Transformer Installation

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 let’s get started.

For this exercise, let’s 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 today’s 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 75C.

Step 1. Determine Transformer Current Ratings: Determine the primary and secondary current rating of the transformers: Figures 1 and 2

    Primary Current   Secondary Current
45 kVA   45,000 VA/(480 x 1.732) = 54A   45,000 VA/(208 x 1.732) = 125A
112.5 kVA   112,500 VA/(480 x 1.732) = 135A   112,500 VA/(208 x 1.732) = 312A

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].

45 kVA   54A x 1.25 = 68A, next size up 70A
112.5 kVA   135A x 1.25 = 169A, next size up 175A

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)].

45 kVA   54A x 1.25 = 68A, 4 AWG rated 85A at 75C, Table 310.16
112.5 kVA   135A x 1.25 = 169A, 2/0 AWG rated 175A at 75C, Table 310.16

Sizing Equipment Grounding Conductor (when required) 250.122(A):
The size of the equipment grounding (bonding) conductor for the transformer primary is based on the primary protection device.

45 kVA   70A Primary Protection (Step 2), Table 250.122, 8 AWG
112.5 kVA   175A Primary Protection (Step 2), Table 250.122, 6 AWG

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).

45 kVA   125A x 1.25 = 156A, 175A protection
112.5 kVA   312A x 1.25 = 390A, 400A protection

Author’s 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 75C terminal rating [110.14(C)]. This means that the next size up rule contained in 240.4(B) does not apply.

45 kVA   125A x 1.25 = 156A, 175A protection = 2/0 AWG, rated 175A
112.5 kVA   312A x 1.25 = 390A, 400A protection = 600 kcmil, rated 420A

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 90C ampacity [110.14(C)], must be no less than 175A for the 45 kVA transformer and 400A for the 112.5 kVA transformer.

45 kVA   3/0 AWG, rated 225A x 0.80 = 180A, greater than 175A protection
112.5 kVA   600 kcmil, rated 475A x 0.80 = 380A, therefore we must use the next size larger conductor
112.5 kVA   700 kcmil, rated 520A x 0.80 = 416, greater than 400A protection

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
112.5 kVA Secondary Conductors 700 kcmil = 2/0 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. Figure 3.

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. Figure 4

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). Figure 5

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)]. Figure 6

45 kVA Secondary Conductors 3/0 AWG = 4 AWG Bonding Jumper
112.5 kVA Secondary Conductors 700 kcmil = 2/0 AWG Bonding Jumper

Author’s 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.
(2) Effectively grounded metal water pipe, within 5 ft from the point of entrance into the building.

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:

  • An electrode encased by at least 2 in. of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 ft of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than in. in diameter, or consisting of at least 20 ft of bare copper conductor not smaller than 4 AWG. See 250.52(A)(3).
  • A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG. See 250.52(A)(4).
  • Rod or pipe electrodes not less than 8 ft in length. See 250.52(A)(5).
  • A plate electrode that exposes not less than 2 sq ft of surface to exterior soil. See 250.52(A)(6)
  • Other local metal underground systems or structures such as piping systems and underground tanks. See 250.52(A)(7).

Mike Holt’s Comment: If you have any comments or feedback, please let me know, Mike@MikeHolt.com


Copyright © 2002 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)