Code Question Of The Day

This question concerns the use of expansion joints as discussed in NEC 300-7(b). The text states that “raceways shall be provided with expansion joints where necessary to compensate for thermal expansion and contraction.” The FPN then discusses the 0.20 co-efficient for steel conduit, which could be applied based on the values in Table 352.44 for Rigid Non-metallic Conduit. Per 352.44 (Expansion Fittings) under Rigid Nonmetallic Conduit, provisions must be made for thermal expansion and contraction when the conditions are such that there could be a ¼” movement.

Does this requirement for thermal expansion and contraction apply to Rigid Metallic Conduit? Are expansion fittings required for Rigid Metallic Conduit?

Ex: If 100 feet of Rigid Metallic Conduit is subject to 100 degrees F. temperature change, then the pipe would expand/contract 4.1” x 0.20 = 0.82.”

I don’t see any thing in the NEC that addresses provisions for the expansion/contraction of rigid metallic conduit, other than the FPN under 300-7(b). I realize that the FPN is for clarification or explanation, but nonetheless, it seems that ¼” movement is the same regardless what the conduit material is. Provisions must be made for Rigid Non-metallic Conduit when the movement is ¼”. Does the same requirement apply to other conduits (specifically, RMC)? NEC 300-7(b) seems to require it but I can’t see anything in the code that spells it out as clearly as 352.44 does for Rigid Non-metallic Conduit. Please clarify this for me.

Yes, the requirements for compensation of thermal expansion and contraction are applicable to all raceways as shown in 300-7(b). The FPN provides information that a nominal number for steel conduit can be determined by multiplying the expansion length shown in Table 352.44(A) by 0.20. The FPN also provides information on the formula, which can be used for determining an exact expansion length.

The NEC has no specific requirement concerning the amount of length change due to thermal expansion or contraction, which requires compensation by means of an expansion fitting, for raceways other than rigid nonmetallic conduit in 352.44. I suggest you follow the requirements shown for rigid metal nonmetallic conduit multiplied by .20 as shown in the FPN to Table 352.44.

Ex: A 100 ft length of PVC conduit subject to temperature variations from 95°F (summer) to -10°F (winter) would be a temperature change of 105°. Based on Table 352.44 this could result in a length change of 4.26 in. per l00 ft. Using 0.20 as a nominal number for steel conduit as shown in the FPN to Table 352.44 would be 4.26 x .20 = .85 in. or slightly over ¾”.

It appears that the rigid metal conduit installation would survive, however the 100’ run of PVC between boxes would be a serious problem.