derating to 310.10 new FPN

[andy32821]

Hi,
I am giving a class on 2005 code revisions and wish to use an example of the effect of derating.
This is an actual installation this facility is getting ready to install.

Twelve 120-volt 20-amp branch circuits to feed individual duplex receptacles to charge utility vehicles.

(I am assuming they are linear loads, subject to change based on manufacturer?s response.)
(Four multiwire circuits from a 120/208 3-phase panel.)

The existing plan is to run the conduit along the roof conduit rack in direct sunlight.
I wish to show the effect of the new FPN 310.10.
A typical summer day here is 98F. That gives an ambient of 128F using the new FPN.

(20 amp rating of branch circuit) divided by (0.76 for a 128F temp factor) divided by (50% for 12 current carrying conductors) equals 52.63 rounds up to 53 amps, which requires number 8 wire.
(Good for 40 amps at 60F)
(Is it correct to round 0.5 and higher up and round below 0.5 down on final derating value?)

If instead they stay inside the building and run along the wall in two separate conduits then:
(20 amp rating of branch circuit) divided by (1.0 for 86F temp factor) divided by (80% for 6 current carrying conductors equals 25 amps, which requires a number 14 wire. (Good for 20 amps at 60F)

However 240.4(D) will require #12 wire.

Is this correct or have I missed something.

Thanks for your help,
Andy

[ July 05, 2005, 06:33 PM: Message edited by: andy32821 ]

 

[infinity]

Re: derating to 310.10 new FPN

The FPN for 310.10 is not a mandatory rule so adding the extra 30 degrees to your calculation may be good practice but it's not required.

[ July 05, 2005, 07:34 PM: Message edited by: infinity ]

 

[charlie]

Re: derating to 310.10 new FPN

FPN 2 was submitted with poor test data. While it is well understood that sunlight will raise the internal temperature of any conduit, the testing was done in a single location. The temperature rise in the northern areas of the country is much less than is in the souther regions. Since the test was done in one location, it was obviously not valid everywhere and was relegated to a FPN and is not enforceable.

The question I have is the number of failures due to overheated service entrance conductors, feeders, and branch circuits. Has this really been a big problem that needs to be addressed?

 

[andy32821]

Re: derating to 310.10 new FPN

I checked the Temperature of the conduit on the roof. Ambient was 95F. Conduit on a non-running vent fan was 136F. (Orlando Florida)

Do inspectors generally not enforce the temperature derating?

I have had them question me on number derating but have never been asked about temperature derating.

Andy

 

[bphgravity]

Re: derating to 310.10 new FPN

Enforcing correction factors is a very difficult proposition for an inspector and the building department. I'm in SW Florida which is roughly the same climate as Orlando. It gets very warm here during the summer but for most of the year we average near the 86? standard of 310.16.

The fact of the matter is that if a failure were to occur due to a conductor's insulation exceeding its temperature rating, the blame and correction will lie with the contractor. The building department will not be held responsible for approving the installation.

There are extreme cases where it absoultely necessary, however in most cases, the installer can take advantage of 310.15(A)(2) Exception to avoid an derating or corrections.

 

[physis]

Re: derating to 310.10 new FPN

I'd probably have to stay up all night to understand these sections well enough to offer much help but I'd be really interested in seeing some real world test results to compare to the NEC standards that we have to use.

Not so much because I have authority issues but just to have a better understanding what really happens.