Guide to Sizing Secondary Conductors for Transformers

Transformer secondary conductors are critical components of electrical systems that provide a safe path for current drawn by utilization equipment. These questions frequently appear on electrical licensing exams because proper sizing of transformer secondary conductors is essential for safety and code compliance. Understanding how to size these conductors is crucial for passing your exam.

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Example Transformer Secondary Conductor Questions on NEC Electrical Exams

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1. A panelboard being protected with a 200 amp main breaker is being fed from the secondary of a 75 kVA 480V-208/120V transformer. What are the minimum size THWN insulated copper conductors permitted to feed the panelboard where the secondary conductors are 8 feet in length?
2. For a commercial application, what are the minimum size copper THWN insulated conductors permitted to supply a load from the secondary of a 75 kVA 480V-208/120V transformer where secondary conductors are 15 feet in length and a 225 amp breaker is protecting the primary conductors?
3. What are the minimum size XHHW aluminum conductors permitted to feed a 100 amp fused disconnect being fed from the secondary of a 45 kVA 480V-240/120V 3-phase transformer, where the disconnect is located 12 feet from the transformer, and the it’s termination block is rated 75^OC? 

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How to Identify a Branch-Circuit Conductor Question on NEC Electrical Exams

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Key phrases to look out for:

• "Secondary conductors"
• "Transformer secondary tap conductors"
• Questions that reference a length for secondary conductors
• References to Section 240.21(C)(1)-(6)

When you spot these elements:

1. Determine all of the parameters involved
2. Determine the length of the secondary conductors
3. Determine the secondary tap rule that corresponds to the parameters
4. Does the question mention termination temperatures or reference termination or equipment temperature limitations or Section 110.14(C)?

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Transformer Secondary Conductor Tap Rules

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Section 240.21 states…

“Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H)” with 240.21(C) pertaining to transformer secondary conductors.

240.21(C)(1) states that unless a transformer is 2-wire single-phase, or 3-wire delta-delta three-phase, they are not considered protected by the primary overcurrent device. In other words, in general, if a transformer has a neutral, we must have a breaker or fuses on the secondary of a transformer.

The tap rules located in Sections 240.21(C)(2)-(4) allow us to bypass this rule, but only under specific conditions. 

So, typically we would need a breaker or fuse at the secondary terminals. The reason we almost never see this done is because of the tap rules found in Section 210.21(C)(2)-(4). Because a transformer creates a separately derived system, the secondary conductors are not considered protected by the breaker or fuses located on the primary side of the transformer.

These tap rules allow us to install our secondary conductors without their own overcurrent protection so long as we meet all of these conditions for the individual tap rules. 

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Walkthrough for NEC Electrical Exam Branch-Circuit Conductor Questions

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Standard Transformer Secondary Conductor Sizing Question

Question: A panelboard being protected with a 200 amp main breaker is being fed from the secondary of a 75 kVA 480V-208/120V transformer. What are the minimum size THWN insulated copper conductors permitted to feed the panelboard where the secondary conductors are 8 feet in length?

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Step 1: Identify Key Information

• Equipment being fed: 200 amp main breaker panelboard
• Conductor material: Copper
• Conductor insulation: THWN
• Secondary conductor length: 8 feet (<10 feet)

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Step 2: Locate and apply appropriate tap rule 

• System has a neutral so 240.21(C)(1) is ruled out
• 240.21(C)(2) covers secondary tap conductors not 10 feet in length
• 240.21(C)(2)(b) states conductor ampacity cannot be less than OCPD rating

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Step 3: Select Conductor Size Using Table 310.16

• THWN insulated copper conductors are rated for 75^OC 
• Conductor ampacity must be  200 amps

Minimum size secondary conductor size = 3/0 AWG THWN

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Where No Specific Loads are Given

Question: For a commercial application, what are the minimum size copper THWN insulated conductors permitted to supply a load from the secondary of a 75 kVA 480V-208/120V transformer where secondary conductors are 15 feet in length and a 225 amp breaker is protecting the primary conductors.

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Step 1: Identify Key Information

• Load: Unknown
• Conductors: THWN insulated copper
• Secondary conductor length: 15 feet (<25 feet)
• 75 kVA, 480V-208/120V
• PRIMARY (not secondary side) OCPD = 225 amps

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Step 2: Choose the appropriate secondary tap rule

• Secondary has a neutral so 240.21(C)(1) is ruled out
• Conductors are too long for 240.21(C)(2) (10 feet max)
• 240.21(C)(3) is specific to industrial installations
• 240.21(C)(4) is specific to outdoor installations
• 240.21(C)(5) applies to transformers whose primary conductors are also taps
• 240.21(C)(6) is the section that fits our scenario

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Step 3: Calculate Minimum Ampacity Using Section 240.21(C)(6)

• No OCPD is given for the secondary so that is not our limiting factor
• We need to use 240.21(C)(1) to find our minimum conductor ampacity
• CA = (Primary Voltage / Secondary Voltage) x (Primary OCPD Rating/3)
• CA = (480v/208v) x (225 amps/3)
• CA = 2.3  x  75 amps
• CA = 172.5

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Step 4: Choose the minimum size conductor per Table 310.16

• THWN conductors are rated for 75C
• Using the 75^OC column of Table 310.16, 2/0 AWG conductors are rated for 175 amps

The minimum size copper branch-circuit conductors required are 2/0 AWG THWN.

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