I was doing some research and found some great calculators online that you can use for the field.
Concrete Materials Calculators
http://www.concrete.com/calculators/concrete-materials-calculators
and
Cubic Inches to Cubic Yards converter
ENJOY!
Showing posts with label electrician testing. Show all posts
Showing posts with label electrician testing. Show all posts
Wednesday, April 13, 2011
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.
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.
Tuesday, December 29, 2009
Wire Ampacity and Conduit Fill Calculation
Wire Ampacity and Conduit Fill Calculation
This week I want to take a minute to review our wire ampacity and conduit fill (wire fill) calculation.
There seems to be a lot of common confusion out there when it comes to wire fill. Annex "C" in the back of the 2208 NEC © is a good tool, but it can be very dangerous to use it without considering the impact of de-rating factors found in Chapter 3.
We normally use a 20 amp circuit breaker to protect #12 AWG cu wire. Annex C tables show that we can safely place a total of 16 of this size (Type THHN, THWN, or THWN-2) in a 3/4" EMT conduit. this is only for the protection of the wire from a physical stand point. Annex C is only concerned with the protection of the wire during installation. It does NOT take the ampacity of the wire into consideration!
When de-rating wire, we begin with table 310.16 (page 147, 2008 NEC ©) and de-rate for our ambient temperature AS WELL AS the number of current carrying conductors. The de-rating table T310.15(b)(2)(a) on page 145 shows us:
"Adjustment Factors for more than Three Current carrying conductors in a Raceway or Cable"
| Number of C.C.C. | % of 310.16 Values |
| 4-6 Conductors | 80% |
| 7-9 Conductors | 70% |
| 10-20 Conductors | 50% |
| 21-30 Conductors | 45% |
| 31-40 Conductors | 40% |
| 41+ Conductors | 35% |
You can quickly see that in our earlier example, if we put 16, #12 THHN conductors in that 3/4" conduit, we would have to drop the circuit breaker to a 15 ampere size! Now seriously, how many electricians in the field really do that? You understand now how dangerous the casual use of Annex "C" can be.
For all practical purposes, only 9 current carrying conductors could be placed in that conduit without having to drop the OCPD (circuit breaker) size: [T310.16 value for #12 THHW=25 Amps. 25 Amps X 70% (the de-rating for 7-9 conductors) would equal 17.5 Amps, and due to 240.3(B) we are allowed to round up to a 20A breaker.]
The grounded conductor (neutral) can also cause confusion sometimes. It counts as a "current carrying conductor" in some cases, but not in others. If it is shared between two different phases in a multi-wire circuit, it usually does not count as a current carrier , as it only carries the unbalanced load in 
the circuit. [For example, a "full boat with circuits #3, #5, #7 in a 30 panel, would not usually have their neutral counted.]
Where non-linear loads are supplied however the neutrals DO count. If you have electronic fluorescent ballasts or heavy computer loads (etc...) on a shared branch circuit neutral, it will have harmonic distortions that will disrupt the cancellation of loads between phases and can actually cause larger loads on a neutral that are found on any single ungrounded phase conductor in the circuit.
If the (neutral) grounded conductor is NOT shared between phases, i.e. a single 120V single phase branch circuit, such as a general purpose receptacle outlet circuit conductor, it counts as a current carrying conductor. In this installation it carries the full load on it back to the distribution point. Two other common issues with Annex C are considerations for different sized EGC/GEC conductors, and differences between Annex C values and manual fill calculations using Chapter 9, tables 4 and 5.
The value of Annex C is a "staff it full" value. Therefore you would have to keep in mind your additional ground wire, which is usually a smaller size than that of the phase conductors. The only way to use Annex C in this case is to always consider your EGC or GEC as a full size conductor count. A manual calculation is more accurate than Annex C and it has been this author's observation that Annex C values sometimes vary quite a bit from a manually calculated fill based on Chapter 9 tables.
All of that considered, going back to our 3/4" raceway, we could put 3 "full boats" in this raceway (3 neutrals not counted, 9 phase conductors, and 1 equipment ground) for a total of 13 conductors. This results in 3 less than our "fill amount" according to our Annex C tables. Thus, our advice is to always carefully consider your ampacity de-rating factors as required by 310.15.
(Author's Note: As evidenced by the above, an electrician would have to refer to several different tables in order to correctly and safely calculate the correct fill and ampacity values for watch and every pipe run. we are excited to provide and industry first for all our members. Electrician Testing has painstakingly prepared a custom table; available only through us, that considers ALL required factors. Everything is brought together in one quick and easy to follow chart. Contact us today to get yours, and look for our upcoming publication that will include this chart along with many, many more custom time saving tables!)
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