Changing the thread of this topic a little, the US NFPA (National Fire Protection Association) has a proposal for change regarding metrication of the National Electrical Code (NEC). This is sponsored by the NEC Technical Correlating Committee, at the direction of the NFPA Standards Council, so I would expect it to be approved.
It is a mix of hard and soft conversions. Essentially it means all dimensions in the code will be printed in SI metric units, occasionally with inch-pound units in parenthesis. Conversion will be done with appropriate rounding in the appropriate direction. Trade size references for conduit will be converted to metric without a physical change in size. The original trade sizes didn't actually mean anything, for example 1/2 inch (rigid) conduit was actually 0.840" outside, 0.636 inside, and other grades had no 0.500 applicable dimension either. As a result, the trade name for 1/2 inch conduit will become 16 mm conduit. The trade size is roughly an underestimate of the internal diameter. Note that wire size (AWG) and horsepower (motor sizes) will not be converted (unfortunately). Getting back to the RE: Update on UK conduit entry dimensions, In North America, conduit or tubing is terminated in a variety of ways including - Threaded openings - mostly in castings - Sleeve type terminations - for electrical metallic tubing (EMT), held with set screw or clamp - Sleeve type glued terminations - for plastic conduit - Compression type terminations - for EMT and assorted flexible tubings - Holes in sheet metal - for threaded connections with nuts on either side - Knockouts - sheet metal holes where the opening is still blocked by metal ready to be punched out - Fittings - adapting sleeve, glued, clamped or other terminations to threaded holes or holes using nuts. Note that the knockout or hole size is the parameter of most interest to equipment manufacturers since they are making the termination for the conduit, not the conduit. Below is a table of North American conduit sizes you might find of use. I was surprised to find there was such a thing as 1/4 and 3/8 conduit. It is only for use in special circumstances. I haven't provided actual conduit dimensions since these are different depending upon type, e.g. rigid, IMT, EMT, flexible, corrosion resistant, plastic, aluminum, etc. and even depending on manufacturer. Note again, the trade size is not a real dimension found anywhere. The knockout dimension is a real diamenter. Trade Knockout inch metric inch metric 1/4 3/8 12 1/2 16 7/8 23 3/4 21 1-3/32 29 1 27 1-11/32 35 1-1/4 35 1-11/16 43 1-1/2 41 1-15/16 50 2 53 2-3/8 60 2-1/2 63 2-7/8 73 3 78 3-1/2 90 3-1/2 91 4 100 4 103 4-1/2 115 5 129 5-5/8 127 6 155 6-5/8 150 The North American knockout sizes already appear in IEC 60950 in metric units (in parentheses). I am on TC74 WG8, but what should be done to upgrade table 3A for Europe and the rest of the world should probably be proposed by Cenelec. I would think that, like North American knockout sizes, the dimension should represent the hole size needed for connection of the conduit to the box, not necessarily any diameter of a particular type of conduit or tubing. I have yet to see any of the European inputs mention standard European conduit opening sizes rather then conduit dimensions. Jim Eichner wrote: From: Jim Eichner <[email protected]> Tue 1:28 PM Subject: RE: Update on UK conduit entry dimensions To: [email protected] Thanks for the update. The final outcome of this thread seems to be that the requirements in EN60950 are out of date. Clause 3.2.2 insists that permanently connected equipment, if it does not have a power, must be provided with either a cable entry or conduit entry. For the normal 3-wire supply circuit of 16A or less, the conduit entry must be "suitable for" conduit with 16mm OD (Table 10). The sources that we've heard from are saying that 16mm OD conduit has become almost unavailable, and everyone is using the next size up, which has 21mm OD (not the 20mm listed in Table 10). I'd like to hear from anyone on one of the xxx950 committees on this subject. Are we really discovering an anachronism in the standard, and if so can something be done about it? Thanks, Jim Eichner
