Electrical Forum - Transformer article, part 1

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Subject - Transformer article, part 1
Ryan_J	Transformer Inspections (part 1 of 2) If I were to make a list of the electrical inspection items that I most commonly fail, I think transformer installations might very well be at the top of the list. Although many people consider transformer installations to be rather simple, there are many rules that govern their installations, some of which are quire subtle. Transformers ratings are measured in Kilovolt-amps, where “K” is 1,000, and volt amps is simply voltage multiplied by amperage. For example, a single phase 75KVA transformer that has a 480 volt primary and a 240 volt secondary would have an amperage of 156 amps on the primary side (75000/480) and 312 amps on the secondary side (75000/240). The same concept applies to three phase transformers, only there is one additional step required, which is incorporating the square root of three (1.732) into the equation. For example, a 3 phase 75KVA transformer with a 480 volt primary and a 208 volt secondary would have an amperage of 90 amps on the primary side [75000/(4801.732)], and would have an amperage of 208 on the secondary side [75000/2081.732)]. The first rule I would like to discuss is the (earth) grounding of transformers. Since transformers are a separately derived system as defined in article 100, the rules of 250.30 must be followed. Sections 250.30(A)(3) and 250.30(A)(7) of the 2005 NEC discuss this in much clearer detail than the 2002, due to the entire section being reformatted. The basic rule is that the metal water piping system of the building (within 5’ of the point of entrance), or the structural metal of the building, whichever one is closer, must be used as the electrode. The grounding electrode conductor from the transformer to the water pipe or structural metal must be sized in accordance with 250.66, based on the “derived phase conductors”, which are the secondary conductors. For example, the three phase 75 KVA transformer in the above example would probably have 3/0 copper secondary conductors. This would require a 4 AWG grounding electrode conductor to the electrode. While checking this on the inspection, be mindful of the rule in 250.64(E) that tells us that if the raceway containing this conductor is metallic, it must be bonded at each end. The next rule I would like to bring up is overcurrent protection. Section 240.21(C) and table 450.3(B) are the sections that are most commonly violated here. 240.21(C) governs the protection of the secondary conductors. The interesting thing about this section is that it prohibits the use of the “next size up” rule of 240.4(B). What this means is that the secondary conductors must be protected at or below their ampacity, without exception. For example, on a 112.5KVA transformer, the secondary has an amperage of 312 amps. This means that generally speaking, a 300 amp circuit breaker would be installed at the termination of the secondary conductors. In most electrical applications, a 300 Kcmil conductor could be used, despite it only being rated at 285 amps. On a transformer secondary, however, these conductors would have to be rated at least as much as the circuit breaker (300), which would require 350 Kcmil conductors. Overcurrent protection on the primary side is a little easier. Table 450.3(B) gives us the values that must be followed in order to protect the primary side. Typically, the “currents of 9 amps or more” column would be used. On the 3 phase 75 KVA transformer that has an amperage of 90, a 125 amp breaker could be used because of the allowance of note 1 to the table (see 240.6) [90*125=112.5]. Please remember that this is a “maximum” value and therefore cannot be exceeded. To be continued next month… Ryan Jackson 2nd VP, Utah IAEI Ryan@Draper.ut.us
blackrd	Now that is the kind of stuff I came here to read and learn about.
Ryan_J	Thank you. I will continue to write these on a monthly basis and post them when they are done.
stedder	Ryan, after reading your second post on this subject I came back to read your first. Thanks very much for this (handbook style) look at this subject, I feel I should be compensating you for the seminar. Thanks again, Bob
Ryan_J	Bob, it's my pleasure, really. Thank you for your kind words, by the way.
wareagle	Ryan It's a very good article. I have to nit pick one statement. You posted "The basic rule is that the metal water piping system of the building (within 5’ of the point of entrance), or the structural metal of the building, whichever one is closer, must be used as the electrode." It sounds as though you have a choice of the water pipe or the steel. If both are available you must use both. I am sure thats what you intended to say.
Ryan_J	Actually, thats not correct. You only have to (earth) ground the closer of the two, but you have to bond them both, as per 250.104(A)(4) and it's exception.