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!

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.

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...

Voltage Drop Formulas

This week we will take a break from Grounding and Bonding to talk about Voltage Drop formulas. Voltage Drop calculations are arguably the most complicated, yet one of the most important items an individual electrician must perform. Total impedances and resistances of conductors may cause a substantial variation of voltages between the service supply and the voltages present at the point of utilization. Overly excessive VD's can severely impair starting and the operational running of equipment. Voltages that are too small for their nominal ratings cause substantially high inefficiencies in equipments, lighting, and heating. Even a small drop of only 10% of the rated voltage causes decreases of 15% for fluorescent lighting, and as much as 30% for incandescent lighting! Motors will run with less torque and operate at higher temperatures. Given the same 10% VD, the Full Load Amperage increases by 11%, the operating temperature rises by 12%, and the torque produced would DROP by 19%!



These figures showcase the field electrician's ability to help (or harm) the overall efficiency of an electrical system's installation. In today's climate, conserving energy is now a MAJOR consideration. Many electricians hate math and granted, voltage drop formulas seem only a step down from an engineer's level of calculation. Few, maybe as small as 10% of us, even remember the formulas off hand, much less apply them on a daily basis. However, the electrical engineer isn't present during an installation, YOU are! The EE has no way of knowing the actual routing or footage a particular circuit conductor may take. Thus the installer MUST be the person responsible for these calculations!



There are essentially five types of voltage drop formulas. These are,



(1):Direct Current VD (the most commonly used and the simplest one) that uses a "constant value" for "k" (specific resistivity),

(2): Direct Current VD that use individual resistance values from Chapter 9, Table 8 of the NEC,

(3): A/C resistance VD using an 85% Power Factor and multiplying factors from Chapter 9, Table 9,

(4): A/C resistance VD using the "Neher-McGrath" method for specific Power Factors other than 85% (the single most difficult formula to use), and finally,

(5): The "Mid-Point" calculation, used for multiple loads over a distance but on a single circuit. Here are the formulas written out:



(1) VD = 2*(K)*I*D

-----------

CM



(2) VD = 2*D*R*I

----------

1000



(3) VD (line to neutral) = Table Value * D * I

--------------------

1000' (Multiply times 2 for 2 pole circuits such as 240V etc...)



(4) Zc = (Rx * cos0) + (XL * sin0)





(5) Includes steps from 1 and 2 with averages (more about this formula next week)...



While none of these are NEC code requirements, they are listed as FPN's in 210.19(A)(1) FPN(4), and 215.2(A)(2) FPN(2). VD calculations are simply good and smart practices to include in your day to day work. The use of any of these formulas will give, in most cases, fairly close values, therefore, many use the simplist of them for ease of daily use. Formula 1 has, for the most part, become the "de facto" one to use in the field. All you must remember is that "k" is a constant value of 12.9 for copper and 21.2 for aluminum. The circular mils value is a quick reference in Table 8 and you're all set. (The 12.9 ohms value is derived by using the ohms per 1,000' of a given stranded wire, divided by 1000 to get the per foot values, then multiplied by the circular mils of the wire size. Interestingly, solid wire has a smaller "k" value of 12.6 ohms. However, use of the 12.9 value will give you the more conservative value.)



One final note to always consider is that all of these formulas and values are based on a temperature rating of 75 degrees C (167 degrees F). Any higher temperature would greatly increase the VD percentages. Temperature increases would use R1 [1 + .00323 (T2 - 75)], where T2 is your higher temp value in degrees and R1 would be the ohms value from Chapter 9, Table 8. (Use this formula for copper only).



All of this being said, VD calculations can be quite the time consuming, confusing, pain in the rear for the field electrician. Electrician Testing has created some very unique and very useful/helpful charts for VD. These charts virtually eliminate the need to remember any of the above formulas, values, and math! Contact us, via email, for a copy of them. Next week we should be back to grounding and bonding! Hope you have a great week, and let us know, as usual, if you have any specific questions....

Grounding and Bonding Part II

PART II

Last week we discussed a discovery where two panels, in a service for a newly converted gym, were not properly grounded. Article 250 is normally considered to cover two specific topics, Grounding and Bonding. However, it really covers three distinctly separate issues, and those are Grounding, "Earthing," and Bonding. Many in the industry are beginning to separate and treat the "Earthing" component as a majorly separate area. This helps to distinguish between grounding WITHIN a system FROM the grounding OF the system. Our issue here is more of an Earthing concern, where we must insure that stray currents and our "intentionally" grounded conductors have a safe and unfettered path to earth.


The initial 2008 NEC section to confer is Art. 250.52. This article explains what your accepted and physical grounding electrodes must be. Under "Electrodes Permitted for Grounding," there are 8 classes of Grounding Electrodes (GE) and 2 that are specifically prohibited. In most retro-fit remodel jobs, we would use two of these classes the most. These are our "Water Pipe" rule and "Ground Rods." We find the first of these two in section 250.52(A)(1) and the rods under 250.52(A)(5) "Rod and Pipe Electrodes."

There are a couple of installations rules to follow with these two types. The biggest of these is the so called "5-foot" rule. A water pipe that is FURTHER than 5 feet from the ENTRANCE of the building CANNOT be used as an acceptable GE. (Note, there is an exception to this; however, very FEW remodel jobs would ever meet the qualifications for this exception). We must also ensure that a minimum of 10' of continuously bonded metal pipe and casings exist that make DIRECT contact with earth for the entire minimum length. Water meters and the like must be jumpered over with a bonding conductor. Under Article 250.53 we also find a combining rule that REQUIRES an additional supplemental GE be used when we utilize the water piping system as a GE. Thus we see that we're required to use a ground rod or something similar in conjunction with our water pipe.


Ground rods must be at least 8' in length, and according to 250.53, 8' of that length MUST be in contact with soil. Most ground rods are manufactured in 10' lengths so that the bonding connector would not have to be buried in soil. Clamps that are buried must be listed for direct burial. The rod must be driven vertically, unless rock bottom is encountered. (In the Austin area, that is an extremely common situation. In that case, a 45 degree drive/burial angle is allowed or it may be completely buried laying flat, under a minimum of 30" of soil. DO NOT bend your ground rod into a 90 degree "stub up" configuration, where the top portion is above ground and the remainder is horizontally flat. This is a poor practice that is not only a code violation but is also a functionality liability! A situation that I've see and run across too many times beforehand)

There are two final rules to consider. The first is that you are not required to use a bonding jumper for the supplemental GE rod that is larger than a #6 AWG CU conductor. The second is that your resistance reading between the rod and earth shall be 25 ohms or less. The use of special grounding meters should be utilized to determine this value. Where it is greater than 25 ohms, additional rods should be installed at a distance NOT LESS than 6 feet apart. (See Art. 250.56).

Next week we will talk about how to apply Table 250.66 and review more installations rules and requirements. If you have specific questions about any grounding or bonding issues, we encourage you to send us an email. We will respond promptly and your question may even be featured on our web site!

Grounding and Bonding

Grounding and bonding are two words that cause a lot of electricians to flinch or roll their eyes. One can hardly blame that kind of reaction due to the complex nature of the extensive rules for "earthing," "grounding," and "bonding." In fact, Article 250, "Grounding and Bonding" in the 2008 NEC is the single most largest section in the entire code book! There were 40 significant code changes from the 2005 NEC edition, making it also one of the most changed articles as well. With 81 separate sub-sections, there is a tremendous amount of material to absorb and understand. It is a foregone conclusion that anyone preparing for ANY type of electrician's exam must study and know Article 250 fluently. There have been many books, articles, and studies published on this subject. There is an entire industry in and of itself surrounding this subject alone. So don't feel bad if you are a little shaky on this subject. Over the next few weeks you will find several blogs on our site dealing with various parts of Article 250.



Last week I visited a job site where an older residential home was being converted into a commercial type workout gym. One of the electricians on the site had made himself a pretty ingenious device to trip circuit breakers. It was a single pole toggle switch mounted in a single gang bell box with a cord cap attached to about 36" of SO cord. He'd simply plug it in and flip the toggle switch, thus causing a purposeful direct short. (he must have had the equipment grounding conductor on one side of the toggle and the ungrounded conductor on the other side) He had also left one end of the bell box unplugged to allow the arc flash or heat to have a convenient path for discharge.



In this building, he mentioned to me that every time he employed his shorting device, he'd noticed a much louder and larger arcing inside his device. I immediately had a suspicion that the system may have a poor grounding system, thereby creating a much larger potential resistance to ground to overcome, "forcing" the circuit breaker to function. There were two separate 100 amp panels in the basement. Each panel had its own service conductors feeding it from an outside utility transformer. (a very rare and odd set up) After removing the dead fronts, I discovered that there was NO grounding electrode conductor what-so-ever! I was extremely surprised and I would have chalked it up to simply being a case of an old service that had not ever been brought up to code. BUT, these were obviously newer panels and it was very apparent that at least one panel was less than two or three years old. I have to assume that it must have just escaped notice, however, this was a serious safety over-site by those technicians in the past!


One panel had a neutral/ground bond, the other did not. The two grounding bars between the panels had been bonded together with a #8 AWG copper conductor. We must treat these two panels as two separate services. Our controlling Articles would be: Art. 250.52, 250.53, 250.56, 250.64, and finally Table 250.66. Next week we'll go into detail on how these specific articles apply to this installation.

Part II

Last week we looked at the proliferate use of service cords in the electrical industry. We left off after looking at our ampacity rating tables. Our 30 A load ultimately needed a size 8/4 service cord (SC). According to T400.5(A), in the 2008 NEC we find an 8/4 rated at 35 amperes as its maximum rated ampacity. SC installations must also follow other de-rating factors for; numbers of current carrying conductors, ambient temperature corrections, as well as other installations restrictions.


In a similar method to how we treat other wire types, we must also de-rate anytime an installation of SC's contains either: more than three (3) current carrying conductors (CCC) or where our ambient temperature rises above 86 degrees F. First begin by obtaining your "Core Ampacity" (as in the case above it was 35A for an 8/4, according to table values). The next step (step II) is to de-rate where there are 4 or more CCC's. SC's come in many different conductor counts, but they usually range from 2 to 6 conductors. Our rule for grounded conductors ("the neutral") follows us from 310.15(B)(4), see 400.5(B). If the "neutral" only carries the imbalance load from different phased conductors (ie 'shared neutrals'), we do NOT have to count it as a current carrying conductor. Where it is the "load side" of a 120V circuit (as an example, or in any other single phase voltage branch circuit) you MUST count it as a full current carrying conductor. T400.5 adjusts by a percentage of our core ampacity value. It is the same exact table as you see in T310.15(B)(2)(a). Ambient temperature factors would be used from T310.16 under the temperature column that corresponds to the temperature rating of the Service Cord. (*Note SC temperature ratings are NOT shown in T400.4 and should be taken from the printed or scribes ratings on the outer jacket of the cable). Portable power cables, types G, PPE, and W, and Flexible stage and lighting cables, types SC, SCT, and SCE; follow their own pre-calculated temp rating/de-rating in T400.5(B).



I have also noticed SC's installed in commercial buildings as substitutes for permanent wiring. One contractor used it for branch circuit wiring to feed under-cabinet lights. His installer had fished it into several wall cavities. He explained that he couldn't get the bend radius neccessary to conceal it using MC Cable. Unfortunately for him, he had to remove it. 400.8 specifies 7 specific uses that are NOT permitted when installing SC's. Essentially the rules are: 1) Not used as fixed wiring of a structure; 2) Not run through walls, doors, windows etc; 3) Not concealed by walls, floors, or suspended ceilings; 4) Not installed in raceways, and finally 5) Not where subject to physical damage. If you are ever tempted to run a SC inside a conduit or raceway, DON'T do it! Not only does that suggest a more "fixed type wiring" scenario, but you are not permitted to encase an SC in a raceway (unless specifically permitted to do so under any other section of the NEC or where in an industrial application, under certain restrictions, a maximum of 50' of raceway may be used as a means to protect a SC; see Art. 400.14).


In our earlier scenario where we had a 3 phase, 480V, 50HP motor, our first step is to find the FLA of the motor in T430.250. Under the 460V, 50 HP column we see that to be 65 amperes. Referring back to T400.5(A), our SC Ampacity chart, we find that to achieve this per code, we'd need a size #2 service cable. That is substantially larger than what was currently installed. The #6 being utilized is only rated for 45 amps, a 20 Ampere Deficit!


Finally, to recap, Service Cords are NOT "ordinary wiring." They follow specific rules and MUST be treated in a different manner. The ampacity ratings are much less than typical wiring. The installation restrictions are numerous. It's even interesting to note that you may not install a NEW SC service that has ANY splices or taps. You may repair an existing usage, but you may NOT reutilize it with the splice remaining. Strain relief to keep tension off of joints and terminations must be employed. The voltage rating on many of them is only a maximum of 300V so be cautious that you match the proper insulation voltage rating with the voltage supplied by the service. For example, a Junior Service Cord, type SJO etc... could NOT be used for a 480V, 3 phase motor application. They cannot be encased in raceways, above suspended drop ceilings, or be used inside walls. I urge you to re-read the entire Article 400 to re-familiarize yourself with the SC installation requirements. As always, be safe and remember your responsibility is to keep others safe from your work!