480 volts and at 12,470 volts). My essential question is whether a single (UL Listed) enclosure that contains an overcurrent device satisfies the requirement that the tap terminate at an overcurrent device, even if, internal to the enclosure, the fuse is downstream of the disconnect switch. Can I treat this as a single component, so that I don't have to care how things are put together inside it?

 

[charlie b]

OK, I now see that I have to withdraw this statement:

I don't think 240.21 distinguishes any voltage rating from any other (i.e., it would apply equally at 480 volts and at 12,470 volts).

240.21 does not make the distinction, but 240 itself does. All of parts II through VII are applicable only to 600 volts or lower.

Back to the books for me. :happyyes:

 

[RB1]

Charlie,

I think your OK. 240.101(B) permits feeder taps to be protected by the feeder protective device. The protection for the feeder is permitted to be anywhere from three to six times the ampacity of the conductor depending on the type of overcurrent protection used. The 4/0 is adequately protected by the 430 amp device.

 

[charlie b]

Thanks for the help. We decided to call it good on the basis that, under engineering supervision, we elected to put the overcurrent protection within a fused disconnect switch at the termination of the tap conductors. This is allowed under 240.100(A). Also, per 240.100(C), we judge that a 125 amp fuse (i.e., one that is rated less than half the ampacity of the tap conductors) would open well before a short circuit event could cause damage to the types of conductors used in medium voltage applications. We gave the client an offer to document that judgement via a formal calculation, if they so desire.