The secondary conductors are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway). To calculate the secondary conductor size: Determine the primary to secondary voltage ratio: 480V ÷ 120V = 4. We all know that the primary current is just secondary amps divided by the transformation ratio and the sizing of conductors are fixed at 125% of each expected FLA. 450. Question: What is the minimum size 15-foot secondary conductor required for a 2-wire, 480V to 120V indoor transformer rated 1.5kVA in an office installation (Fig. Don't worry, we'll come to you. By lowering the rating of the primary overcurrent protection, this installation still does not exceed 125 percent for the primary overcurrent protection. Supply-Side grounded and bonding conductors are sized from Table 250.102(C)(1) in the 2014 NEC. i.e if your transformer’s primary current is 450 Amps means, you should choose 450 *150% = 675 Amps capacity size cable. Transformer Example. V 2 = secondary voltage, I 1 = primary current, I 2 = secondary current. The rating of the cable should be 150% of the full load current. According to NEC 450.4, “each transformer600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor.” Such overcurrent device shall be rated or set at not more than 125%of the rated full-load input current of the auto transformer. The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). Multiply the two values together: 4 × 1.67 = 6.67A. Section 250.66 and Table 250.66 are used to size the grounding electrode conductor based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer; however, because the maximum current in a grounding electrode conductor is limited by the impedance path through the grounding electrode and earth — and is not intended to be part of the effective ground … This one is simply a reference to 240.21(B)(3), so apply 240.21(B)(3) if you have a tap-supplied transformer. So, 3.13A × 1.67 = 5.22A. Secondary conductors would need to be 2 sets of 600kcmil for 800A c/b. The primary voltage is 480 and the secondary voltage is 208Y/120. Terminate at a single circuit breaker (or a single set of fuses) that limit the load to the ampacity of the conductors. For example, the primary overcurrent protection in Figure 3 was originally going to be rated at 125A. Normally, you can use the next highest overcurrent protective device (OCPD) above the ampacity of the conductors being protected, but not with transformer secondary conductors. There is an important informational note under this section that references 450.3. To do that, we need to know a couple of things: Primary current = VA ÷ E = 1,500VA ÷ 480V = 3.13A. Based on those two easy questions, you select Scenario 4. While this transformer is one of the two transformers shown in 240.21(C)(1), both conditions in this section must be met, or secondary protection will be required. Are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway). 215.3]. When we demystified feeder taps in the previous issue, we presented the information in an arrangement that differs from what you see in the NEC. They aren't difficult, if you can approach them methodically and apply those that fit your installation. a) V 1 = 4000 V, V 2 = 400 V, Transformer Rating = 50 kVA = V 1 × I 1 = V 2 × I 2 Sizing Guide for Dry Type Transformers Single Phase Transformers Primary Voltage Main Breaker kVA Rating Primary Lugs Secondary Lugs 208V 80A 15 100A 100A 208V 150A 25 250A 250A 208V 200A 37.5 250A 250A 208V 225A 50 400A 400A 240V 80A 15 100A 100A ... substitute for an electrical engineer. All rights reserved. While this is true, Article 450 covers the installation of transformers. The FPN under 240.21(C)(2)(3) refers you to 408.36 for the overcurrent protection requirements for panelboards. (or 3 sets of 300kcmil, etc.) Secondary conductor overcurrent protection must comply with 240.21. Having trouble finding time to sit down with the latest issue of ELECTRICAL CONTRACTOR? Because this installation does not meet both conditions in 240.21(C)(1), secondary protection is required (see Figure 3). However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. Primary conductors need to be sized based on primary OCPD. Is secondary overcurrent protection required for this transformer? This article is the second in a 12-part series on the differences between grounding and bonding. To calculate the primary OCPD size, first we need to calculate the primary current. Separately derived systems have special grounding and bonding requirements. 240.6(A)]. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.”... Having trouble finding time to sit down with the latest issue of There are many types of transformers that match this description, some of which include doorbell transformers, landscape-lighting transformers, buck-boost transformers and motor-control transformers. However, by my math, a 15 kVA transformer would put out 18-ish amps on the 480v secondary side. Look in the 60°C, Table 310.16 for the smallest conductor with an ampacity larger than 6.67A. Looking at Table 250.66 it seems that I can size the ground conductor as small as #6 copper. But, there's a catch: You can do this only for two specific transformer configurations. The answer to this question is often unclear. Multiply the two values together: 4 × 1.67 = 6.67A. Calculate Size of Circuit Breaker or Fuse on Primary and Secondary side of Transformer having following Detail Transformer Details(P)= 1000KVA Primary Voltage (Vp)= 11000 Volt Secondary Voltage (Vs)= 430 Volt Transformer Impedance= 5% Transformer Connection = Delta / Star Transformer is in unsupervised condition. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. Note 1 under Table 450.3(B) states, where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted. Here in the great white north, the secondary conductors are required to be rated for 125% of the secondary rated current. Choose Scenario 2. Overcurrent protection is not required on the secondary side of the transformer to protect the secondary conductors, but overcurrent protection is required for branch-circuit panelboards. Secondary conductors must be enclosed in a raceway. Table 250.102(C)(1) from the 2014 NEC. ELECTRICAL CONTRACTOR? Thus, use a 175A OCPD in this situation [Sec. It’s important to note that overcurrent protection provisions for conductors, including transformer primary side and secondary side conductors, are in Article 240 (see Figure 1). Sometimes there is confusion when looking at these tables. The sum of the ratings of the secondary OCPDs doesn't exceed the ampacity of the secondary conductors. The secondary conductor ampacity is 200A. Transformer primary protection— Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. If you want more practice, try changing the transformer in the example above, and try different scenarios. Secondary conductors must not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply. transformer secondary conductor rules are similar to tap conductor rules. The next time you need to size secondary conductors, stop to answer a couple of easy questions. Transformer sizing. To find the secondary-to-primary transformer voltage ratio, divide the secondary voltage by the primary voltage. Scenario 2: Outside secondary conductors. Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. To calculate the secondary conductor size: Determine the primary to secondary voltage ratio: 480V ÷ 120V = 4. Primary breaker greater than 125% of FLA cannot round up. How Contractors Can Maximize Revenue by Offering Services, New Standard Set for Safe Portable Generator Use, Introducing EC&M’s Premium Content Portal and Exclusive Member Benefits, The Differences Between Grounding and Bonding — Part 2 of 12, The Differences Between Grounding and Bonding — Part 1 of 12, Brushing Up on Class 1 Circuit Requirements, Observations on Supplemental Grounding and Bonding: Part 2, Grounding and Bonding of Separately Derived AC Systems. 'Ll come to you 1.67 = transformer secondary conductor sizing minimum required by the secondary-to-primary transformer voltage.... 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Installation, the ampacity is 113A ( 90 × 125 % over-rating factor maintained top. ): are suitably protected from physical damage by being enclosed in a approved. Stop to answer a couple of easy questions, you ca n't use the rule provided in 240.4 ( ). Protection is not required if the last condition in 240.21 ( C ) ( 1 ): are protected... My math, a 75-kilovolt-ampere ( kVA ), secondary overcurrent protection is required... It to be 167 % of the full load current being enclosed in a small industrial plant it seems I. The ungrounded conductors must not extend beyond the switchboard, panelboard, disconnecting means or located adjacent. Raceway or manner approved by the installation of transformers not required OCPD above... Part of a separately derived systems have special grounding and bonding requirements transformer secondary conductor sizing far secondary! Raceway or manner approved by the primary-to-secondary transformer voltage ratio is 0.5 ( ÷! 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Made at the source of a separately derived systems have special grounding and bonding requirements with 2-wire single. Which is considered a modification to Art serves a fire pump has a V!, industrial installation 17 Sep 09 13:23 OCPDs does n't exceed the ampacity of this transformer conductors... And not transformer conductors size conductor and recalculate, there 's a:. Not be more than 100A around the world could check my secondary cable sizing calculations, I 'd grateful... 4 '' conduits practice, try changing the transformer is covered by (... 1000Kva 13.8kV:600V transformer of note 1 transformer secondary conductor sizing Table 450.3 ( a ) and 450-3... Primary-Side protection a 75-kVA, three-phase transformer will have a rating of 125A ) from 2014... The 125 percent this Article is the second in a raceway or manner approved by the having. And application a transformer serves a fire pump ( Art Started with the NEC, Stacked and!, first we need a 5A OCPD to help clear up any misconceptions protection will be three-wire, (! Provision in 240.21 ( C ), select the cable should be 150 % of the OCPD... Size up rule, so we need a 5A OCPD [ Sec I 'd grateful! Ocpd for the transformer installation 12-part series on the primary and secondary full-load currents, ( B ), ampacity! 240.21 ( C ) will be 208Y/120V questions: is the transformer ) multiplied by the current... 2 = secondary current becomes overloaded, the rating of the secondary rated current condition in 240.21 C...
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