45 kVA Transformer Feeding a Remote Panel

Primary full-load current:

I = 45,000 VA / (480 V x 1.732) = 54.1 A

125% of primary current:

54.1 A x 1.25 = 67.7 A

• 67.7 A is not a standard rating in 240.6, so Note 1 to Table 450.3(B) permits the next higher standard rating: 70 A.
• The 70 A device is 129% of primary current. It is acceptable as primary-only protection only because the next-standard-size rule is expressly allowed; it is not an invitation to go further.
• Be aware that if a larger primary device were used, the secondary would then have to be protected at not more than 125% of secondary current.
• The primary feeder conductors still must have an ampacity of at least 70 A, and the transformer must be protected against overcurrent per 450.3(B) on at least one side.

Result: 70 A is compliant, but it is the top of the permitted range.

Secondary full-load current:

I = 45,000 VA / (208 V x 1.732) = 124.9 A

Secondary protection check: 125% of 124.9 A = 156 A. The 150 A main breaker is below that, so the secondary is protected within 125%. Compliant.

Conductor check: 2/0 copper at 75 C = 175 A, which exceeds the 150 A device. Compliant.

Length check: 18 ft exceeds the 10 ft allowance of 240.21(C)(2), so the 25 ft rule of 240.21(C)(6) must be met. Conditions:

1. The secondary conductors terminate in a single overcurrent device - satisfied by the 150 A main.
2. Ampacity is not less than the calculated load.
3. Ampacity is not less than one-third of the primary overcurrent device rating multiplied by the secondary-to-primary voltage ratio: (70 / 3) x (480 / 208) = 23.3 x 2.31 = 53.9 A. 175 A clears this easily.
4. The conductors are enclosed in a raceway - the EMT satisfies this.
5. The conductors do not extend beyond the panelboard.

Result: compliant, but the 25 ft rule is a package deal. If the panel were relocated past 25 ft, or if the conductors ran as exposed cable, the tap would fail and a secondary device or a full-size conductor run would be required.

Two bonding jumpers is a defect.

250.30(A)(1) requires the system bonding jumper to be installed at a single point - either at the transformer or at the first disconnecting means on the secondary, never both. Two connection points put the neutral-to-ground bond in parallel and create objectionable current on the enclosure, the raceway, and the grounding electrode conductor.

Pick one:

• Installing the bonding jumper in the transformer and running a full-size supply-side bonding jumper to the 150 A panel with a floating neutral is the common choice. The neutral bar in the panel stays isolated.
• Or leave the transformer neutral unbonded and install the bonding jumper in the 150 A panel, with a supply-side bonding jumper between the transformer and panel.

Grounding electrode conductor:

• 250.30(A)(5) sizes the GEC per Table 250.66 based on the largest derived phase conductor. With 2/0 copper secondary conductors, Table 250.66 gives 4 AWG copper.
• The GEC must originate at the same point as the system bonding jumper and run to the grounding electrode system - building steel, metal water pipe, or a concrete-encased electrode. Building steel is acceptable if it qualifies as a grounding electrode under 250.52(A)(2).
• A grounding electrode conductor must be run to the electrode system; it is separate from the equipment grounding conductor and must not be used to carry neutral current.

The plan is correct in concept but must be reduced to a single bonding point.