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.