NEC Transformer Secondary Issues

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tecorp

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Location
SoCal
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Electrical Engineer
Hello,

I have a 480V panel with a 225A breaker feeder a 150kVA transformer that provides 208/120V on the secondary. The primary feeder is 4/0 conductors and the secondary is 500 conductors. Secondary feeder of less than 10' goes to a distribution board with a 500A breaker.

The secondary breaker is properly sized per 450.3(B).

The inspector is having issue with the secondary feeder. Yes there is no overcurrent protection for it (500A breaker on 380A feeder), but doesn't 240.21(C)(2) relieve me of secondary overcurrent protection anyways? The feeder is less than 10', the ampacity of the conductor meets the calculated load of 357A, the conductors end at the switchgear, and conductors are in EMT conduit, and no the conductors do not leave the enclosure. Since I meet all those requirements, shouldn't 240.21(C)(2) basically say the main breaker isn't for secondary overcurrent protection? It is for mechanical loads so we don't want nuisance tripping anyways.

What am I missing?
 
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Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
I don't know what you are reading, or I am not understanding what you are writing. 240.21(C)(2)(1)(b) tells you you are in violation. The equipment containing the overcurrent device is obviously larger than 500A and the overcurrent protection is larger than the ampacity of the conductors.

(2) Transformer Secondary Conductors Not over 3 m (10 ft)
Long. If the length of secondary conductor does not exceed
3 m (10 ft) and complies with all of the following:
(1) The ampacity of the secondary conductors is
a. Not less than the combined calculated loads on the
circuits supplied by the secondary conductors, and
b. Not less than the rating of the equipment containing
an overcurrent device(s) supplied by the secondary
conductors or not less than the rating of the overcurrent
protective device at the termination of the secondary
conductors.
 
X

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
tecorp said:
Hello,

I have a 480V panel with a 225A breaker feeder a 150kVA transformer that provides 208/120V on the secondary. The primary feeder is 4/0 conductors and the secondary is 500 conductors. Secondary feeder of less than 10' goes to a distribution board with a 500A breaker.

The secondary breaker is properly sized per 450.3(B).

The inspector is having issue with the secondary feeder. Yes there is no overcurrent protection for it (500A breaker on 380A feeder), but doesn't 240.21(C)(2) relieve me of secondary overcurrent protection anyways? The feeder is less than 10', the ampacity of the conductor meets the calculated load of 357A, the conductors end at the switchgear, and conductors are in EMT conduit, and no the conductors do not leave the enclosure. Since I meet all those requirements, shouldn't 240.21(C)(2) basically say the main breaker isn't for secondary overcurrent protection? It is for mechanical loads so we don't want nuisance tripping anyways.

What am I missing?
Click to expand...
See 240.21(C)(2) - the secondary conductors must have an ampacity that is equal to or greater than the OCPD supplied by the secondary conductors.
 
T

tecorp

Member
Location
SoCal
Occupation
Electrical Engineer
oh crap you are right. welp. i guess the feeder is incorrectly size. is the easiest thing to swap out the 500A with a 400A? or is there some exception out there that says we are good?
 
X

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
tecorp said:
oh crap you are right. welp. i guess the feeder is incorrectly size. is the easiest thing to swap out the 500A with a 400A? or is there some exception out there that says we are good?
Click to expand...
IMHO.... does the secondary load require a neutral? Is the Neutral considered a CCC? If so, after derating, I believe 300 is the largest breaker.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
tecorp said:
oh crap you are right. welp. i guess the feeder is incorrectly size. is the easiest thing to swap out the 500A with a 400A? or is there some exception out there that says we are good?
Click to expand...
Can't use 500 kcmil as a secondary conductor with a 400 amp ocpd
(C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). Section 240.4(B) shall not be permitted for transformer secondary conductors.
Click to expand...
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Seemingly the simplest correction would be to install secondary conductors rated at or above 500 amps.
 
T

tecorp

Member
Location
SoCal
Occupation
Electrical Engineer
augie47 said:
Seemingly the simplest correction would be to install secondary conductors rated at or above 500 amps.
Click to expand...
yup. i think we're just gonna do this.

so with 500A mcb ahead, we must have wire sized for 625A (500A/0.8) since we have 4 current carrying conductors.

that means we'll need 2 parallel sets of 400MCM wire.

using 250.66 we'll need 2/0 ground on each set of the feeder.

sound about right?
 
T

tecorp

Member
Location
SoCal
Occupation
Electrical Engineer
Krusscher said:
But that is something you may want to ask the AHJ since this issue is something they will be paying close attention to.
Click to expand...
Yea I was leaning on the heavy side since I don't want to piss off the inspector anymore. But yes most loads are nonlinear.

So then I'll go with 2 sets of 250MCM wire with 1/0 ground in each parallel set
 
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