I have completed the online Table 310.16 wire size, ocpd, and voltage
drop calculator. I can't find anymore bugs but maybe you can. It took
me about 10 weeks during the last 10 years to build this program.
It is at
Any feed back about problems will be appreciated. Send to electrician
I have a question for you...in the HVAC business wiring to
units on the roof is often long distances and in the sun,
where tools can get way too hot to pick up... yet commonly
one sees wire sized for normal inside conditions, not derated
for this temperature extreme (those units burn out the
compressors a lot also)... I do not see the city code
compliance inspectors considering that issue either... I dont
think Ive ever seen it considered on engineering projects I
get involved in.
The calculator has ambient derating for higher than normal
temperatures. Table 310.16 is based on 30 degrees C. or 86 degrees F.
Select a higher ambient and the calculator will find the correct size
conductor and ocpd. Maybe I should convert degrees C to degrees F in
the calculator. I may do that. The conversion is done in the right
column at the bottom of Table 310.16. For instance, for a 125 degrees
F ambient that I would expect on a roof top in a warm climate the
derating factor is 76 per cent for a copper conductor with 90 degree
insulation. Also, there are derating conditions based on the length
given in Section 310.15(2) Exception. It is important to remember that
ampacity tables assume an infinite length where there is no axial heat
transfer but only radial heat transfer. However in the real world we
know there is axial heat transfer (commonly called heat-sink effect)
and that is why there is no derating for over three current carrying
conductors in a raceway or bundle that is 24 inches or less in length.
The 10 foot and 10 per cent rule given below applies to ambient
derating and also assumes that there is axial heat transfer from the
hotter part of the run to the cooler part. The 2005 NEC handbook
explains this more. Basically it says where the length of a
particular run is greater than 10 feet and more than 10 per cent of the
circuit length where the derating applies, then the derating applies to
the entire run.
(2) Selection of Ampacity. Where more than one calculated
or tabulated ampacity could apply for a given circuit
length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance
equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.
I saw all that...its very good. My comment was though its
not commonly applied at all and it really should be... I was
wondering what you thought about that and why most electrical
engineers or contractors Ive seen a least ignore these solar
heated conduit runs in that regard.
I do not know why many engineers and contractors do not apply derating
correctly as you have indicated. I have learned that the application
of all the NEC rules for determining the size of conductors and the
over load protection is confusing, anyway it was for me. For instance,
I have provided the choice of option A or B in the calculator because
to me the NEC is unclear about how the ampacity is limited when a
terminal temperature is lower than the conductor insulation
temperature. The examples in the NEC Handbook are also unclear about
this. The NEC concentrates on the conductor protection, but Section
110.14 is not really clear on protecting the terminal. I suppose many
engineers and electricians are unclear as to how all the rules apply.
It has taken me many hours to come up with an algorithm that seems to
work. I am certain that many engineers and contractors simply do not
have enough time to dedicate to this subject. I am now in process of
flow charting this process and including this subject in a continuing
education course. There are several NEC changes that I hope to propose
on this subject.
For us HVAC/ refrigeration guys low voltage, particularly
to our 3 phase motors is a big issue...even with single phase
motors. these rely on the refrigerant suction gas for
cooling...in the summer the suction gas is a little
warmer...the loads are heavier (higher amps), and when the
line voltage goes low, or out of balance phase to phase which
is also common, the amperage draw on our compressors goes up
dramatically, the motors over heat.
These have thermal over loads built into the windings in most
cases, but those are set to trip only when the thing is about
to burn up... damage is done by the low voltage...which is
often 10% below peak voltage levels, and 10 to 20% below the
ideal voltage at the motor... so they run hot.
Undersized feeders, on a hot roof, running 190 degrees inside
the conduit, takes the already low voltage issues in the
summer to much more serious levels if its a long run.
We do care about the terminals of course also, those burn off
at the compressor connections ruining the compressor in some
You have done some superb work... thank you very much.
you should be able to sell the program, along with a mini
book on the issues.
I agree with you Phil. I have seen the same thing in the early days ~70's in
It is a dry heat.
Then the city changed to anything outside for more than 5 feet has to use
the 75 degree table not the 90.
For feeders we use the 75 degree table and then use the de-rating on the
bottom for 108 F. ~80% if I remember correctly. Still some installations
slip by but not as many as in the before times. 4/0 copper is the standard
for a 200 amp single phase overhead service. Fortunately there are not to
many of them now days.
Exposed wiring IS inspected for THHN,THWN or the preferred XHHW insulation.
I have not seen TW for a while now.
I am working on a project ~million square foot building. The owner is
installing 105 swamp coolers, 480v 3 phase. The electrical contractor is
running 10 stranded THWN (just under the metal roof) some 300 feet to the
MCC. ( longest run ). By the time they install the control power transformer
for the pumps, and the control power transformer for the outlet, the owner
does not want to run a separate circuit for outlets on the roof, the wire is
pretty much at the limit. Hey it ok, no permits, and no inspections, yet.
I asked politely if anyone thought that the roof would get hot during the
summer? They replied that as long as the coolers were running the wiring
would be ok. Sounds like a plan to me........
those would be small cheapie swamp coolers...a bad way to go,
see if you can get him to look at 10 or 20 larger industrial
rated evaporative coolers with stainless sumps, and cooling
tower rated fill that will not have to be changed.
with 105 small coolers of that type maintenance after a few
years will be nasty.
The electrical contractor is
amazin' ain't it.... such logic just blubbers my brain.
You will stay employed that way... myself I end up pissing em
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