Table 310.16 service entrance @ 90?C

[mweaver]

Greetings,


In a discussion with two other electrical contractors, I was told that (on two separate occasions from two separate engineers) the engineers calculated a service entrance utilizing six disconnects and sizing the service entrance conductors from the 90?C column. When questioned on this practice the engineers cited that because of the diversification of six throws of the hand, they could utilize the 90?C column for their service entrance cable sizing. They both were unable to provide additional details regarding either installation. They both said they were baffled but just went along with the engineers drawings.

I was asked if I had ever heard of this. I had to respond that I was not aware of any instance where the 90?C column could be used for anything other than deration of conductors. Service equipment terminations are limited to 75?C under 110.14(C). I do not see how six disconnects affect this.

Has anybody ever ran into anything like this?

Thanks in advance

mweaver

 

[ryan_618]

Re: Table 310.16 service entrance @ 90?C

I've never heard of it, and I also that you are right.

 

[caosesvida]

Re: Table 310.16 service entrance @ 90?C

I agree with you also, the disconnects are rated at 75.

 

[bphgravity]

Re: Table 310.16 service entrance @ 90?C

Maybe the engineers were invoking their 310.15(C) rights which resulted in the same value of column C, but was not just arbritarily selected from that column.

 

[mweaver]

Re: Table 310.16 service entrance @ 90?C

Thanks to all who replied.

I have been trying to think of some situation where this would be feasable.

The only thing I can think of is example:


500Kcmil service entrance conductors sized from the 90?C column (tapped with 90?C tap fittings) feeding 75?C conductor taps to six disconnects.

Even with this..... the other end of the 500Kcmil would have a utility transfrmer connection of 75?C. I am stumped......


Thanks again to all who responded. If I ever get any additional info on this I will post it.

mweaver

 

[charlie b]

Re: Table 310.16 service entrance @ 90?C

Originally posted by bphgravity:
Maybe the engineers were invoking their 310.15(C) rights which resulted in the same value of column C. . . .

It's a posibility, but I think it's unlikely. Every time I have used the Neher McGrath methodology to calculate ampacity, the results have been lower than the values posted in the 75C column of Table 310.16. My read is that the "engineers" did not understand what they were doing.

 

[roger]

Re: Table 310.16 service entrance @ 90?C

mweaver, could the installation actually be in compliance with 230.90(A)exception No. 3.

Is the question because the combined OCPD's exceed the conductors 75 deg rating.

Forgetting the face value of the six disconnects, does the "calculated load" exceed the conductors ampacity if the 75 deg column was used?

You have probably already considered this, but I thought I'd ask anyways.

Roger

 

[bob]

Re: Table 310.16 service entrance @ 90?C

MWeaver

500Kcmil service entrance conductors sized from the 90?C column (tapped with 90?C tap fittings) feeding 75?C conductor taps to six disconnects.

Even with this..... the other end of the 500Kcmil would have a utility transfrmer connection of 75?C. I am stumped......

The utiltiy service and connectors are not covered in the NEC. I think yout idea of using 90C connectors might work if they are available. Are 90C connectors available?
The problem would be that you have 75C and 90C conductors connected together. This no different than 90C conductors connected to as 75C panel.

[ April 29, 2004, 01:40 PM: Message edited by: bob ]

 

[jschultz]

Re: Table 310.16 service entrance @ 90?C

110.14(C)(1)(b)(2)

Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75 degree C ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors.

 

[jschultz]

Re: Table 310.16 service entrance @ 90?C

the cabinet has to be rated for higher than 75 degrees also, not just the lugs and wire.

 

[jryan]

Re: Table 310.16 service entrance @ 90?C

I am having just such an issue now with a 1200A panel, 3-500kcmil will NOT work, the contractor either runs 600kcmil or parallels a smaller wire.


NEC-2002
ART 110.14 Section C
"The temperature rating associated with the ampacities of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."
&
Sub part 1(b)
(Over 100A & Wire larger the #4 AWG) only one of the following:
"(1) Conductors rated 75 deg C"
"(2) Conductors with higher temperature ratings, provided the ampacities of such conductors does not exceed the 75 deg C ampacities of the conductor size used , or up to their ampacity if the equipment is listed and identified for use with such conductors."

I do not believe the equipment will be rated 90 deg C. The substation switchgear might be rated for 90 deg. C, but to my knowledge, there are no panelboards currently listed by UL with terminations rated for higher than 75 deg C. Since the conductors have to be the same size and type at both ends (assuming that it is not spliced somewhere in the middle), the 75 deg C rating wins.


Even if the lugs are rated for 90 deg. C, it is the UL listing of the panelboard that limits it to 75 deg. C. Putting in higher rated lugs does not increase the rating of the panelboard.