Transformer Primary Conductor Size Calculator

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Transformer Primary Conductor Size refers to the thickness or cross-sectional area of the conductors connected to the transformer’s primary winding.
It establishes how well the conductors can move electrical current without experiencing significant voltage drop or resistance.

Ensuring secure and effective electrical power transmission is the main goal of transformer primary conductor size specifications.
A suitable conductor size reduces power losses, guards against overheating, and keeps voltage regulation within reasonable bounds.

Understanding Transformer Primary Conductor Size:

When determining the appropriate size for the primary conductor of a transformer, it is crucial to take into account multiple factors to guarantee both safety and optimal performance. Below is a condensed overview of the critical considerations:

Step 1: To ascertain the primary current rating, it’s derived by dividing the kVA rating of the transformer by both the primary voltage and the transformer’s primary-to-secondary voltage ratio. For instance, consider a 45kVA transformer with a primary voltage of 480V and a primary-to-secondary voltage ratio of 1:1. In this scenario, the primary current rating would amount to 90 amperes (45,000 ÷ 480 ÷ 1.732 = 90.2 = 90).

Step 2: Ensure that the primary conductor is sized at 125% of the primary current rating. This adjustment accommodates for inrush current and additional considerations. For instance, in this scenario, the primary conductor should ideally have an ampacity of no less than 113A (90 × 125% = 112.5, rounded up to 113).

Step 3: “The sizing of the main overcurrent protection device (OCPD) depends on the primary current rating and the kVA rating of the transformer. For instance, if the primary OCPD is rated at 70A, the primary conductor should be sized to match this specification.”

Step 4: Determine the appropriate size for the equipment grounding (bonding) conductor by referring to the rating of the primary protection device, as outlined in section 250.122(A). For instance, if a 45kVA transformer is equipped with a 70A primary protection device, it necessitates the use of an 8 AWG copper conductor.

Additional considerations:

The main cable should be chosen according to the continuous load capacity of the transformer, as outlined in the manufacturer’s instructions. It should also be able to handle any sudden increases in current or initial power surges. Moreover, it needs to adhere to the specifications laid out in the National Electric Code (NEC) and local electrical regulations. By adhering to these guidelines and taking these variables into account, you can guarantee that the main conductor is appropriately selected for your transformer, satisfying safety and operational standards.


  • Power Distribution Networks
  • Industrial Installations
  • Commercial Buildings
  • Residential Applications
  • Renewable Energy Systems

To get the right conductor size for your electrical system, use our online electrical wire or cable size calculator. A safe and effective flow of electric current depends on the conductor size, commonly referred to as wire or cable thickness. You can choose the appropriate conductor size for your particular requirements by using our calculator, which offers precise gauge measurements.

Note : Don’t end with comma ( , )

Current (I)
Current Density (A)


\[δ = \frac{I}{A}\]


  • δ = Conductor Size
  • I= Current
  • A = Current density

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