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Article 501 provides the requirements for Class I locations, such as the refinery shown in Photo 1. In the May issue, we described the Division system outlined in the NEC, which further breaks down the class. If the ignitable material is normally present (in sufficient quantities to present a hazard), then it's a Div. 1 location. Less stringent requirements apply for a Div. 2 location, because ignitables are present only in abnormal circumstances.
Article 501 gives you the option of using the Zone system (Art. 505) instead of the Division system [501.2]. Articles 502 and 503 also give you this option, but inconsistent with Art. 501 they do it in Subsection 6 [502.6, 503.6]. We'll discuss Art. 505 requirements in a future installment.
Article 501 presents most of its requirements with a pattern of "Div. 1 in subsection A" and "Div. 2 in subsection B." This first example occurs in Part II. There, we can see that 501.10(A) provides the wiring requirements for Div. 1, while 501.10(B) provides the wiring requirements for Div. 2. This pattern repeats many times, though it's not a hard and fast rule.
Wiring methods
If a wiring method is allowed for Div. 1, it's also allowed for Div. 2 [501.10(B)(1)(1)]. There are four general differences in wiring method requirements for Div. 1 versus Div. 2:
1) Fittings and flexible cords used in Class I, Div. 1 areas must be listed for Class I, Div. 1.
2) You must run conductors in RMC or threaded steel IMC raceways in Div. 1 (there's an exception for concrete-encased PVC), but you can use any gasketed enclosed busways or wireways for Div. 2.
3) More cable types are allowed for Div. 2.
4) In Div. 2, you can install nonicendive field wiring using any of the wiring methods permitted for unclassified locations.
Sealing requirements
Nearly half of Art. 501 is devoted to presenting sealing requirements, a fact that visually distinguishes it from Art. 502 and Art. 503 (see SIDEBAR: The Class Sealing). The purpose of the seals (Photo 2) is to "minimize the passage of gases and vapors and prevent the passage of flames from one portion of the electrical installation to another through the conduit." [501.15 Informational Note 1].
You must install seals: