NEC 450.3(A) · 450.3(B) · Transformers

How to size transformer overcurrent protection

Primary-only vs primary-and-secondary protection, next-size-up rules, impedance-based limits over 1000 V, and a worked 75 kVA 480-208Y/120 V example — with a free link to the official tables.

Need the actual tables? Look up Tables 450.3(A) and 450.3(B) in the free NFPA viewer →

Which table and which column?

Situation	Table	What sets the limit
Transformer rated 1000 V or less (typical 480-208Y/120 V dry-type)	Table 450.3(B)	Protection scheme (primary only, or primary + secondary) and primary current range
Transformer rated over 1000 V (e.g. 12.47 kV-480 V pad-mount)	Table 450.3(A)	Transformer impedance, device type (breaker vs fuse), and location
Over 1000 V in a supervised location	Table 450.3(A), supervised rows	Qualified persons monitor and service the installation, so different limits apply
Conductors on either side of the transformer	Art. 240 (240.4, 240.21(C))	Conductor ampacity — the 450.3 device does not automatically protect the wire

Our own summary of the 2023 NEC, not the code text. Look up Tables 450.3(A) and 450.3(B) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ. Not engineering advice — medium-voltage protection should be designed and coordinated by an engineer.

How to read Table 450.3(B), step by step

Step	What you do	Watch out for
1	Compute primary and secondary rated current from kVA: 3-phase I = kVA × 1000 ÷ (1.732 × V); 1-phase I = kVA × 1000 ÷ V	Use the transformer’s rated voltages, not the panel nominal
2	Choose the scheme: primary only, or primary with a secondary device	Primary-only is only an option if the secondary conductors are otherwise protected per 240.21(C)
3	Find the row for the primary current range	Small transformers (low primary current) have different percentages
4	Multiply rated current × the percentage for your scheme and side	These are maximums — smaller is always allowed
5	Apply Note 1: where the 125% value is not a standard size, the next standard size up is allowed	Note 1 does not apply to the higher primary ceiling in the primary + secondary scheme — round down there
6	Separately confirm every conductor is protected per Art. 240	The most common inspection failure is a secondary conductor that is not protected at its ampacity

Worked example — 75 kVA, 480 V delta to 208Y/120 V, dry-type

Input / step	Value	Source
Primary rated current	75,000 ÷ (1.732 × 480) ≈ 90.2 A	Physics
Secondary rated current	75,000 ÷ (1.732 × 208) ≈ 208.2 A	Physics
Primary only: max device	90.2 × 125% ≈ 112.8 A → 125 A	Table 450.3(B) + Note 1
Primary + secondary: secondary device	208.2 × 125% ≈ 260 A → 300 A	Table 450.3(B) + Note 1
Primary + secondary: primary ceiling	90.2 × 250% ≈ 225.5 A → 225 A (round down)	Table 450.3(B)
Then check conductors	Secondary conductors protected per 240.21(C); primary feeder per 240.4	Art. 240

Worked example values only — the 125% and 250% factors are the two this scheme uses. Look up the full table in the free NFPA viewer.

Common field mistakes

Mistake	What to do instead
Sizing the primary breaker at the 250% ceiling with no secondary main	The higher primary limit only applies when a secondary device is also installed
Treating 450.3 compliance as conductor compliance	Check 240.21(C) tap rules and 240.4 for the conductors separately
Rounding up the 250% value	Next-size-up (Note 1) applies to the 125% cells; the higher primary ceiling is a hard cap
Setting the primary breaker too close to FLA	Transformer inrush can trip a tight breaker on energizing — pick a breaker with suitable trip characteristics
Using Table 450.3(B) on a medium-voltage unit	Over 1000 V uses Table 450.3(A), driven by impedance and location

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Protecting the transformer vs protecting the wire

Look up Tables 450.3(A) and 450.3(B) in the free NFPA viewer. This page explains how to apply them; it does not reproduce the tables.

Section 450.3 protects the transformer. For units rated 1000 V or less, Table 450.3(B) gives you two schemes: protect the primary only, with a tight limit, or protect both sides, which lets the primary device be much larger because the secondary device is doing the overload work. Over 1000 V, Table 450.3(A) bases the limits on the transformer’s nameplate impedance, the device type, and whether the location is supervised by qualified persons.

Worked example. A 75 kVA, 480-208Y/120 V dry-type draws about 90.2 A on the primary and 208 A on the secondary (kVA × 1000 ÷ 1.732 ÷ volts). Primary-only protection at 125% gives 112.8 A, and Note 1 lets you use 125 A. If you add a secondary main at 125% (260 A, next size up 300 A), the primary may go as high as 250% — 225.5 A, rounded down to 225 A. Standard sizes are covered in the standard breaker sizes guide.

Then protect the conductors. Passing 450.3 does not clear the wire. Secondary conductors need a 240.21(C) path — the 10 ft or 25 ft rules, or a secondary main at the transformer — and the primary feeder must be protected at its ampacity. Size those with the NEC 310.16 ampacity chart, and ground the derived system with the grounding electrode conductor guide.

Field PM keeps one-lines, transformer submittals, and inspection records together on the job, so the breaker in the panel matches the one on the drawing.

FAQ

How do I size a transformer primary breaker?+

Compute the primary rated current from kVA and voltage, then apply the percentage from NEC Table 450.3(B) for your scheme. For a transformer 1000 V or less with primary-only protection and a primary current of 9 A or more, the limit is 125%, with the next standard size up permitted. For a 75 kVA, 480 V three-phase transformer (about 90.2 A), that allows a 125 A primary breaker. Look up the table in the free NFPA viewer at nfpa.org/freeaccess.

When can the primary breaker be larger?+

When the transformer also has secondary protection sized per Table 450.3(B), the primary device can go up to 250% of primary rated current. In that scheme you do not round the primary up to the next size. The conductors must still be protected per Article 240.

How do I calculate transformer full-load amps?+

For three-phase: amps = kVA × 1000 ÷ (1.732 × line-to-line volts). For single-phase: amps = kVA × 1000 ÷ volts. A 75 kVA transformer is about 90.2 A at 480 V and about 208 A at 208 V.

What is different about transformers over 1000 volts?+

Table 450.3(A) sets limits based on the transformer impedance, whether the device is a breaker or fuse, and whether the location is supervised by qualified persons. It is typically applied by an engineer as part of a coordination study, so we do not reproduce its values here — look them up in the free NFPA viewer.

Does the transformer breaker protect the secondary conductors?+

Not automatically. Except in limited cases (such as a two-wire single-voltage secondary or a three-phase delta-delta, three-wire system under 240.4(F)), the secondary conductors need their own protection or must qualify under a 240.21(C) secondary-conductor rule.

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