Table 310-16 design

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mweaver

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Greetings from Kansas, I am new to the board and there appears to be a wealth of information here. What I was looking for I was unable to find by searching so I created a new topic in hopes of getting an answer or confirmaiton.

I was taught in the past that Table 310-16 (1999 NEC) was designed so that (for example): a #8 THHN CU (90?C) operating in an ambient of 30?C with ZERO load will be 30?C but at the max allowable load (55 amps AND two additional Current-Carrying conductors in conduit) this conductor will reach a temperature of 90?C (the maximum allowable temperature for this condutor)

Every time I say this it just seems mind boggeling and impossible. Is this how this table was designed?

Can anybody lead me to documentation on this issue?


thanks in advance,
Mweaver
 
Re: Table 310-16 design

Bob,

Thanks for the reply. I found this article prior to posting and just didn't feel it actually answered my question. You do seem to agree with my question, though and any confirmation was what I was hoping for.

Again, Thanks for the reply

mweaver
 
Re: Table 310-16 design

mweaver

As Bob has stated, now you know that these temperatures are possible to be reached by conductors. That is one of the reasons for box fill, bundling, and conduit fill. Also how about termination connections?

Pierre
 
Re: Table 310-16 design

Pierre,

Thanks for the reply.

110-14(c)should always be your last criteria in determining conductor size, but I was not really attempting to go there.

I was just looking for additional understanding of Table 310-16 itself. I believe, I (in part) understand how to REALLY read this table, I just gasp everytime I say outloud: that a #8 THHN in conduit with 2 additional current-carry conductors at an ambient of 86F loaded to 55 amps will produce a conductor temp of (more than double) 194F. .....(Gasp)


catching my breath,

mweaver
 
Re: Table 310-16 design

Originally posted by mweaver:
that a #8 THHN in conduit with 2 additional current-carry conductors at an ambient of 86F loaded to 55 amps will produce a conductor temp of (more than double) 194F. .....(Gasp)
Click to expand...
I do not think that is the case, one 8 loaded at 55 amps will not reach 90C but three together each loaded 55 amps will reach 90C (194F) or close to it, that still seems hot but that is what the insulation is rated for.

Go to table 310.17 for single conductors in free air and you will see it takes 80 amps to get a single 8 AWG to 90C or close to it.

Something that I find surprising is UL allows the terminals on breakers loaded 100% to get 50C above a max ambient of 40C.

[ August 10, 2003, 08:34 AM: Message edited by: iwire ]
 
Re: Table 310-16 design

Another thing to consider is how often in the real world do you apply a 55 amp load to a 55 amp rated conductor, issues like voltage drop and the fact that it is usual for a load to match a conductor rating that close means that most times the conductor is rated above the load current by a nice margin.
 
Re: Table 310-16 design

If it helps you avoid gasping, let me offer the suggestion that the table is not declaring that the temperature will hit 90C, but rather is giving assurances that the temperature will not exceed 90C. In other words, you are safe :) if you keep the current below 55 amps, and "safe" is all the NEC is about anyway.
 
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