Sunday, July 25, 2010

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



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



(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....

Monday, July 19, 2010

Grounding and Bonding 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!

Wednesday, July 14, 2010

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.

Friday, July 9, 2010

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!

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