Sunday, August 22, 2010

Part III "Circuit Considerations for Calculating Wire Sizes."

****     Earlier this week I received the following code question. I will post this, and the very first person who can respond with the correct answer, including ALL code references, will get a FREE laminated wallet card. This wallet card is a special tri-fold sheet with a unique way of cramming a lot of commonly utilized field calculations into one simple card.

              "When or would it be allowable to use a plastic [or similar non-conductive material] old-work box with MC cable installations?"

           ***Remember, the answer MUST be complete and ALL code references should be cited. I will take the first complete and correct answer or the best answer of all responses, whichever is first. The winner will be announced the next day! Good Luck!

    Part III "Circuit Considerations for Calculating Wire Sizes."

     The past few weeks we have examined several of the 6 'rules' for circuit sizing. The final three, Sizing for economic considerations, Sizing for voltage drop considerations, and Sizing for "copper losses," are our final ones. We will look at these last three issues briefly. None of these are code considerations, but they are all three important for a complete and efficient system. Always remember that, even though the "Code" may be considered the "electrician's bible," it is STRICTLY concerned with safety ONLY! There are many other items outside its scope that are still very important to consider.

     Rule "C" says that "It should be sized for economic considerations so as not to exceed those limitations." The simple meaning of that is, be wary of the expense when making calculations (or rather, when NOT making them....) Most often, technicians will make "seat of their pants" decisions and simply 'up size' an installation rather than trouble themselves with the calculations necessary to ascertain the correct minimum sizing. While this method is overlooked in small branch circuit sizing on a limited scale, it can have HUGE impacts over a span of time. If the same electrician did this here and there over the course of his career, he could ultimately cost his employer and/or clients tens of thousands of dollars in wasted material and the additional labor units it costs to install them. In addition, on larger runs, (for example,) calculations over a 200' fun of 250 KC MIL of copper feeders can add up higher costs extremely quickly. You would not want to guess at up sizing these feeders to a 300 KC MIL as the cost difference could be well over $2,000.00! The bottom line is that 15 minutes of the technicians careful planning and calculations may save hundreds of dollars.

      Voltage drop considerations have been exhaustingly examined in earlier blogs. As such, I only wish to remind you that they are a very crucial consideration and I invite you to go back and review those postings.

     The final consideration is for "copper losses." This has also been covered in earlier blogs. It is a caution to remind you, the installer, that where high amperage loads are encountered, a smaller wire size will have a higher resistance. This resistance will cause heating in the conductor. In the same way that a small heat strip will spin your utility meter's dial, it will cost money in heat losses over the entire time the conductor is in use! Of course, the cost of the next higher size conductor may outweigh the losses over five to ten years, so BOTH should be considered. There is a formula to utilize for this calculation, however, due to its complexity, we will cover it in a future edition.

   Okay everyone, that's all for the 5 rules (plus the bonus rule!) of wire sizing for circuit conductors. Let me have some feedback and questions if you have them! Have a great week!~

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