The correct wire size for a 45 kVA transformer is the specific conductor cross-section required to safely carry the calculated primary and secondary full-load currents multiplied by a 125% continuous load safety factor, without exceeding the termination temperature rating. For a standard 3-phase 480V Delta to 208Y/120V 45 kVA transformer, you typically need 4 AWG copper on the primary (480V) side and 2/0 AWG copper on the secondary (208V) side, assuming 75°C terminations and standard NEC continuous load sizing.

Quick Reference (3-Phase, 480V to 208V):
Primary (480V): 54.1A FLA → 67.6A Sizing → 4 AWG Copper (70A Breaker)
Secondary (208V): 124.9A FLA → 156.1A Sizing → 2/0 AWG Copper (175A Breaker)

The Math Behind the Gauge (Worked Example)

Sizing transformer conductors is not about guessing based on the kVA number alone; it requires calculating the Full Load Amps (FLA) on both sides of the transformer and applying National Electrical Code (NEC) multipliers. We will use the most common commercial configuration: a 3-phase, 480V primary to 208Y/120V secondary step-down transformer.

Step 1: Calculate Primary Full Load Amps (FLA)
The formula for 3-phase current is I = (kVA × 1000) / (V × √3).
Primary I = (45 × 1000) / (480 × 1.732) = 45,000 / 831.36 = 54.1 Amps.

Step 2: Calculate Secondary Full Load Amps (FLA)
Secondary I = (45 × 1000) / (208 × 1.732) = 45,000 / 360.25 = 124.9 Amps.

Step 3: Apply the 125% NEC Continuous Load Rule
Per NEC Article 215.2 and standard transformer sizing practices, conductors supplying continuous loads must be sized at 125% of the FLA to prevent thermal degradation of the insulation over time.

  • Primary Sizing Current: 54.1A × 1.25 = 67.6 Amps.
  • Secondary Sizing Current: 124.9A × 1.25 = 156.1 Amps.

Step 4: Select Wire from NEC Table 310.16
Assuming standard THHN/THWN-2 copper wire and 75°C rated terminations (the default for most commercial breakers and transformer lugs under 100A, and a safe baseline for larger ones):

  • Primary Wire: We need a wire rated for at least 67.6A. Looking at the 75°C column, 4 AWG copper is rated for 85A, which safely covers the load and pairs with a 70A breaker.
  • Secondary Wire: We need a wire rated for at least 156.1A. In the 75°C column, 2/0 AWG copper is rated for exactly 175A, making it the perfect fit for a 175A secondary breaker.
Bench Note: Never size your wire based on the 90°C column of Table 310.16 unless the equipment is explicitly rated for 90°C terminations. Almost all standard commercial transformer lugs and molded-case breakers are rated for 75°C. Using the 90°C column will result in undersized wire that overheats at the connection point, even if the wire itself stays cool.

What Wire Sizing Changes in a Real Installation

Choosing between 4 AWG and 2/0 AWG isn't just a numbers game on a spreadsheet; it fundamentally alters the physical execution of the job. Here is what wire sizing changes on the jobsite:

1. Conduit Fill and Physical Space
NEC Chapter 9 limits conduit fill to 40% for three or more conductors. Three runs of 4 AWG THHN easily fit inside a 1-inch EMT conduit. However, three runs of 2/0 AWG THHN require a minimum of 1.5-inch EMT, and pulling 2/0 around multiple 90-degree sweeps requires significantly more physical effort, pulling lubricant, and a larger junction box for bending radius.

2. Termination Lug Compatibility
A 175A breaker or a 150A secondary busbar lug might physically accept a 2/0 AWG wire, but if you miscalculated and tried to double up smaller wires (like two 2 AWG) to achieve the ampacity, you would violate NEC 310.10(H) regarding paralleled conductors, which generally prohibits paralleling wires smaller than 1/0 AWG.

3. Voltage Drop Over Distance
The calculations above assume the transformer is located relatively close to the panel it feeds. If your secondary feeder runs 200 feet to a subpanel, 2/0 AWG copper will experience roughly a 1.8% voltage drop at full load (well within the NEC recommended 3% maximum for feeders). If the run extends to 400 feet, you must step up to 3/0 AWG or 4/0 AWG to maintain voltage regulation, regardless of the ampacity tables.

Where You Meet This in Practice

You will rarely see a 45 kVA transformer in a standard residential setting; this is a staple of commercial and light-industrial electrical work. You will encounter this specific sizing scenario in:

  • Commercial Tenant Build-Outs: When a new retail space or office is carved out of a larger warehouse, a 45 kVA transformer is frequently dropped from the 480V overhead busway to create a new 120/208V lighting and receptacle panel.
  • EV Charging Depot Expansions: Adding a cluster of Level 2 (19.2kW) EV chargers often requires a dedicated 45 kVA step-down transformer to isolate the heavy charging loads from the building's main lighting panels.
  • CNC and Manufacturing Upgrades: Small machine shops upgrading from single-phase to 3-phase power use transformers of this size to run 208V 3-phase motors while maintaining 120V for control circuits and computers.

Common Confusions: kVA vs kW and Temperature Columns

The most frequent mistake made by junior electricians and DIYers is confusing kVA (kilovolt-amperes) with kW (kilowatts).

Think of a highway: kVA is the total number of vehicles on the road (apparent power), while kW is the number of vehicles actually carrying cargo (real power). The highway (your wire) must be built wide enough to handle every single vehicle, regardless of whether they are empty or full. Therefore, transformer wire sizing is always based on kVA, ignoring the power factor (kW) of the specific loads connected downstream.

The second major confusion involves aluminum vs. copper terminations. Many modern commercial transformers feature aluminum busbars. Even if you are pulling copper wire, you must use 75°C rated compression lugs and apply an antioxidant compound (like Noalox) to prevent galvanic corrosion. If you decide to use aluminum wire to save money on the secondary side, you must consult the aluminum column of NEC Table 310.16, which will require a much larger physical wire (typically 4/0 AWG aluminum for the secondary of a 45 kVA unit) to achieve the same 175A ampacity.

Frequently Asked Questions

What size breaker do I need for the primary side of a 45 kVA transformer?

For a 3-phase 480V primary, the full load amps are 54.1A. Multiplying by 125% for continuous load sizing gives 67.6A. According to NEC 240.6, you must round up to the next standard breaker size, which is a 70 Amp, 3-pole breaker. If the transformer has a high inrush current (common with large core-and-coil designs), you may be permitted by NEC 450.3 to size the primary breaker up to 250% of the FLA (a 125A or 150A breaker) to prevent nuisance tripping during energization, but the wire must still be sized based on the 125% continuous rule or the breaker rating, whichever governs the specific installation layout.

Can I use aluminum wire for a 45 kVA transformer secondary?

Yes, aluminum is highly cost-effective for heavy secondary feeders. To carry the required 156.1A (sizing current) on the 208V secondary, you must look at the 75°C aluminum column in NEC Table 310.16. A 3/0 AWG aluminum wire is rated for 155A, which is just under your 156.1A requirement. Therefore, you must step up to 4/0 AWG aluminum, which is rated for 180A at 75°C. Ensure your transformer lugs are rated for aluminum (AL/CU) and use a proper wire brush and antioxidant paste during termination.

Does the wire size for a 45 kVA transformer change if it is single-phase?

Yes, drastically. While 45 kVA is predominantly a 3-phase rating, if you are using a single-phase 45 kVA transformer (e.g., 480V to 120/240V), the current is calculated without the √3 multiplier.
Secondary I = 45,000 / 240V = 187.5 Amps.
Applying the 125% rule yields 234.3 Amps.
This requires a 250A breaker and 250 kcmil copper wire (rated 255A at 75°C) or 350 kcmil aluminum wire. Always verify the phase count on the transformer nameplate before pulling wire.