The code is somewhat useful as a guide for wire ampacity but it does not apply
inside products. It is meant to cover situations like wiring inside insulated
walls, in buried conduit and many other applications including the complicated
Neher-McGrath formula (see NEC 310.15).
Inside your product the requirement is to keep the application safe. You can
put whatever you want through the wire including having it glow red or even
fuse as long as the safety of the product is maintained. When insulation is
important, keeping below previously approved temperatures for rated wire is a
simplified practice. You can even exceed these temperatures if you can assure
safety is not impaired. As a crude example, you could fan cool the wire.
You can do some pre-design estimates using the derating factors, but
ultimately the insulation temperature and protection is what you will have to
demonstrate for standards compliance and to the approval agencies, not the
current level.
Bob Johnson
ITE Safety <http://www.itesafety.com>
Radojicic, Marko wrote:
With my company being acquired and the subsequent e-mail address
change, the
listserver didn’t recognize me so I’m sending.
________________________________
From: Radojicic, Marko
Sent: Monday, February 02, 2009 1:24 PM
To: 'Kunde, Brian'; [email protected]
Subject: RE: De-Rating Hookup Wire
Brian,
To address your questions:
- Your derating process is correct according to NEC. You have to add
all the
derating factors at the same time.
- I don’t believe that you have to derate for self-heating (the 10C
factor
in your calculation). The correction factors at the bottom of Tables 310-16
and 310-17 specifically state “ambient temperatures”. The self-heating is
what the ampacity calculation takes into account.
- Tie-wrapping wires together = bundling.
- No further derating for length of wire. Voltage drop is an issue,
like you
state.
- Make sure you are using the correct table if the wires are in
raceways. For
example, Table 310-16 is for “Raceway, Cable, or Earth (Directly Buried)”
while 310-17 is for “Free-Air”.
- I looked through my 1999 version of NEC and was unable to find how
you got
the initial ampacity rating. I don’t see any mention of 105C insulation at
10 AWG. Could you let me know how you got that?
…Marko
________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of Kunde,
Brian
Sent: Monday, February 02, 2009 11:48 AM
To: [email protected]
Subject: De-Rating Hookup Wire
I want to make sure I’m properly de-rating the current capacity of
hook-up
wire.
A 105º hookup wire size 10 AWG is rated 58 amps at 30ºC.
At 50ºC (40ºC out side temp and 10ºC internal temp rise) I have to
de-rate
by .86 which would now make it handle only 49.9 amps.
If I bundle 2 to 5 wires together, I have to de-rate another .8 which
would
make it only rated for 39.9 amps.
So do I de-rate the de-rated value or is there some other method that
is more
correct?
What is considered a “Bundle”? If I cable tie 2 wire together is this
considered a bundled?
Is there an additional de-rating for wire length of hookup wire? I
assume
wire length only affected voltage drop, not current rating.
Thanks to all for your help.
The Other Brian
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