1) Correct, this cannot be used in a multiwire branch circuit since it may eventually be removed without being bypassed. The concern is that an open neutral in a multiwire (e.g. 120/240 3w or 120/208 4w) circuit leaves loads connected in series phase to phase. The voltage across a product intended to operate phase to neutral then becomes dependent on the other series loads. Note also that measuring the current in the neutral of a multiwire circuit will not provide power measurement for any currents flowing phase to phase. For example when loads on each phase are equally balanced, neutral current would be zero (ignoring harmonics). I see no other code complications. I wonder however why you need to make the current measurement in the neutral rather than the phase conductor.
2) The 15 or 20 amp receptacles must be protected with Listed devices (not recognized devices such as supplementary protectors). This protection however may be provided by the branch circuit. If the product with receptacles has a plug of the same size (e.g. 15 or 20 amps) then you can assume it is connected to a circuit with the proper protection and therefore avoid additional protection. If it plugs into a larger circuit such as a 30 amp receptacle, then you will need to add Listed 15 or 20 amp overcurrent protection within the product for protection of the smaller receptacles. Note that UL is reconsidering this requirement, but standards changes have not been made yet. Note also that Canada does not permit 15 amp receptacles on 20 amp circuits (and therefore does not permit the T-slot type 5-20R receptacles), although this may have changed recently. Maybe Canadians can clarify. My responses are based on UL/CSA 60950, but I think you will find this subject is consistent across other standards. Bob Johnson ITE Safety -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Richard Meyette Sent: Tuesday, April 30, 2002 1:05 PM To: emc-p...@majordomo.ieee.org Subject: NEC Questions I have a couple of questions regrading the National Electrical Code: 1) Consider a power measuring device that is intended to be installed into a residential 120V branch circuit, protected by a single 15 A or 20 A circuit breaker. The device would be installed into a conduit outlet box on the load side of the breaker to measure the power loads on the outlet receptacles or lights connected to the branch circuit. The device has two neutral terminals with a 12 AWG copper wire connected between them to sense the load currents. This device would be wired in series with the neutral (grounded) conductor for the branch circuit. The live (ungrounded) conductor of the branch circuit would have a common connection on this device. Based on Section 300-13(b) of the NEC (NFPA 70), this device could not be used in a multi-wire branch circuit, since the removal of the device would interrupt the continuity of the grounded conductor. However, I cannot find any requirements that would prohibit the use of this device in a single phase branch circuit, protected by one circuit breaker. Since the resistive loss in the neutral circuit through this device would be negligible, would this device comply with the NEC? My guess is yes, that it meets the NEC code requirements for this branch circuit. Any comments? 2) Consider the same power measuring circuit installed into a direct plug-in (NEMA 5-15P) with a single outlet receptacle (NEMA 5-15R) or into an outlet strip with a 16 AWG power cord with several outlet receptacles. Assuming that the measuring circuit is provided with a suitable fuse for overcurrent protection, are the outlet receptacles required to be protected by a 15 A fuse or circuit breaker in the device or can they rely on the circuit breaker for the branch circuit for overcurrent protection? The UL product standard for this device is UL 3111-1 (Electrical Measuring and Test Equipment), which is harmonized with IEC 61010-1, requires an overcurrent protector to be fitted within the equipment for all devices connected to the mains supply (9.6.2). There are no US deviations in this standard that would allow the circuit breaker to provide this protection, so based on this I would assume that a 15 A circuit breaker of fuse would be required for the NEMA 5-15R receptacles. I would also assume that a 20 A overcurrent protector would be required for a NEMA 5-20R outlet receptacle. However, the UL product standard for household appliances (UL 60335-1) does have a US deviation to a similar requirement for overcurrent devices (19.1, Note 2) that states the "The PROTECTIVE DEVICE in the fixed wiring does not provide the necessary protection." However, the US deviation states "The circuit protection device is permitted to provide necessary protection". If I am interpreting this correctly, a household appliance in the USA could rely on the panel breaker for overcurrent protection. Any comments? Thanks in advance for anyone willing to wade through this and send me a response. Richard A. Meyette. PE meye...@pacbell.net ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
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