[RE-wrenches] Expansion fittings?

[Andrew Truitt]

Quick (I hope) question:

What do you guys use to calculate spans between expansion fittings?


NEC2008 300.7(B):

Raceways shall be provided with
expansion fittings where necessary to compensate for thermal
expansion and contraction.

FPN: Table 352.44 and Table 355.44 provide the expansion
information for polyvinyl chloride (PVC) and for reinforced
thermosetting resin conduit (RTRC), respectively.
A nominal number for steel conduit can be determined by
multiplying the expansion length in Table 352.44 by 0.20.
The coefficient of expansion for steel electrical metallic
tubing, intermediate metal conduit, and rigid conduit is
1.170 × 10−5 (0.0000117 mm per mm of conduit for each
°C in temperature change) [0.650 × 10−5 (0.0000065 in. per
inch of conduit for each °F in temperature change)].


352.44:

Expansion Fittings. Expansion fittings for PVC
conduit shall be provided to compensate for thermal expansion
and contraction where the length change, in accordance
with Table 352.44, is expected to be 6 mm (1⁄4 in.) or greater in
a straight run between securely mounted items such as boxes,
cabinets, elbows, or other conduit terminations.



By my calculations this leads to 3-4 expansion fittings per 100' of EMT for
a temperature range of 100 degrees F, which seems like a lot.  The rule of
thumb I have heard is to use e-fittings for runs over 100' - why are these
two so different?  Is there really no variability with conduit diameter or
thickness?



Grassyass,

Andrew Truitt
Standard Solar Inc.




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to fission. And it just so happens that there's an enormous fusion reactor
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~William McDonough
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