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!~

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 for your test preparation material.  Check back often since we give away helpful stuff for use on the field and the test room!

Wednesday, August 11, 2010

Circuit Calculations (The 5 Major Considerations)

     This week I'd like to take a quick look at the "Big 5" considerations when calculating a branch circuit. Often times in our rush to complete a project, we will over look one or more of the following items. However, each single item is a very important part of a successful, economical, intelligent, and efficient installation.

     There are actually five separate and distinct considerations (rules) when sizing wire for an electrical installation. These 5 rules are:

A)     (Insulation Rating and  Ampacity Sizing)
         The insulation must be able to withstand the heat generated by the current flow without damage or risk of fire.

B)     (Conduit Fill and Physical Strength)
         It shall be of sufficient mechanical tensil strength in order to withstand the stresses incurred within a normal and expected (reasonable) installation.

C)     (Cost per Sizing)
         It should be sized per economic considerations so as not to exceed those limitations.

D)     (Voltage Drops)
         It should be sized large enough to accommodate losses due to voltage drop considerations.

E)      (Energy/Heat Losses)
          It should be sized so that the cost of the Current (squared) Resistance energy (I2R) losses would not be excessive.

     There is a further consideration for "future expansion or load requirements." This is not a "Code" requirement by any definition. It is also something that the average "low bidder" does NOT want to have to bear the burden of cost, as it is an upfront investment for the owner of the property. However, a good design, upfront with potential and foreseeable future loads, having been taken into consideration, ultimately gives the end user a tremendous value in their property's ability to expand easily. It also can potentially eliminate situations whereby future installers would be tempted to "overstress" the current system due to the cost and expense of having to completely replace an existing electrical installation. Therefore, even though code calculations would allow for an 80% lighting load on a single 20A circuit (for example), it might be more prudent to limit the installation loads to only a 50% capacity. That would allow for a much more flexible system for easy additional lighting to be moved, added, or taken out. This additional expense upfront is negligible, however it would be a considerable expense for a later technician to rip it all out and install larger provisional circuits in the future. So one can easily see how some common sense and minimum expense upfront may make a enormous positive difference to the client's property.

     Rule A) is where our Tables in 310.15 and 310.16 come into effect. This is a major reason why we instruct our students to rely on their table 310.16 on page 147 of the 2008 NEC from "Temperature Rating of CONDUCTOR" to "Temperature Rating of INSULATION." It makes the application of this table more sensible and easier to grasp it's correct usage. I have gone over Ampacity rating of conductors in previous blogs, so I won't re-visit it in this edition. It is one of the more commonly recognized and applied rules of the 5 above rules.

     Next week we will take a longer look at the other 4 major rules. I will take you step by step through them so that you'll be able to see the reasons behind each of the rules, and why they not only make sense, but they're MUSTS in creating a good, safe, and usable electrical installation.

Sunday, August 1, 2010

Grounding & Bonding, Part III

This week we finish our Grounding and Bonding installation for the Gym remodel project. In previous blogs we discussed two separated services that enter the building, supplying two separate panels, each with its own set of feeders.

     Our initial grounding choice might be the "Metal Underground Water Pipe" as is specified in 250.52(A)(1). I hesitate to utilize this method however, due to the fact that it can be very difficult to verify the "minimum 10' of direct earth contact" rule. There is simply no easy method to verify that the cold water service doesn't change over to PVC, two feet or so after it goes underground. Therefore, we might bond it regardless, but we would not want to consider it our primary grounding electrode.

     Thus, we're left with a Rod, Pipe, or Plate electrode type. Ground rods are the most commonly installed, and we'll most likely use these in this application. The only draw back to this, is that we MUST then perform a final resistance to ground test, when the installations are completed, in order to comply with 250.56's rule of 25 ohms or less.

     We would size our Grounding Electrode Conductor (GEC) from table 250.66, ignoring the maximum rule of 250.53(E). That rule allows us a maximum of a #6 AWG, but it only applies when the ground rod is a supplementary electrode -- not the sole electrode as is true in this application.

     T250.66 sizes the GEC based on the "Largest Ungrounded Service Conductor." I found 2/0 CU feeders in each panel, thus we would be in the "2/0 to 3/0" row and find a #4 AWG CU would be required. We would encase it to protect against physical damage in accordance with 250.64(B).

     Because there are two separate services, each would receive its own identically sized GEC installation based on the service conductors present in each panel. We would not be allowed to tap the two together. Article 250.28(D)(2) instructs us to supply a main bonding jumper to each service disconnect. We bond each panel per 250.24(A)(1).

     One final note, if we use a metal conduit to encase the #4 GEC, we MUST bond EACH end of the conduit with a grounding type clamp or fitting, attached to the same size bonding jumper as we used for the GEC. This prevents dangerous "choking" potential during high voltage discharge releases, such as found in lightening strikes etc...

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