[RE-wrenches] Temperature Compensation calcs for wire

Bill Brooks billbrooks7 at yahoo.com
Wed Jan 22 17:23:20 PST 2014


William,

 

One way to think about this is that temperature correction is used based on
the location of the ambient temperature you are concerned about.

 

Mostly, we are worried about the very high temperature of conductors in a
sunlit raceway on a roof. This gives us very high temperatures and we use
the 90C table and 90C temperature correction factors.

 

In no case can we have an ampacity that is greater than the 75C table at 30C
since the terminal is rated that way. It does not matter what kind of
insulation we have on the wire—the wire can’t get hotter than 75C at the
terminal. 

 

What the code is not very good at is telling us what to do with a combiner
box on a rooftop with 75C terminals. What is the ambient temperature of a
75C terminal in a rooftop combiner box? Kinda, really, pretty hot, I guess

That’s about what we have to go on. If the box is white it might be less.
Generally, the current on the string conductors is so small that it is not
an issue.

 

However, for an inverter at ground level, the ambient temperature to use for
the terminals is probably around 40C—nothing like a rooftop conduit.

 

At the end of the day, the short answer is that you use 90C table with 90C
temperature correction factors and the temperature adders for rooftop
conduit. That will generally be more aggressive than a 75C terminal at 40C
with the 75C temperature correction factors. You are welcome to do the
calculation, however. Does this make sense? You can’t double-dip and use 75C
temperature correction factors on the roof where there are no terminals.
That would make it impossible to install PV since these temperatures can
occasionally exceed 75C.

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of William
Miller
Sent: Wednesday, January 22, 2014 4:32 PM
To: RE-wrenches
Subject: [RE-wrenches] Temperature Compensation calcs for wire

 

Friends:

 

I try to be rigorous in application of NEC codes to everything I do,
including wire sizing.  I understand that even though I am using conductors
rated at 90°C, the breakers I use have terminals rated at 75°C so when
deriving the values for ampacity for a given gauge from the tables, I have
to use the 75°C column.

 

What is not clear, however, is which column I use when applying temperature
derating.  Table 315(B)(2)(b) has a column for 75 and a column for 90.  I am
using 90° wire.  The values for 90° are much more generous than the 75°
values and I would like to use them.  Which is correct?

 

As always, thanks to everyone on this list for all of the help and advice.

 

William

 

 



Lic 773985

millersolar.com <http://www.millersolar.com/> 

805-438-5600

 



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