The recent code change in NFPA 70 in article 725-41(a)(4) permits "Listed information technology (computer) equipment limited power circuits" to be considered Class 2 circuits (not to be confused with Class II). This in turn permits building wiring in accordance with clause 725 for these circuits which means CL2 (or better) wire for general wiring, CL2R for riser applications and CL2P for plenum applications. While these are very relaxed wiring requirements, those not familiar with the section should read clause 725 sections A and C, because there ARE restrictions.
The code specifically calls out the limited power circuits of UL 1950 (see clause 2.11) as examples of (read: criteria for) the Listed ITE limited power circuits mentioned above. Exempt are circuits "In nonconcealed spaces where the exposed length of cable does not exceed 10 ft (3.05m)," see clause 725-61(e)Exception 1. These wires are not considered building wiring wiring and the requirements fall back on the equipment standard, UL 1950. Again, check the code because this exemption does not apply to even short lengths in plenums, risers, cable trays, hazardous locations, etc. If the circuits involved are not limited power circuits, other wiring sections of the code apply, usually Class 1 requirements. Class 1 circuits require much more stringent wiring methods, including those in Chapters 3 and 4 of the code. The use of DP-1 or DP-2 cables (see UL 1950 clause 3.1.12) would generally be called for with these circuits. Note also the recent UL 1950 change to clause 7.2 requiring identification of any connectors (including telecom circuits) NOT supplied by a limited power source. This was done to ease the job of installers and wiring inspectors. Additional comments regarding the question: I see no exemption to the requirements of 2.11 permitting abnormal testing or single fault conditions. To have a Class 2 circuit and use the installation practices it permits you must either meet the requirements of UL 1950 clause 2.11 or meet the other power requirements for Class 2 circuits called out in the code clause 725-41. Bob Johnson Bob Brister wrote: > > Hello, > > I am trying to understand the relationship between IEC950 (US/CSA deviations) > and NFPA 70 in regards to what I believe would be defined as "interconnecting > cables". In particular, I am looking at the circuit between a device port on > the system and the external device which connects to that port. I am not > considering direct connection to telecommunications, but interface connections > to devices such as printers, scanners, and other things which exist. Assume > that these products are desktop or deskside. They are not in a computer room. > > It appears that UL/CSA section 3 covers this and either the circuit meets > the limited power source requirements of tables 8 or 9 section 2.11 or it > must be shown through abnormal testing that no hazard exist under single fault > conditions. Do you agree? > > Table 8 and 9 come from Article 725 of the NEC for Class 2 and Class 3 > circuits. If the limits of these tables are not met, what does that mean as > far as the NEC is concerned. Does it simply mean the circuit is not Class 2 > or 3? > > I hope this makes some sense. If not ask some questions and I will see if I > can clarify what I am asking for. > > Regards, > > Bob Brister > Digital Equipment Corp. > Corp. EHS
