Don’t let secondary rules get you wound up
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Similar to feeder taps, covered in the last issue, transformer secondary conductors can be every bit as confusing. Let's take a closer look at 240.21(C) to help clear up any misconceptions.
As with feeder taps, you can't use the “next-size-up-OCPD” rule provided in 240.4(B) for any transformer secondary conductor. Normally, you can use the next highest overcurrent protective device (OCPD) above the ampacity of the conductors being protected, but not with transformer secondary conductors. The rule is the same because the physics are the same. As with feeder taps, the sizing of that transformer secondary conductor depends on its length and application.
When we demystified feeder taps in the previous issue, we presented the information in an arrangement that differs from what you see in the NEC. We also described an easy way to pick the correct scenario from the five possible. That same tip applies to transformer secondary conductors, which also have five scenarios.
Scenario 1: Secondary conductors from a feeder-tapped transformer.
Scenario 2: Outside secondary conductors.
Scenario 3: Secondary conductors not over 10 feet long.
Scenario 4: Secondary conductors 10 to 25 feet long.
Scenario 5: Secondary conductors 10 to 25 feet long, industrial installation.
To pick the right one, answer three easy questions:
Is the transformer outside? Choose Scenario 2.
Is the secondary conductor under 10 feet long or between 10 and 25 feet long? Choose Scenario 3 or 4, but choose Scenario 5 if it's an industrial installation.
Is the transformer supplied by a feeder tap? Choose Scenario 1.
Scenario 1: Secondary conductors from a feeder-tapped transformer
This one is simply a reference to 240.21(B)(3), so apply 240.21(B)(3) if you have a tap-supplied transformer.
Scenario 2: Outside secondary conductors
Outside secondary conductors can be of unlimited length, without overcurrent protection at the point they receive their supply, if they (Fig. 1):
Are suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction (AHJ).
Terminate at a single circuit breaker (or a single set of fuses) that limit the load to the ampacity of the conductors.
The OCPD for the ungrounded conductors must be an integral part of a disconnecting means or located immediately adjacent to it.
The disconnecting means must be located at a readily accessible location near the point of entrance of the conductors.
Scenario 3: Secondary conductors not over 10 feet long
You can install secondary conductors up to 10 feet long, without overcurrent protection at the point they receive their supply, if they have an ampacity that is not less than (Fig. 2):
The calculated load per Art. 220.
The rating of the device supplied by the secondary conductors or the OCPD at the termination of the secondary conductors, and
One-tenth the rating of the OCPD (protecting the primary of the transformer) multiplied by the primary-to-secondary transformer voltage ratio.
Secondary conductors must not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
Secondary conductors must be enclosed in a raceway.
Overcurrent protection is not required on the secondary side of the transformer to protect the secondary conductors, but overcurrent protection is required for branch-circuit panelboards. This OCPD must be on the secondary side of the transformer, and typically it's within the panelboard. The FPN under 240.21(C)(2)(3) refers you to 408.36 for the overcurrent protection requirements for panelboards.
Scenario 4: Secondary conductors 10 to 25 feet long
You can install secondary conductors up to 25 feet long, without overcurrent protection at the point they receive their supply, if they (Fig. 3):
Have an ampacity that is at least the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the OCPD that protects the primary of the transformer.
Terminate in a single circuit breaker (or set of fuses) rated no more than the secondary conductor ampacity per 310.15 [Table 310.16].
Are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway).
Scenario 5: Taps 10 to 25 feet long, industrial installation
In an industrial application (only), you can install secondary conductors up to 25 feet long, without overcurrent protection at the point they receive their supply if:
The ampacity of the secondary conductors is at least the value of the secondary current rating of the transformer.
The sum of the ratings of the secondary OCPDs doesn't exceed the ampacity of the secondary conductors.
The secondary OCPDs are grouped.
The secondary conductors are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway).
Now, let's see if you can put all this together to solve a practice problem. Question: What is the minimum size 15-foot secondary conductor required for a 2-wire, 480V to 120V indoor transformer rated 1.5kVA in an office installation (Fig. 4)?
Based on those two easy questions, you select Scenario 4. Then, you size the secondary to comply with Scenario 4 requirements.
To calculate the primary OCPD size, first we need to calculate the primary current. To do that, we need to know a couple of things:
VA = 1,500VA
E = 480V
Now we can calculate primary current:
Primary current = VA ÷ E = 1,500VA ÷ 480V = 3.13A
Now we can calculate primary protection. Per 450.3(B), it has to be 167% of the primary current. So, 3.13A × 1.67 = 5.22A. We can't apply the next size up rule, so we need a 5A OCPD.
To calculate the secondary conductor size:
Determine the primary to secondary voltage ratio: 480V ÷ 120V = 4.
Multiply the primary OCPD size by 1/3: 5 × 1/3 = 1.67.
Multiply the two values together: 4 × 1.67 = 6.67A.
Look in the 60°C, Table 310.16 for the smallest conductor with an ampacity larger than 6.67A.
If you want more practice, try changing the transformer in the example above, and try different scenarios. For example, what if this is an industrial installation? Then, do the same thing but choose Scenario 5. You might find it useful to spend a few minutes trying different variations. This will help you better understand transformer tap conductor rules. They aren't difficult, if you can approach them methodically and apply those that fit your installation.
If those confusing transformer secondary rules have ever tripped your personal breaker, that's understandable. But now you can steer through those rules with confidence. The next time you need to size secondary conductors, stop to answer a couple of easy questions. Then, select the correct scenario for your installation.
With some transformer secondary conductors, you can use the primary OCPD to provide the overcurrent protection (just as you use the feeder OCPD to protect the feeder tap conductors). But, there's a catch: You can do this only for two specific transformer configurations. Those are:
Single-phase transformer with 2-wire (single voltage) secondary.
3-phase delta-delta transformer with 3-wire (single voltage) secondary.
The OCPDs for those transformers must be sized per Art. 450.
The secondary conductors must meet the sizing minimum required by the installation type described in 240.21(C).