Dry-Type Transformer OCPD, Wire Size, and EGC (2026 NEC)

Transformer OCPD sizing, conductor sizing, and grounding and bonding are separate steps.

Article 450 protects the transformer. Article 240 determines whether the primary and secondary conductors are properly protected. Article 250 determines how the transformer and separately derived system are grounded and bonded.

This article uses a common example: a 75 kVA, three-phase, dry-type transformer with a 480V primary and a 208Y/120V secondary.

The examples assume:

• Copper conductors
• 75°C-rated terminations
• No ambient-temperature correction or conductor-adjustment factors
• A typical two-winding transformer
• A separately derived 208Y/120V secondary system
• A transformer and first secondary disconnect located in separate enclosures where noted

Always verify the transformer nameplate, equipment listing, terminal temperature ratings, local amendments, and AHJ requirements.

The Key Code Sections

• NEC 450.5(B) and Table 450.5(B) — transformer OCPD sizing for transformers 1,000V and under
• NEC 240.4 and 240.21(C) — primary and secondary conductor protection
• NEC 240.4(F) — when transformer secondary conductors can be considered protected by the primary OCPD
• NEC 240.6(A) — standard OCPD ratings
• NEC 250.30 — grounding and bonding separately derived systems
• NEC 250.30(A)(2) — supply-side bonding jumper
• Table 250.102(C)(1) — system bonding jumper and supply-side bonding jumper sizing
• Table 250.66 — grounding electrode conductor sizing
• Table 250.122(A) — equipment grounding conductor sizing

How to Calculate Transformer Full-Load Current

Before sizing anything, calculate the full-load current on each side of the transformer.

Three-Phase Transformer Formula

Full-load current = kVA x 1,000 ÷ (V x 1.732)

Single-Phase Transformer Formula

Full-load current = kVA x 1,000 ÷ V

For three-phase transformers, use the line-to-line voltage.

On a 208Y/120V secondary, use 208V — not 120V.

Example — 75 kVA, 480V Primary / 208Y/120V Secondary, Three-Phase

Primary full-load current:

75,000 ÷ (480 x 1.732) = 90.3A

Secondary full-load current:

75,000 ÷ (208 x 1.732) = 208.2A

Option 1 — Primary Transformer OCPD Only

This option uses the primary-only transformer protection calculation in Table 450.5(B).

The primary OCPD is limited to 125% of the transformer primary full-load current.

If the calculation does not land on a standard OCPD rating, Note 1 to Table 450.5(B) permits the next higher standard rating from NEC 240.6(A).

Primary OCPD maximum = Primary full-load current x 125%

75 kVA Example — Primary-Only OCPD Calculation

90.3A x 125% = 112.9A

112.9A → next standard size → 125A breaker

Important: this is a transformer-protection calculation. It does not automatically mean that every transformer secondary conductor is protected by the 125A primary breaker.

Under NEC 240.4(F) and 240.21(C)(1), the primary OCPD can be treated as protection for secondary conductors only in limited configurations, including single-phase two-wire systems and three-phase delta-delta three-wire systems, when all applicable conditions are met.

A 208Y/120V, three-phase, four-wire secondary cannot rely on the 125A primary breaker alone for secondary-conductor protection.

The secondary conductors must comply with an applicable NEC 240.21(C) method. A common installation is to run the transformer secondary conductors to equipment with a properly sized secondary main breaker or fused disconnect.

Primary-Only Protection Summary — 75 kVA, 480V Primary

Option 2 — Primary and Secondary OCPD

When secondary transformer protection is provided, Table 450.5(B) allows the primary OCPD to be sized up to 250% of primary full-load current.

This larger primary OCPD helps prevent nuisance tripping caused by transformer inrush current.

The 250% primary value is a hard ceiling.

Do not round up above 250%. Use the largest standard OCPD rating at or below the calculated 250% value.

The secondary OCPD is limited to 125% of the transformer secondary full-load current. Note 1 to Table 450.5(B) permits the next higher standard OCPD rating when the calculated value is not a standard rating.

Primary OCPD maximum = Primary full-load current x 250% → round down

Secondary OCPD maximum = Secondary full-load current x 125% → round up permitted

75 kVA Example — Primary and Secondary OCPD Calculation

Primary OCPD:

90.3A x 250% = 225.75A

225.75A → largest standard size not over 250% → 225A breaker

Secondary OCPD:

208.2A x 125% = 260.25A

260.25A → next standard size permitted → 300A breaker

Primary Conductor Sizing With Primary and Secondary Protection

The 225A primary breaker is permitted to protect the transformer under Table 450.5(B).

The primary conductors must also be protected under Article 240.

Because the 225A breaker directly protects the primary conductors, those conductors must have ampacity sufficient for the 225A breaker.

225A primary OCPD → conductors must be rated at least 225A

Table 310.16 at 75°C → 4/0 AWG copper rated 230A

Result: 4/0 AWG copper primary conductors

A common mistake is to use only 125% of transformer full-load current for the primary conductor size.

For this transformer:

90.3A x 125% = 112.9A

At 75°C, #3 AWG copper is rated only 100A.

#3 copper cannot be protected by a 225A breaker.

Secondary Conductor Sizing With a Secondary OCPD

Transformer secondary conductors must comply with the applicable rule in NEC 240.21(C).

The exact secondary conductor size depends on the installation method, conductor length, location of the first secondary OCPD, and equipment supplied.

For this example, assume the transformer secondary conductors terminate in a separate disconnect enclosure containing a 300A breaker and are installed under an applicable NEC 240.21(C) arrangement.

300A secondary OCPD → conductors must be rated at least 300A

Table 310.16 at 75°C → 350 kcmil copper rated 310A

Result: 350 kcmil copper secondary conductors

The next-standard-size permission applies to the secondary OCPD where Table 450.5(B) Note 1 permits it.

It does not automatically allow undersized secondary conductors.

NEC 240.4(B) does not apply to transformer secondary conductors.

Grounding and Bonding

A typical two-winding 480V to 208Y/120V transformer is a separately derived system.

A separately derived system requires grounding and bonding under NEC 250.30.

Verify the transformer configuration. An autotransformer is not a separately derived system.

When the transformer enclosure and first secondary disconnect/OCPD enclosure are separate, the transformer secondary conductors are on the supply side of the first secondary OCPD.

The bonding conductor run with those secondary conductors is a supply-side bonding jumper.

It is not an equipment grounding conductor.

An equipment grounding conductor is used on the load side of an OCPD.

Grounding and Bonding Example — Primary and Secondary Protection

For this example, assume:

• 4/0 AWG copper primary phase conductors
• 350 kcmil copper secondary phase conductors
• 225A primary breaker
• 300A secondary breaker/disconnect
• Transformer and first secondary disconnect in separate enclosures
• System bonding jumper installed at the transformer X0 bonding point

Primary EGC:

225A primary OCPD → Table 250.122(A) → #4 AWG copper

Supply-side bonding jumper:

350 kcmil copper derived ungrounded conductors → Table 250.102(C)(1) → #2 AWG copper

System bonding jumper:

350 kcmil copper derived ungrounded conductors → Table 250.102(C)(1) → #2 AWG copper

Grounding electrode conductor:

350 kcmil copper derived ungrounded conductors → Table 250.66 → #2 AWG copper

Note: the required grounding electrode conductor can change based on the electrode used. A grounding electrode conductor connected only to rod, pipe, or plate electrodes is not required to be larger than #6 copper.

Secondary feeder EGC:

On the load side of the 300A secondary OCPD → Table 250.122(A) → #4 AWG copper

Primary and Secondary Protection Summary — 75 kVA Example

The grounding electrode conductor result can change depending on the electrode used and the applicable limitations in Article 250. A grounding electrode conductor connected only to rod, pipe, or plate electrodes is not required to be larger than #6 copper.

Common Mistakes

Treating Primary-Only Protection as Complete Secondary Protection

A 125A primary breaker can protect this 75 kVA transformer under Table 450.5(B).

But it does not automatically protect the secondary conductors of a 208Y/120V, three-phase, four-wire secondary.

Those secondary conductors still need to comply with an applicable NEC 240.21(C) method.

Using the Old 450.3(B) Reference

In the 2026 NEC, the transformer OCPD rule is Table 450.5(B), not Table 450.3(B).

Update calculations, reference materials, and permit drawings to the current section number.

Rounding the 250% Primary Value Up

The 250% primary OCPD value is a hard maximum.

For this example:

90.3A x 250% = 225.75A

The correct primary OCPD is 225A, not 250A.

Only the 125% calculations can use the next higher standard OCPD rating when Note 1 to Table 450.5(B) applies.

Sizing Primary Conductors Only From Transformer Full-Load Current

If a 225A primary breaker directly protects the transformer primary conductors, those conductors must have sufficient ampacity for the 225A breaker.

Do not stop at 125% of transformer full-load current.

Calling the Secondary Supply-Side Bonding Jumper an EGC

The bonding conductor between the transformer secondary enclosure and a first secondary disconnect/OCPD in a separate enclosure is a supply-side bonding jumper.

It is sized from Table 250.102(C)(1), based on derived ungrounded conductors.

It is not a secondary feeder EGC.

Sizing Every Grounding and Bonding Conductor From the Primary Breaker

The primary EGC, supply-side bonding jumper, system bonding jumper, grounding electrode conductor, and secondary feeder EGC are not all sized the same way.

• Primary EGC: Table 250.122(A), based on primary OCPD
• Supply-side bonding jumper: Table 250.102(C)(1), based on derived conductors
• System bonding jumper: Table 250.102(C)(1), based on derived conductors
• Grounding electrode conductor: Table 250.66
• Secondary feeder EGC: Table 250.122(A), based on secondary OCPD

Treating All Transformer Secondary Conductors the Same

Transformer secondary conductors must comply with the applicable NEC 240.21(C) rule.

The required conductor size depends on the installation arrangement, conductor length, location of the first secondary OCPD, and equipment supplied.

Do not use the 350 kcmil conductor size from this example as a universal transformer-secondary answer.