After recalculating, the result is 115 (230 × 0.5 = 115). I am assuming copper conductors, rated for 75 deg C. My calculations are as follows: (1) Secondary Current I_sec = (1000kVA/sqrt(3)/600V) = 963A Outside secondary conductors can be of unlimited length, without overcurrent protection at the point they receive their supply, if they (Fig. The secondary conductor ampacity shall not be less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer [240.21(C)(6)(1)]. The secondary conductors are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway). Primary conductors need to be sized based on primary OCPD. 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 The secondary conductors must meet the sizing minimum required by the installation type described in 240.21(C). After multiplying the primary current by 125 percent, the ampacity is 113A (90 × 125% = 112.5 = 113). 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. Look in the 60°C, Table 310.16 for the smallest conductor with an ampacity larger than 6.67A. This segment takes a look at some significant revisions in chapters 2 and 3 (with the comment or proposal number cited... © 2003-2021. While secondary overcurrent protection is not required for the transformer itself, overcurrent protection is required for the conductors. To determine if the last condition in 240.21(C) will be met, recalculate with the next larger size conductor. A: Overcurrent protection for the transformer is covered by 450.3(B) and Table 450-3(B). The feed will be underground in 4" conduits. Normally, you can use the next highest overcurrent protective device (OCPD) above the ampacity. Article 240.21 of the National Electrical Code (NEC) covers the location and sizing requirements of overcurrent protection for tap conductors and transformer secondary conductors. Introduction A common question is whether transformers, transformer secondary conductors, and overcurrent protection are independent components or parts of a system. V 2 = secondary voltage, I 1 = primary current, I 2 = secondary current. But, there's a catch: You can do this only for two specific transformer configurations. Secondary conductors must be enclosed in a raceway. Multiply the primary OCPD size by 1/3: 5 × 1/3 = 1.67. © 2021 Endeavor Business Media, LLC. We also described an easy way to pick the correct scenario from the five possible. Assuming an ideal transformer, determine (a) the primary and secondary full-load currents, (b) the transformer turns ratio. Are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway). 695), its primary OCPD s… ELECTRICAL CONTRACTOR? There is an important informational note under this section that references 450.3. All rights reserved. I have a transformer with a secondary full load current. If a transformer serves a fire pump (Art. The primary or input voltage will be 480V, and the primary conductors will be 1 AWG THHN conductors. A transformer is a static electrical machine that transfers AC electrical current between two more more circuits. Where installed in accordance with one of these six rules, a set of conductors feeding a single load, or each set of conductors feeding separate loads, can be connected to a transformer secondary without overcurrent protection at the secondary [240.21(C)]. All you need to know about the witness test, Access in Drywall Ceilings, Type P Cable and More, The Cycle Begins: The author looks at the 2023 NEC process from the outside after three decades, Psychiatrist’s Office, Pool Inspection and Testing and More. 1): Are suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction (AHJ). 3-phase delta-delta transformer with 3-wire (single voltage) secondary. Per 450.3(B), it has to be 167% of the primary current. Terminate in a single circuit breaker (or set of fuses) rated no more than the secondary conductor ampacity per 310.15 [Table 310.16]. The next larger size is 250 kcmil and the ampacity of this conductor is 255A. Then, do the same thing but choose Scenario 5. The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). The primary voltage is 480 and the secondary voltage is 208Y/120. To calculate the secondary conductor size: Determine the primary to secondary voltage ratio: 480V ÷ 120V = 4. Scenario 4: Secondary conductors 10 to 25 feet long. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. To calculate the secondary conductor size: Determine the primary to secondary voltage ratio: 480V ÷ 120V = 4. Choose Scenario 3 or 4, but choose Scenario 5 if it's an industrial installation. This transformer is rated 75,000 volt-amperes (75 kVA × 1,000 = 75,000). transformer secondary conductor rules are similar to tap conductor rules. 240.6(A)]. This starts with estimating or calculating the demand load using NEC Article 220 and then applying and applicable demand factors. To calculate the primary OCPD size, first we need to calculate the primary current. Secondary conductors must not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply. If the transformer installation does not meet both conditions, secondary protection will be required. Subrules (1) and (2) require transformer primary and secondary conductors to have an allowable ampacity of not less than 125% of the rated current. Secondary conductor overcurrent protection must comply with 240.21. The rule is the same because the physics are the same. Looking at Table 250.66 it seems that I can size the ground conductor as small as #6 copper. This starts with estimating or calculating the demand load using NEC Article 220 and then applying and applicable demand factors. If someone could check my secondary cable sizing calculations, I'd be grateful! Terminate at a single circuit breaker (or a single set of fuses) that limit the load to the ampacity of the conductors. A summary of electrocution incidents from around the world. 2): The rating of the device supplied by the secondary conductors or the OCPD at the termination of the secondary conductors, and. This one is simply a reference to 240.21(B)(3), so apply 240.21(B)(3) if you have a tap-supplied transformer. It would also be permissible to increase size of the secondary conductors instead of lowering the primary overcurrent protection. All rights reserved. Don't worry, we'll come to you. 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. In this video we run through the sizing of primary and secondary conductors required by the 2015 23rd edition of the Canadian Electrical Code. With some transformer secondary conductors, you can use the primary OCPD to provide the overcurrent protection (just as you use the feeder OCPD to protect the feeder tap conductors). Conductors connected to the secondary side of a transformer are not protected with an overcurrent device and, therefore, are not protected against overcurrent in accordance with their ampacities specified in 310.15. Separately derived systems have special grounding and bonding requirements. 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. A three-phase, fused disconnect with 125-ampere (A) fuses will be installed on the primary side of this transformer. 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. The secondary for this transformer will be three-wire, 240V (single-voltage), and the conductors will be 3/0 AWG THHN. Secondary Conductor Size: Secondary conductors must have an ampacity rating not less than the rating of the overcurrent protective device at the termination of the conductors in accordance with Table 310.16 based on 75°C terminal rating [110.14(C)]. While this is true, Article 450 covers the installation of transformers. Then, select the correct scenario for your installation. For example, a 75-kVA, three-phase, delta-delta connected transformer will be installed in an industrial facility. If the primary overcurrent protection is lowered to 100A, the installation would meet all the conditions in 240.21(C)(1). Calculations: Transformer Primary Current (Ip)= P/1.732xVp Transformer … Now we can look at the requirements for sizing conductors for transformers. Thus, use a 175A OCPD in this situation [Sec. Title: 139-0553-00 Transformer Sizing Guide.indd Tables 450.3(A) and (B) pertain to overcurrent protection for transformers and not transformer conductors. Article 240.21 of the National Electrical Code (NEC) covers the location and sizing requirements of overcurrent protection for tap conductors and transformer secondary conductors. Your secondary breaker can be 800A. The first rule for transformer secondary conductors pertains to only two types of transformers; the first is a single-phase transformer having a two-wire (single-voltage) secondary. Choose Scenario 1. Multiply the two values together: 4 × 1.67 = 6.67A. In accordance with Table 450.3(B), secondary overcurrent protection is not required if the primary protection does not exceed 125 percent. To do that, we need to know a couple of things: Primary current = VA ÷ E = 1,500VA ÷ 480V = 3.13A. Now, let's see if you can put all this together to solve a practice problem. According to Table 310.16 the ampacity of #6 is 65 amps but on the seconary size, the transformer will draw (75000kva)/ (480V) (1.73) = 90.32 amps at full load. I'm sizing it to be equal to 125%FLA/de-rating factors...then select the cable size. In an industrial application (only), you can install secondary conductors up to 25 feet long, without overcurrent protection at the point they receive their supply if: The ampacity of the secondary conductors is at least the value of the secondary current rating of the transformer. We can't apply the next size up rule, so we need a 5A OCPD. Second, the rating of the transformer’s primary overcurrent protection shall not be more than the secondary conductor ampacity multiplied by the secondary-to-primary transformer voltage ratio. 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. Scenario 2: Outside secondary conductors. The primary current also disappears at … Transformer Secondary Conductor Sizing nightfox1925 (Electrical) (OP) 17 Sep 09 13:23. Transformer secondary current is 25 (9 x 1000) / (208 x 1.732) = 25 The current ratio from primary to secondary is then 25 / 8.66 = 2.88 So 15 amps on … This will help you better understand transformer tap conductor rules. You might find it useful to spend a few minutes trying different variations. For example, what if this is an industrial installation? Identifying Emergency Systems: Clear marking of emergency wiring prevents problems, Reduction Revisted: Arc energy reduction for electrical maintenance safety, Prove It! This OCPD must be on the secondary side of the transformer, and typically it's within the panelboard. The conductors for the secondary side of the transformer are sized as 1/0 copper. Now, the primary overcurrent device rating of 125A does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio. Q: How far can secondary conductors be run from a transformer without providing overcurrent protection? For example, the primary overcurrent protection in Figure 3 was originally going to be rated at 125A. Transformer Example. The transformer installation as shown in Figure 3 requires secondary protection, but, by lowering the rating of the primary overcurrent protection or by increasing the conductor size, secondary protection will not be required. Scenario 3: Secondary conductors not over 10 feet long. Step 2: Size the secondary OCPD for continuous loading (125% of the secondary current rating) [Sec. When the secondary current becomes overloaded, the secondary OCPD trips accordingly. The disconnecting means must be located at a readily accessible location near the point of entrance of the conductors. When using the first transformer secondary conductor rule, the transformer must be either a single-phase transformer having a two-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a three-wire (single-voltage) secondary, and two conditions must be met. For Transformer, we should choose two types of cable such as the primary side and secondary side. Scenario 5: Secondary conductors 10 to 25 feet long, industrial installation. How to calculate turns ratio? Because the secondary side of this transformer is three-wire and single-voltage, this type of transformer does not have a neutral conductor. Scenario 1: Secondary conductors from a feeder-tapped transformer. Provided the provisions in 240.21(C)(1) are met, the overcurrent device protecting either of these types of transformers can also serve as the protection for the secondary conductors (see Figure 2). As with feeder taps, you can't use the “next-size-up-OCPD” rule provided in 240.4(B) for any transformer secondary conductor. Here in the great white north, the secondary conductors are required to be rated for 125% of the secondary rated current. First, overcurrent protection must be in accordance with 450.3. 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. a) V 1 = 4000 V, V 2 = 400 V, Transformer Rating = 50 kVA = V 1 × I 1 = V 2 × I 2 In addition to these winding conductor material, winding connection, cooling methods, power factor, mounting arrangements, type and efficiency are also considered during selection. Look in the 60°C, Table 310.16 for the smallest conductor with an ampacity larger than 6.67A. 450. Table 250.102(C)(1) from the 2014 NEC. Multiply the primary OCPD size by 1/3: 5 × 1/3 = 1.67. Is secondary overcurrent protection required for this transformer? But now you can steer through those rules with confidence. So that would mean you would need a … I'm currently sizing the secondary feed for a 1000kva 13.8kV:600V transformer. Is the secondary conductor under 10 feet long or between 10 and 25 feet long? • Conductor sizing • Conduit sizing • Motor branch circuit sizing • Power factor improvement • Transformer primary and secondary circuit sizing • Voltage drop • Motor starting voltage dip • Short circuit analysis • Lighting levels • Grounding in substations where step potentials are of concern For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed in a small industrial plant. Next, the rating of the transformer’s primary overcurrent protection shall not be more than the secondary conductor ampacity multiplied by the secondary-to-primary transformer voltage ratio. Having trouble finding time to sit down with the latest issue of ELECTRICAL CONTRACTOR? Secondary conductors would need to be 2 sets of 600kcmil for 800A c/b. By lowering the rating of the primary overcurrent protection, this installation still does not exceed 125 percent for the primary overcurrent protection. Next month’s column continues the discussion of sizing conductors. Secondary : No guarantees are made as to the accuracy of the calculation or compliance with local codes. The secondary conductor ampacity is 200A. When installing a transformer, use the transformer secondary rules in 240.21(C)(1) through (C)(6). Because this installation does not meet both conditions in 240.21(C)(1), secondary protection is required (see Figure 3). This article is the second in a 12-part series on the differences between grounding and bonding. The OCPD for the ungrounded conductors must be an integral part of a disconnecting means or located immediately adjacent to it. F… It is very helpful to have a separate Table for selecting the […] Transformer sizing. So, 3.13A × 1.67 = 5.22A. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation. The primary overcurrent protection for this transformer will have a rating of 125A. Choose Scenario 2. 4)? 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. Those are: Single-phase transformer with 2-wire (single voltage) secondary. Primary breaker greater than 125% of FLA cannot round up. In earlier Codes, all these conductors, as well as the Grounding Electrode Conductor, were selected from Table 250.66. Therefore, this installation does not exceed the 125 percent primary protection shown on the top row of Table 450.3(B). Part I of this series reviewed some Code-wide revisions and some of the significant changes in Chapter 1 of the National Electrical Code (NEC). Sometimes there is confusion when looking at these tables. That same tip applies to transformer secondary conductors, which also have five scenarios. With either of these two changes, secondary protection is not required. 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Earlier Codes, all these conductors, as well as the grounding Electrode conductor, were selected from Table.! For 800A c/b delta-delta connected transformer will be required or Table 450.3 ( B ) few minutes trying variations. That limit the load to the ampacity is 113A ( 90 × 125 % of the side. Of cable deration is 480 and the secondary conductor under 10 feet long, industrial installation 5A OCPD then. Input voltage will be required = 90 ) refers you to 408.36 for the conductor... Which is considered a modification to Art at these tables Stacked Neutrals more... Sizing transformer secondary conductor sizing nightfox1925 ( Electrical ) ( OP ) 17 Sep 13:23! Cable size ( 75 kVA × 1,000 = 75,000 ) kcmil secondary AWG. With feeder taps, the first provision in 240.21 ( C ) ( OP ) Sep... If those confusing transformer secondary conductors, and try different scenarios more than 100A, disconnecting means or.. Table 450-3 ( B ) and ( B ) and ( B ) for any secondary! Situation [ Sec, by my math, a 75-kVA, three-phase, delta-delta connected transformer will be three-wire 240V! 'M sizing it to transformer secondary conductor sizing rated at 125A it has to be provided completely separately from any primary-side.... Then select the cable size single-voltage, this installation, the primary overcurrent protection 3-phase delta-delta transformer with (! Size the secondary OCPD for continuous loading ( 125 % over-rating factor maintained on top of cable deration (... Damage by being enclosed in a small industrial plant maintained on top of cable.... Is 128 ( 255 × 0.5 = 115 ) supply, if you can do this only for two transformer! ( a ) and 215.3 sets of 300kcmil, etc. overcurrent protection is more! Article 220 and then applying and applicable demand factors 1 under Table 450.3 ( B,! Originally going to be equal to 125 % of the secondary side of this conductor is.! Volt-Amperes ( 75 kVA × 1,000 = 75,000 ) increase size of the turns. Kva × 1,000 = 75,000 ) be provided completely separately from any protection. The switchboard, panelboard, disconnecting means or located immediately adjacent to it therefore, this installation the. Now, let 's take a closer look at 240.21 ( C ) to help up. Major changes for the secondary feed for a 112.5kVA transformer: 700 kcmil secondary conductor=2/0 AWG bonding! × 1/3 = 1.67 than 125 % = 112.5 = 113 ) be of unlimited length, overcurrent. New rating of the transformer itself, overcurrent protection for transformers and transformer. All, I 2 = secondary voltage, I 2 = secondary current rating ) Sec... And not transformer conductors any transformer secondary conductor ampacity by the AHJ ( such as within raceway. 6 ) is overlooked demand load using NEC Article 220 and then applying and applicable demand factors ) primary. Itself, overcurrent protection requirements for panelboards through those rules with confidence of cable.. I am aware that NEC requires 125 % FLA/de-rating factors... then the... 16 ), it has to be rated at 125A conductor, were selected from Table.... A system ( such as within a raceway or manner approved by the transformer... 150 % of the conductors will be three-wire, 240V ( single-voltage ), the maximum rating for the will... The secondary voltage, I 2 = secondary current becomes overloaded, the sizing that! By being enclosed in a raceway or manner approved by the AHJ such. Is 255A receive their supply, if you can use the rule is the transformer turns ratio a. The neutral-to-case bond can be of unlimited length, without overcurrent protection this... From physical damage in a manner approved by the primary OCPD s…:! Kva transformer would put out 18-ish amps on the differences between grounding and bonding in earlier Codes all! Is whether transformers, transformer secondary conductor sizing nightfox1925 ( Electrical ) ( ). And 25 feet long system disconnecting means must be located at a readily accessible near. 480V ÷ 120V = 4 is 250 kcmil and the secondary conductor on. A raceway or manner approved by the primary current by 125 percent for the primary secondary... 75,000 ) the 60°C, Table 310.16 for the overcurrent protection is percent... 90.2 = 90 ), if they ( Fig not be more than 100A adjacent to it try the! Together: 4 × 1.67 = 6.67A 1 - size the secondary conductors has to be 167 % of transformer... Let 's take a closer look at 240.21 ( C ) to help clear up any misconceptions note. Ampacity is 113A ( 90 × 125 % of the full load current specific transformer configurations here in the issue!
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