Sizing Transformer Primary and Secondary Conductors

Written by Mark Delle Bovi

Sizing transformer Primary and Secondary conductors can be an import concept to understand when working with transformers in general. There are many rules that tell us how to size these conductors so lets jump in see what it takes to properly size both the primary and secondary conductors of a transformer.

Primary Conductors

Transformer primary conductors are rather simple to size. We size the conductors using NEC table 310.15(B)(16). If the transformer is feeding a continuous load, we also need to factor in 125% into our sizing. Let’s look at an example.

Example: size primary feeders for a 30kVA transformer with an input voltage of 480V 3phase that will be used to feed continuous load. 

Step 1: Find the Primary Current of transformer:

for 3 phase transformers: Transformer kVA*1000Primary Voltage x 1.73      = 30*1000480V x 1.73=36 amps

Step 2: multiply Primary current by 125% to size wire:

36A x 125%=45A

Step 3: Use Table 310.15(B)(15) 75 Deg C column to pick conductor size. Choose conductors with at least a 45A rating or higher

8 AWG copper = 50A 

6 AWG Aluminum = 50A 

Either one of these sizes would be acceptable. 

Secondary Conductors

Sizing Secondary conductors is a little more complicated as there can be a few different situations that come up. These situations are all covered in 250.21(B) and 250.21(C). Let us break them down 1 by 1 and cover each in detail. Diagram, schematic
Description automatically generated

  1. Transformer Fed from a tap [250.21(B)(3)]. Primary plus secondary conductors cannot be longer than 25 feet. Secondary conductors cannot have a value of less than the primary to secondary voltage ratio multiplied by 1/3 of the rating of the overcurrent protection device protection the feeders.

Example:  Exhibit 240.9 from the NEC handbook shows an example of a transformer that is fed from a tap. We know this is a tap because the fuse is 110A and the primary conductors are 8AWG copper rated for 50A. Lets check the transformer secondary size:

Primary VoltageSecondary Voltage*(13*OCPD rating)OCPD rating

480208*13*100=77A  . Based on this calculation we know that our secondary conductors can not have an ampacity less than 77A. 3 AWG copper is chosen which is above 77A with a rating of 100A. This meets the requirements. 

  1. Transformer secondary conductors not over 10 feet long [250.21(C)(2)]. Ampacity of secondary conductors shall be as follows:
    1. Not less than the combined calculated loads on all circuits suppled by the secondary conductors. 
    2. Not less than the ampacity rating of equipment containing OCPDs or not less than the ampacity rating of OCPD at termination of secondary conductors. 
  1. Transformer secondary conductors not over 25 feet long for industrial facilities only [250.21(C)(3)]. The ampacity of secondary conductors is not less than the secondary current rating of the transformer and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors. 

Example: Find the secondary conductor size for a 480V primary 208V secondary 3phase 45kVA transformer in industrial building feeding a 225A panelboard with a 150A main breaker. The load is continuous. 

Step1: Find the secondary current: 45kVA*1000208V*1.73=125A

Step 2: Size the Secondary Conductors based on continuous load and using table 310.15(B)(16):  125A*125%=156A. We go down to the next size which is a 1/0 copper rated at 150A. 

Step 3: Check OCPD on the secondary to verify it is not larger than conductor rating: 150A OCPD matches 150A of conductor value. We have a code complaint installation. 

  1. Transformer secondary conductors not over 25 feet long [250.21(C)(6)]. Secondary conductors cannot have a value of less than the primary to secondary voltage ratio multiplied by 1/3 of the rating of the overcurrent protection device protection the feeders. This is the same requirements as feeder tapped transformer so refer to that example in item #1. 
  1. Outside Secondary Conductors [250.21(C)(4)]. Secondary conductors considered to be outside the building can be run for an unlimited length without overcurrent protection at the point they receive their supply. The Secondary conductors terminate to a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors. This situation we would size our secondary conductors to match the ampacity of the overcurrent protection device. The month shown in example from item #3 would apply here. 


Sizing primary conductors is rather simple while sizing secondary conductors can be more involved. However, there are a few easy take away we can get from here: 

  1. Start by observing the application. Are we an industrial facility? 
  2. Measure the length of the primary and secondary feeders to determine which rule must be followed. 
  3. Are the feeders considered outside the building? 
  4. Remember to factor in 125% for continuous load. 
  5. Utilize NEC table 310.15(B)(16) for Conductor sizing. 

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button