Transformer secondary conductors and overcurrent protection sizing

[alej27]

It's my understanding that the NEC has no rules on sizing the capacity (power) of generators, transformers, PV arrays, etc., only on sizing their conductors and protections.

What are the rules for sizing the secondary conductors and the secondary OCPD of a transformer? Is the secondary circuit considered a feeder? Should the secondary conductors and OCPD be sized based on the load current, or the transformer rated current?

A feeder is defined as "All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device" [art. 100.] So it seems the conductors downstream the secondary side of a customer-owned transformer is certainly are a feeder. So, art. 215 should apply for sizing the secondary conductors and secondary OCPD, right?

If so, then we shall size the secondary conductors like a feeder, i.e. with a minimum ampacity of the sum of 100% of noncontinuous load plus 125% of continuous load [sec. 215.2(A)(1)(a).]

However, then there's art. 450 (related to transformers). The informational note #1 of sec. 450.3 states that such section provides rules for protecting the transformer winding (i.e. internal transformer conductors), not the transformer incoming/outgoing conductors. What's the difference?

If treating the transformer secondary conductors like a feeder, then the secondary OCPD shall be sized with a minimum current rating of the sum of 100% of noncontinuous load plus 125% of continuous load [sec. 215.3.] Yet sec./table 450.3(B) states to size the secondary OCPD with a maximum current rating of 125% of the rated secondary current of the transformer. Which one shall we use for protecting the secondary conductors?

Example

Suppose I calculated a continuous load of 10.23 kVA, in a single-phase three-wire 240/120-V system. I choose the next standard size that I can find, say 12 kVA, and there's no NEC rule for this (though I know I should consider altitude derrating, ambient temperature derrating, K-factor/harmonics derrating, load growth factor, motor starting, etc.) Now I think there are two possible ways to proceed, and I don't know which one is code-compliant, or if neither or both are:

1. Next, the continuous load current is (10 230 VA)/(240 V) = 42.625 A [apparent power formula]. Then, I size the secondary OCPD with a minimum rated current of 1.25 × 42.625 A = 53.28 A [sec. 215.3]; I choose the next standard size, 60 A [table 240.6(A)]. After that, I size the secondary conductors with a minimum ampacity of 1.25 × 42.625 A = 53.28 A [sec. 215.(A)(1)(a)]; however, to protect them with the previous OCPD, I size the conductors with an ampacity equal to or greater than the OCPD rated current (60 A), so I choose 1 set of #6 AWG THW conductors (with an ampacity of 65 A) [table 310.16].
2. Next, the secondary rated current is (12 000 VA)/(240 V) = 50 A [apparent power formula]. Then, I size the secondary OCPD with a maximum rated current of 1.25 × 50 A = 62.5 A [sec. 450.3(B)]; I choose the next standard size, 70 A [table 240.6(A), note 1]. After that, I size the secondary conductors with a minimum ampacity of 1.25 × 50 A = 62.5 A [sec. 215.(A)(1)(a)]; I choose 1 set of #6 WG THW conductors (with an ampacity of 65 A) [table 310.16]; however, to protect them with the previous OCPD, I size the conductors with an ampacity equal to or greater than the OCPD rated current (70 A), so I choose 1 set of #4 AWG THW conductors (with an ampacity of 85 A) [table 310.16].

Which of the two previous methods are correct? Or are both wrong? For me the first option seems wrong because I could overload the Tx with 55 A (5 A above its capacity) and the OCPD would not trip, which might cause a fire overtime. This article by Mike Holt at EC&M treats the transformer secondary conductors as a feeder, so the first method is used.

 

[jim dungar]

The output capacity of the transformer is not important as a starting point.
I usually suggest following these steps. Remembering that any single protective device may cover several areas (such asa panel board main also providing secondary conductor protection).
The conductors must be sized to feed to load.
The conductors must be properly protected, see transformer secondary conductors in 240.21(C).
Choose a transformer to feed the secondary load(s).
Protect the primary windings of the transformer per 450.3.
Select conductors to feed the transformer.