Tuesday, August 17, 2010

Part II

Last week we discussed the first few rules of calculations, when sizing wire for circuit considerations. In Part II we will cover rule "B," which states that wire sizing shall be of sufficient mechanical tensile strength in order to withstand the stresses incurred within a normal and expected installation. Within the scope of this, we'd be concerned over the complete raceway sizing, pathways, and fill rates.

             All electrical wires MUST be physically protected against damage during the installation. To that end we carefully safeguard against using too many degree's worth of bends between pull points. This rule is exhibited in individual raceway articles, such as 358.26 "Bends - Number in One Run," which states that no more than 360 degrees shall be allowed. This keeps the amount of force, required to pull the conductors through the points, below that of an amount that could cause irreversible damage due to strain or stretching of the actual copper or aluminum of the wire. In this Author's opinion, there should also be a maximum footage rule between pull points for the branch circuits. As the length of the circuit increases, so to does the sheer weight of the wire itself. A #10 AWG Copper conductor, for example, weighs about a pound per every 31 feet. Thus a 200' run would add almost 7 pounds of drag to the pull, not even considering the additional amount of side wall friction that the 200' of wire adds. Hence, our suggested rule of thumb would be no more than 100' of raceway between pull points, in addition to the 360 degree rule.

         An installer would also need to be wary and mindful of always reaming out all cuts of raceway, to minimize sharp edges that could cause damages to the insulation of the conductor. As well as provide bushings where Articles 300.4(G), 342.46, 344.46, and 352.46 requires. Thus ANY raceway containing #4 AWG or larger wire MUST have bushings, and where the raceway is of IMC type, Rigid type, or PVC type.

       Our installation should also comply with the maximum fill percentages of 53% for a single conductor, 31% for 2 conductors, or 40% for 3 or more (applies to all runs over 24"). We find these table values and rules in Chapter 9, Tables 1, 4, and 5. Actual conduit fill applications and calculations are outside of the scope of this blog. However, do keep in mind that these are MAXIMUM values, and in order to mitigate the adverse effects of longer conduit run lengths, severe bends, or complicated and difficult pulls, and to facilitate the ease of conductor installations, the electrical technician should consider applying smaller percentage fill values, or increasing the size of the raceway itself.

       Finally, FPN No. 2 in Table 1 of Chapter 9, warns against what is called a "Jam" ratio, higher than 3.2%. This is a seldom considered issue that usually occurs only with 3 conductor installations. Essentially what happens is that a raceway may not be completely "round" at a bend. When the conductors enter the bend, the middle one may slip between the two outer ones, and when they exit the bend, it may cause a jam. The Jam ratio is simply the Inner Diameter of the raceway (found in Table 4) divided by the Outer Diameter of the conductor (found in Table 5). {Hint: this value will ALWAYS be greater than 1, so ALWAYS divide the Biggest # by the Smallest #.}

      All of these cautions, when used together, insure a mechanically safe installation of electrical wire. Don't forget that ALL conductors count - grounds etc... when calculating conduit fill! More next week, have a safe week everybody!

Don’t forget to check out our website at www.ElectricianTesting.com for your test preparation material.  Check back often since we give away helpful stuff for use on the field and the test room!

No comments:

Post a Comment

Popular Posts