Whether or not using cable on the secondary side of a 30kVA transformer in a high bay is "code" depends on several factors, including: The type of cable

The cable must be rated for the voltage and current it will carry. It must also be suitable for the environmental conditions of the high bay (temperature, moisture, exposure to chemicals, etc.).

The ampacity of the cable

The cable's ampacity must be sufficient to handle the calculated load on the secondary side of the transformer, with appropriate derating factors applied.

Overcurrent protection

The secondary side of the transformer must have appropriate overcurrent protection (fuses or circuit breakers) to protect the cable from overload and short circuits. The overcurrent protection must be sized according to the ampacity of the cable and the transformer's secondary current rating.

Installation method

The cable must be installed according to the NEC and manufacturer's instructions. This includes proper support, protection from physical damage, and secure terminations.

Local codes and regulations

Some local jurisdictions may have additional requirements or restrictions on the use of cable.

Type of occupancy

The type of occupancy can impact the wiring methods allowed.

Here's a breakdown of why it might be right or wrong, and what to check



Potential Reasons Why It Could Be Correct



Appropriate Cable Type

The cable used might be THHN, XHHW, or another type suitable for the environment and application.

Correct Ampacity

The cable size might be sufficient to carry the calculated load on the secondary side, considering derating factors for ambient temperature, number of conductors in a raceway, etc.

Proper Overcurrent Protection

The secondary side overcurrent protection is correctly sized to protect the cable and the transformer.

Proper Installation

The cable is properly supported, protected, and terminated.

Potential Reasons Why It Might Be Incorrect (and what to look for)



Incorrect Cable Type

Is the cable type rated for the voltage of the secondary? Is the cable type rated for the temperature environment of the high bay (often hot near the ceiling)? Is the cable type suitable for wet locations (if applicable)?

Insufficient Ampacity

What is the secondary voltage? Calculate the secondary current (30kVA / Secondary Voltage). Is the cable ampacity rated for that current, after derating for ambient temperature and number of conductors in a raceway/conduit? High bays can get very hot, requiring significant derating.

Improper Overcurrent Protection

NEC 450.3 outlines the protection requirements. The primary and secondary overcurrent protection must be coordinated.

Installation Issues

Is the cable properly supported and secured? Is the cable protected from physical damage? High bays often have moving equipment. Are the terminations properly made?

Key Things to Check


Cable Type

Identify the cable type (e.g., THHN, XHHW, MC cable). Verify that it is suitable for the voltage, temperature, and environment.2.

Cable Size

Determine the cable size (e.g., 4 AWG, 6 AWG).3.

Secondary Voltage

What is the transformer's secondary voltage (e.g., 480V, 208V, 120V)?4.

Calculated Load

Determine the actual load connected to the secondary side of the transformer.5.

Ampacity Calculation

Calculate the required ampacity of the cable, considering the calculated load and any applicable derating factors.6.

Overcurrent Protection

Verify that the secondary side overcurrent protection (fuses or circuit breakers) is properly sized according to NEC 450.3 and the ampacity of the cable.
7.

Installation Method

Inspect the cable installation to ensure it meets NEC requirements and manufacturer's instructions.8.

Local Codes

Contact your local electrical inspector or authority having jurisdiction (AHJ) to verify compliance with local codes and regulations.

In summary, using cable on the secondary side of a 30kVA transformer in a high bay can be code-compliant, but it's crucial to verify that the cable is appropriately sized, rated, and installed according to the NEC and local codes. A thorough inspection and calculations are necessary to determine compliance. Flag for review