Power Transformer Question 2

[TheDawg]

I am working on a Power Distribution Unit where the ETL Listed Equipment has a 500A Main CB feeding
a 300kVA Delta-Wye Transformer (480: 208/120). The Transformer Secondary Feeds Six 225 Amp Molded Case
Circuit Breakers (ie. Primary Protection Only). What NEC Sections would pertain to the sizing and length
of the secondary conductors feeding the six circuit breakers on the secondary side? I am questioning the
size of the conductors used.

 

[dpc]

See Article 240.21

 

[TheDawg]

Thank you for the NEC reference.
Based upon condition of acceptability 240.21 (C)(2)(1),
I am still confused by the requirement for the secdondary
conductor ampacity to be not less than the combined calculated
loads AND not less than the rating of the device supplied by the
secondary conductors OR not less than the rating of the Overcurrent
Protective Device at the termination of the secondary conductors.

The THHN conductors maintain separation and the manufacturer is using NEC
table 310.17 for the ampacity calculations (75 Deg C). The wires terminate
into an 80% rated 225 Amp MCCB, so the ampacity being used is 180 Amps
(80% or 225). Does this jive with the later half of 240.21 (C)(2)(1),
or do the conductors need to be sized for the full 225 Amps? I think the intent
of the code is what I am missing.

 

[dpc]

Conductors must be sized based on the breaker's actual rating. The 80% doesn't enter into the conductor sizing. The 80% accounts for the requirement to size the breaker based on 125% of the continuous load.

(If you have already taken 125% of the continuous load for the sizing calculation, you do not need to apply the 80% de-rating. The 125% and 80% factors are two sides of the same coin. 1/1.25 = 0.8)

 

[TheDawg]

I agree with your comments, and this is how I would approach
conductor sizing for these applications. But I did not design it
and I was hoping to site the NEC for a clear definition on this.
The calculated load is less than 180 Amps, the MCCB is correctly
sized at 225 Amps, the unprotected transformer secondary conductors
feeding the 225A MCCB are #1/0 AWG THHN CU. Using 310.17 (or 310.15(B)(17)
in the 2011 NEC) the 75 DEG Ampacity is 230 Amps at 30 Deg C.
Based upon your comments, this is sound as long as there are no
correction factors required for temperature.

When would the NEC condition: "not less than the rating of the
device supplied by the secondary conductors OR..." apply? Why is this
statement necessary if only the OCPD rating is required? I cannot
think of an application where this would apply.

 

[dpc]

I think you have a typo. For 225 A breaker, you'll need at least #4/0 copper, not #1/0.

"Device" in the NEC is not the same as an "Overcurrent Protecive Device"

From the NFPA commentary:

Components (such as switches, circuit breakers, fuseholders, receptacles, attachment plugs, and lampholders) that distribute or control but do not consume electrical energy are considered devices. Devices that consume incidental amounts of electrical energy in the performance of carrying or controlling electricity are also considered devices. Some examples of these components include a switch with an internal pilot light, a GFCI receptacle, and even a magnetic contactor.

 

[TheDawg]

Thank you for the comments dpc. I am also thinking that the intent of the code would
pertain to a situation where the secondary of a transformer is tapped to feed 2 ea.
Main Lug 225 Amp Rated Load Centers, then the wire would need to be sized per the full
"rating" of the device (ie. 225 Amps) as there would be no OCPD until the branch circuit level.
There is no typo on the 1/0 AWG, this comes from table 310.17 in the 75 Deg C column for
ampacity in free air. (The conductors are not in conduit).

 

[dpc]

Regardless of the NEC ampacity tables, UL requirements for molded case circuit breakers require a minimum wire size for a given breaker size. You can find these requirements in the manufacturer's data. You may want to review this - the breaker manufacturers have tables for this. For a 225 A breaker, the breaker thermal trip elements are (supposedly) calibrated for use with #4/0 copper since the wire conducts a lot of heat away from the breaker. For smaller wire sizes, there is a "wire size correction factor" that you can apply to derate the breaker's continuous current rating.

I have an old GE publication, GET-2779J. I assume something similar is still available.