By Mike Holt for EC&M Magazine
Installing transformers in accordance with the NEC requires the designer and installer to be up-to-date with the current Code requirements. Most installations can be a NEC challenge and transformers can extend that challenge to a new level. A properly designed, installed and safe installation will ensure that the conductors and equipment are properly sized and protected. In addition, grounding is also an overriding issue. So lets get started.
For this exercise, lets assume we are installing a 45 kVA and a 112.5 kVA 480V delta primary to a 208Y/120V 3-phase wye secondary. Each transformer supplies a lighting and appliance branch circuit panelboard with continuous nonlinear loads, typically found in todays office buildings. The length of the conductors from the transformer secondary to the lighting and appliance branch-circuit panelboard is not in excess of 10 ft, and all terminals are rated 75°C.
Step 1. Determine Transformer Current Ratings: Determine the primary and secondary current rating of the transformers:
Step 2. Primary Protection [450.3]: The primary winding of transformers shall be protected against overcurrent in accordance with the percentages listed in Table 450.3 and all applicable notes. Where 125 percent of the primary current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker as listed in 240.6(A), the next higher rating can be used [Note 1].
Step 3. Size Primary Conductor: Feeder conductors supplying continuous loads shall be sized no less than 125 percent of the continuous loads based on the conductor ampacities as listed in Table 310.16, before any ampacity adjustment in accordance with the terminal temperature rating [110.14(C) and 215.2(A)(1)].
Sizing Equipment Grounding Conductor (when required) 250.122(A):
Step 4. Size Secondary Conductor: Secondary conductors can be run without secondary overcurrent protection at the point of supply for 10 ft, if the ampacity of the conductor is not less than the rating of the overcurrent protective device at the termination of the tap conductors. This means that the next size up rule contained in 240.4(B) does not apply.
Secondary overcurrent protection is not required, but overcurrent protection is required for lighting and appliance branch-circuit panelboards, and this protection is required to be located on the secondary side of the transformer in accordance with 408.16(A) and (D).
Overcurrent Protection Device Size: Where a feeder supplies continuous loads, the rating of the (secondary) overcurrent device shall not be less than 125 percent of the continuous load [215.3] as listed in 240.6(A).
Authors Comment: Secondary overcurrent protection is not required for the transformer, but overcurrent protection is required for the lighting and appliance branch-circuit panelboard. Since secondary overcurrent protection is provided in this example, the primary protection device can be sized up to 250% of the primary current rating in accordance with Table 450.3(B) and 240.21(B)(3).
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)]. This means that the next size up rule contained in 240.4(B) does not apply.
But . Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity shall be reduced in accordance with Table 310.15(B)(2)(a). For our examples, there are four current-carrying conductors on the secondary [neutral considered current carrying 310.15(B)(4)(c)], therefore the conductor ampacity after adjustment [based on 90°C ampacity [110.14(C)], must be no less than 175A for the 45 kVA transformer and 400A for the 112.5 kVA transformer.
Step 5. Grounding and Bonding [250.30(A)]. Transformer secondarys that operate at over 50 V [250.20(A) and 250.112(I)] must be bonded to an effective ground-fault current path to ensure that dangerous voltage from ground-faults will not remain [250.2(A)(3)]. In addition, separately derived systems shall be grounded to the earth to stabilize the system voltage to earth during normal operation [250.4(A)(1)].
250.30(A)(1) Bonding Effective Fault Current Path. To provide the low impedance path necessary to clear a ground-fault on a separately derived system, the metal parts of electrical equipment must be bonded together (equipment grounding conductor) and connected to the system grounded conductor (X0 Terminal). The bonding jumper used for this purpose must be sized in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor as follows:
45 kVA Secondary Conductors 3/0 AWG = 4 AWG Bonding Jumper
The neutral-to-case bond can be made at the source of a separately derived system or at the first system disconnecting means or overcurrent device.
When there is no secondary side disconnecting means or overcurrent device(s), the neutral-to-case bond is made at the source of the separately derived system.
DANGER: Failure to provide a low impedance ground-fault path (no neutral-to-case bond) for the separately derived system can create a condition where a ground-fault (line-to-case fault) cannot be removed. The result is that all electrical metal parts, as well as the building structure, will remain energized with dangerous line voltage if a ground-fault (line-to-case fault) occurs.
CAUTION: The neutral-to-case connection for a separately derived system cannot be made at more than one location if so doing would result in a parallel path for neutral current flow [250.30(A)(1) Exception No. 1]. Such multiple neutral current return paths to the grounded (neutral) conductor of the power supply can create fire and shock hazards, as well as power quality problems from electromagnetic interference. See 250.6 and 250.142(A).
250.30(A)(2)(a) Grounding Single Separately Derived Systems. A grounding electrode conductor for a single separately derived system must be sized in accordance with 250.66, based on the total area of the largest secondary ungrounded (hot) conductor. This conductor shall connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). The grounding electrode conductor must terminate at the same point on the separately derived system where the neutral-to-case bonding jumper is installed [250.30(A)(1)].
45 kVA Secondary Conductors 3/0 AWG = 4 AWG Bonding Jumper
Authors Comment: The grounding electrode conductor must connect directly to the grounded neutral conductor terminal. It cannot be terminated to the case of the transformer.
250.30(A)(4) Grounding Electrode. The grounding electrode conductor must terminate to a grounding electrode that is located as close as practicable to, and preferably in the same area as the nearest:
(1) Effectively grounded metal member of the structure.
Exception: The grounding electrode conductor can terminate at any point on the water pipe system for industrial and commercial buildings where (1) conditions of maintenance and supervision ensure that only qualified persons service the installation, and (2) the entire length of the interior metal water pipe that is being used for the grounding electrode is exposed.
(3) Where an effectively grounded metal member of the building structure or an effectively grounded metal water pipe is not available, one of the following electrodes must be used:
Mike Holts Comment: If you have any comments or feedback, please let me know, Mike@MikeHolt.com
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