9.2 Transformer Overcurrent Protection & Grounding

9.2 Transformer Overcurrent Protection & Grounding

Quick Reference:

• Primary-Only Protection (NEC Table 450.3(B)):
  • Primary FLC $\ge 9\text{A}$: Max 125% of FLC (Note 1 permits next higher standard OCPD per 240.6).
  • Primary FLC $2\text{A}$ to $< 9\text{A}$: Max 167% of FLC (No rounding up).
  • Primary FLC $< 2\text{A}$: Max 300% of FLC (No rounding up).
• Primary & Secondary Protection (NEC Table 450.3(B)):
  • Primary OCPD: Max 250% of primary FLC (Rounding up prohibited).
  • Secondary OCPD ($\ge 9\text{A}$): Max 125% of secondary FLC (Note 1 permits next higher standard rating).
• Separately Derived System (NEC 250.30): Premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection to supply conductors originating in another system.
• System Bonding Jumper (SBJ) (NEC 250.30(A)(1)): Connects neutral to equipment ground; installed at source OR first disconnect, NEVER at both!
• Sizing SBJ: Sized from NEC Table 250.102(C)(1) based on largest derived ungrounded phase conductor.
• Sizing SDS GEC: Sized from NEC Table 250.66 based on derived ungrounded conductors.

Transformers present unique challenges to overcurrent protection and system grounding. When energized, a transformer draws a massive instantaneous magnetizing inrush current that can reach 10 to 20 times its rated full-load current for several cycles. Protective devices must be sized high enough to ride through this harmless inrush without nuisance tripping, yet low enough to protect internal windings against sustained thermal destruction. Furthermore, because a two-winding transformer creates an electrically isolated electrical system on its secondary, the National Electrical Code classifies it as a Separately Derived System (SDS), requiring meticulous grounding and bonding under NEC 250.30 to prevent lethal ground-fault voltages and dangerous circulating currents.

---

1. Transformer Overcurrent Protection (NEC Article 450)

NEC Article 450 protects the transformer windings against damage from overcurrent. It is critical to recognize that Article 450 does not protect secondary conductors; secondary conductor protection is separately governed by the tap rules in NEC 240.21(C).

Primary-Only vs. Primary-and-Secondary Protection

Under NEC 450.3(B), designers and installers can choose between two methods for transformers operating at 1,000 volts or less:

1. Primary-Only Protection: A single overcurrent protective device on the primary side protects the transformer. This requires a relatively tight overcurrent setting (125% of FLC) to guard against secondary overloads reflecting back to the primary.
2. Primary and Secondary Protection: Overcurrent protection is provided on both primary and secondary sides. Because the secondary device prevents sustained overloads, the primary device can be set significantly higher (up to 250% of FLC) to easily accommodate inductive inrush.

NEC Table 450.3(B) Summary (Transformers Rated 1,000V or Less)

Note 1 of Table 450.3(B) (The Round-Up Rule): Where 125% of the rated full-load current does not correspond to a standard ampere rating of a fuse or nonadjustable circuit breaker listed in NEC 240.6, the next higher standard rating shall be permitted. Note 1 applies only to devices governed by the 125% limit where FLC is 9 amperes or greater. It never applies to the 250% primary device!

---

2. Step-by-Step Sizing Calculations under Table 450.3(B)

Scenario A: Primary-Only Protection of a 45 kVA Transformer

Problem: A 45 kVA, 480V to 208Y/120V 3-phase dry-type transformer is installed with primary-only overcurrent protection. Determine the maximum standard inverse-time circuit breaker permitted on the primary.

1. Calculate Primary Full-Load Current ($I_{\text{pri}}$): $I_{\text{pri}} = \frac{45,000\text{ VA}}{480\text{ V} \times 1.732} = \frac{45,000}{831.38} = 54.13\text{ Amperes}$
2. Apply Table 450.3(B) Primary-Only Percentage: Since $I_{\text{pri}} \ge 9\text{ A}$, the multiplier is 125%: $\text{Max OCPD} = 54.13\text{ A} \times 1.25 = 67.66\text{ Amperes}$
3. Apply Note 1 (Next Higher Standard Rating): $67.66\text{ A}$ does not match a standard breaker rating in NEC 240.6(A) (standard ratings: 50A, 60A, 70A, 80A...). Note 1 explicitly permits rounding up to the next higher standard rating: $\text{Permitted Standard Breaker} = \mathbf{70\text{ Amperes}}$

Scenario B: Primary and Secondary Protection of a 75 kVA Transformer

Problem: A 75 kVA, 480V to 208Y/120V 3-phase dry-type transformer experiences nuisance tripping during startup with a 125% primary breaker. The contractor elects to install primary and secondary protection per Table 450.3(B). Determine:

1. The maximum standard overcurrent device rating permitted on the secondary.
2. The maximum standard overcurrent device rating permitted on the primary.

1. Calculate Primary and Secondary Full-Load Currents: $I_{\text{pri}} = \frac{75,000\text{ VA}}{480\text{ V} \times 1.732} = 90.21\text{ Amperes}$ $I_{\text{sec}} = \frac{75,000\text{ VA}}{208\text{ V} \times 1.732} = 208.18\text{ Amperes}$
2. Size Secondary Overcurrent Protection: Per Table 450.3(B), secondary protection is limited to 125% with Note 1 rounding permitted: $\text{Secondary Calculation} = 208.18\text{ A} \times 1.25 = 260.23\text{ Amperes}$ Standard ratings per 240.6(A) include 225A, 250A, 300A. Rounding up via Note 1 permits a 300-ampere secondary OCPD (typically installed as the main breaker in the secondary panelboard).
3. Size Primary Overcurrent Protection: Per Table 450.3(B), with secondary protection provided, the primary OCPD multiplier increases to 250%: $\text{Primary Calculation} = 90.21\text{ A} \times 2.50 = 225.53\text{ Amperes}$ Important: Note 1 does NOT apply to the 250% primary limit! Rounding up to 250A is illegal. The installer must round down to the next lower standard rating: $\text{Maximum Primary Breaker} = \mathbf{225\text{ Amperes}}$

---

3. Grounding Separately Derived Systems (NEC 250.30)

An isolation transformer with a 208Y/120V Wye secondary establishes a Separately Derived System (SDS) because there is zero direct electrical connection between the incoming 480V phase/neutral supply conductors and the secondary conductors.

                    SEPARATELY DERIVED SYSTEM GROUNDING
   
   Transformer Enclosure                         Secondary Panelboard
   +------------------------------+             +--------------------------+
   |  Secondary Coils (Wye)       |             |                          |
   |  X1 (Phase A) ---------------\|------------|--> Phase A Bus           |
   |  X2 (Phase B) ---------------\|------------|--> Phase B Bus           |
   |  X3 (Phase C) ---------------\|------------|--> Phase C Bus           |
   |  X0 (Neutral)                |             |                          |
   |         |                    |             |                          |
   |         +--------------------\|------------|--> Neutral Bus           |
   |         |                    |             |                          |
   |   [ SBJ Connection ]         |             |   [ Alternate SBJ ]      |
   |         |                    |             |   (If NOT at Trans!)     |
   |         +-------+            |             |                          |
   |                 |            |             |                          |
   |   Enclosure     |            |             |   Enclosure              |
   |   Ground Lug    |            |             |   Ground Bus             |
   +--------+--------+------------+             +-------------+------------+
            |        |                                        |
            |        +--- Grounding Electrode Conductor (GEC) | (Equipment
            |             To Building Steel / Water Pipe      |  Ground)
            +-------------------------------------------------+
              Supply-Side Bonding Jumper (SSBJ) in Conduit

The Golden Rule: Single-Point Bonding

The neutral terminal ($X_0$) must be bonded to the equipment grounding enclosure at exactly ONE location (NEC 250.30(A)(1)):

• Option A (At the Source): Inside the transformer enclosure. The System Bonding Jumper (SBJ) connects $X_0$ to the transformer case. The Grounding Electrode Conductor (GEC) must also terminate at this same transformer enclosure.
• Option B (At the First Disconnecting Means): Inside the secondary panelboard main breaker enclosure. The SBJ bonds the neutral bar to the panel enclosure, and the GEC terminates at the panelboard.

Lethal Code Violation: Dual Bonding!
If an electrician installs a bonding jumper inside the transformer AND leaves the bonding screw/strap in the secondary panelboard, the neutral current splits between the neutral conductor and the metallic raceway/equipment grounding conductor! This creates continuous objectionable circulating neutral currents (NEC 250.6), heats up conduits, induces electromagnetic interference (EMI), and exposes personnel touching panels to shock hazards.

---

4. Sizing System Bonding Jumpers & Grounding Electrode Conductors

System Bonding Jumper (SBJ) Sizing (NEC 250.28 & 250.102(C)(1))

The System Bonding Jumper ensures that if an ungrounded secondary phase conductor faults to a metallic raceway or enclosure, fault current returns instantly to the $X_0$ neutral terminal, tripping the protective device. The SBJ is sized from NEC Table 250.102(C)(1) based on the size of the largest derived ungrounded phase conductor (or equivalent area for parallel conductors):

Grounding Electrode Conductor (GEC) Sizing (NEC 250.30(A)(5) & 250.66)

The SDS Grounding Electrode Conductor references the derived system to earth potential. It is sized from NEC Table 250.66 based on the largest derived ungrounded phase conductor:

Key Difference Between Table 250.102(C)(1) and Table 250.66:
For very large services exceeding 1100 kcmil, Table 250.66 caps the copper GEC at 3/0 AWG copper, whereas Table 250.102(C)(1) for the SBJ never caps out—it requires 12.5% of the total cross-sectional circular mil area of the phase conductors!

Grounding Electrode Hierarchy for SDS (NEC 250.30(A)(4))

The grounding electrode conductor for a separately derived system must connect to the nearest available grounding electrode from the following hierarchy:

1. Metal Building Frame / Structural Steel (NEC 250.52(A)(2)): Substantial structural metal in direct contact with earth or encased in concrete.
2. Metal Underground Water Pipe (NEC 250.52(A)(1)): Within 5 feet of point of entrance into the building.
3. Concrete-Encased Electrode (Ufer) (NEC 250.52(A)(3)): At least 20 feet of 1/2-in. rebar or 4 AWG bare copper embedded in foundation concrete.
4. Other Electrodes: Ground rod, pipe, or ring per 250.52 if structural steel or water pipe is not present.

Common Grounding Electrode Conductor (NEC 250.30(A)(6))

In large commercial facilities with multiple transformers installed on different floors, running individual GECs from every transformer all the way back to the service ground is impractical. NEC 250.30(A)(6) permits a single Common Grounding Electrode Conductor (minimum 3/0 AWG copper or 250 kcmil aluminum) routed vertically through the building. Individual transformer SDS grounding taps are connected to this common riser using irreversible crimp connectors or exothermic welding.