Tap Conductors

[lectricboy]

I am looking at a One Line for a clothing store. It shows an 800 amp safety switch feeding a wireway, from which two 400 amp "MLO" panelboards are being fed.
Am I reading 240-21 (b&c) correctly? I see it as meaning overcurrent is needed, and the "MLO"
panelboards being a code violation.
Am I correct?

 

[don_resqcapt19]

Re: Tap Conductors

If the panel and the tap conductors are both rated at 400 amps and the length of the conductors is 10' or less the installation is permitted by 240.21(B)(1). Look at the wording in 240.21(B)(1)(1)(b). The panel is a "device". Note that rounding up to the next standard size is not permitted here and the tap conductors would have to be at least 600kcmil copper.
Don

 

[charlie b]

Re: Tap Conductors

The 800 amp safety switch is not necessarily ?the point where the conductors receive their supply,? a phrase taken from 240.21. If there is some type of overcurrent device upstream, and if it is adequately sized for the 800 amp load, and if you satisfy the ?Tap Rules? as Don has suggested, then there is no code violation.

 

[engy]

Re: Tap Conductors

How do you discount 408.16?

 

[charlie b]

Re: Tap Conductors

Good point. But the original question did not describe the loads. Nor, for that matter, did it give the voltage rating of the panels. WE also don't know the location (i.e., impact of local weather on load requirements). 800 amps is enough power for a big store. Conceivably, a large percentage of the load could be for heating and cooling, for computer systems, and for receptacle outlets. If the number of breakers serving lights doesn?t exceed 10% of the number of breakers on the panel, then 408.16 would not come into play.

 

[gregoryelectricinc]

Re: Tap Conductors

(1) Taps Not Over 3 m (10 ft) Long. Where the length of the tap conductors does not exceed 3 m (10 ft) and the tap conductors comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined computed loads on the circuits supplied by the tap conductors, and
b. Not less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent-protective device at the termination of the tap conductors.


"B" seems to imply that an overcurrent device will be installed at the termination point of tap conductor. Another wording issue in the NEC. Personally, I would agree that this installation, without the mains, does not meet the "intent" of the code even if it does meet the letter of the code. After this job is complete, what would prevent me from installing say, for sake of arguement, 800 amps worth of branch breakers in one panelboard? Would this not put the integrity of the 400 amp tap conductor in serious jeopardy?


Had a lttle time this AM so looked up some code articles. 408-16, A & B require an OCD noy greater than the rating of the panelboard. That is the section of NEC that would be a violation.

[ October 10, 2003, 08:45 AM: Message edited by: gregoryelectricinc ]