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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
Figure 01
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Figure 01
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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
Note: The answers to these questions are based on the 2020 NEC.
Q1: What is the minimum and maximum RMC trade size permitted?
A1: 344.20 Trade Size
(A) Minimum. RMC smaller than trade size ½ is not permitted.
(B) Maximum. RMC larger than trade size 6 is not permitted to be used.
Q2: What is the maximum degrees of bends permitted for RMC in one run?
A2: 344.26 Number of Bends (360°)
To reduce stress and friction on conductor insulation, the total bends (including offsets) between pull points are not permitted to exceed 360°.
Author’s Comment:
►There is no maximum distance between pull boxes because this is a design issue, not a safety issue.
Q3: What are the requirements for securing and supporting RMC?
A3: 344.30 Securing and Supporting
RMC must be securely fastened in place and supported in accordance with (A) and (B).
(A) Securely Fastened. RMC must be secured in accordance with any of the following:
(1) Fastened within 3 ft of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Figure 01
Author’s Comment:
►Fastening is required within 3 ft of terminations, not within 3 ft of each coupling.
(2) When structural members do not permit the raceway to be secured within 3 ft of a box or termination fitting, the raceway must be secured within 5 ft of the termination.
(3) Where approved, RMC is not required to be securely fastened within 3 ft of the service head for an above-the-roof termination of a mast.
(B) Supports.
(1) General. RMC must be supported at intervals not exceeding 10 ft.
(2) Straight Runs. Straight runs made with threaded couplings can be supported in accordance with the distances contained in Table 344.30(B)(2).
Table 344.30(B)(2) Supports for Rigid Metal Conduit |
Trade Size |
Support Spacing |
½–¾ |
10 ft |
1 |
12 ft |
1¼–1½ |
14 ft |
2–2½ |
16 ft |
3 and larger |
20 ft |
(3) Vertical Risers. Exposed vertical risers for fixed equipment can be supported at intervals not exceeding 20 ft if the conduit is made up with threaded couplings, firmly supported, securely fastened at the top and bottom of the riser, and if no other means of support is available.
(4) Horizontal Runs. Conduits installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members are considered supported, but the raceway must be secured within 3 ft of termination.
Author’s Comment:
►RMC must be provided with expansion fittings where necessary to compensate for thermal expansion and contraction [300.7(B)]. The expansion characteristics for metal raceways are determined by multiplying the values from Table 352.44 by 0.20, and those for aluminum raceways are determined by multiplying the values from Table 352.44 by 0.40 [300.7 Note].
Q4: What are the requirements for installing RMC couplings and connectors?
A4: 344.42 Couplings and Connectors
(A) Installation. Threadless couplings and connectors must be made up tight to maintain an effective ground-fault current path to safely conduct fault current in accordance with 250.4(A)(5), 250.96(A), and 300.10.
Author’s Comment:
►Loose locknuts have been found to burn clear before a fault was cleared because loose connections increase the impedance of the ground-fault current path.
If buried in masonry or concrete, threadless fittings must be of the concrete-tight type. If installed in wet locations, fittings must be listed for use in wet locations and prevent moisture or water from entering or accumulating within the enclosure in accordance with 314.15.
(B) Running Threads. Running threads are not permitted for the connection of couplings, but they are permitted at other locations.
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