OCPD FOR LOW VOLTAGE SECONDARY TRANSFMORMER

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acoron01

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Albuquerque, N.M., USA
Hello,

i would like some advice/direction as to whether our main OCPD for our secondary side of our transformer is sized correctly. I do not think it is protecting the equipment. We have a , 480V, 225kva, 3 Phase westinghouse transformer with the sub panel in the secondary side of the transformer. There are 6 feet of parrallel 4/0 feeding the backfed main in the subpanel. The main is a cuttler hammer KD3400F breaker. I did a quick calc, to size the breaker correctly and came up with 270Amps available from the secondary side, with a 250Amp breaker, 4-250kcmil conductors & 1-#2AWG EGC rated at 75 degree C. The available short current at a 4.14% Z was 6.5KA. The existing configuration is a 400 amp main, with 6 feet of parrallel 4/0, and 1-4/0 EGC.
 
Out of curiosity, what is the primary voltage on this transformer?

JAP>
 
I would think at most a 350 amp breaker for the secondary.

JAP>
 
acoron01 said:
Hello,

i would like some advice/direction as to whether our main OCPD for our secondary side of our transformer is sized correctly. I do not think it is protecting the equipment. We have a , 480V, 225kva, 3 Phase westinghouse transformer with the sub panel in the secondary side of the transformer. There are 6 feet of parrallel 4/0 feeding the backfed main in the subpanel. The main is a cuttler hammer KD3400F breaker. I did a quick calc, to size the breaker correctly and came up with 270Amps available from the secondary side, with a 250Amp breaker, 4-250kcmil conductors & 1-#2AWG EGC rated at 75 degree C. The available short current at a 4.14% Z was 6.5KA. The existing configuration is a 400 amp main, with 6 feet of parrallel 4/0, and 1-4/0 EGC.
Click to expand...

Take a look at table 450.3(A). You may not even need transformer secondary protection.

The secondary conductors will need protection and (2) 4/0 in parallel are rated at 460 A of ampacity if you have 75 deg C conductors.
 
Table 450.3(a) is for 1000v nominal? I only have 480V? Wouldn't i use Table 450.3(B) and multiply it by 125%, i am confused by this table as well. Is it 9 amps or More on the Primary side correct ? i get 9.8 amps available on the primary side, which would let me use the 167% multiplier which is 450.9 amps, and the 400 amp breaker is still protecting the equipment.
 
acoron01 said:
Table 450.3(a) is for 1000v nominal? I only have 480V? Wouldn't i use Table 450.3(B) and multiply it by 125%, i am confused by this table as well. Is it 9 amps or More on the Primary side correct ? i get 9.8 amps available on the primary side, which would let me use the 167% multiplier which is 450.9 amps, and the 400 amp breaker is still protecting the equipment.
Click to expand...

Look at table A closer. See what it sctually says. Not what you think it says.
 
acoron01 said:
Hello,

i would like some advice/direction as to whether our main OCPD for our secondary side of our transformer is sized correctly. I do not think it is protecting the equipment. We have a , 480V, 225kva, 3 Phase westinghouse transformer with the sub panel in the secondary side of the transformer. There are 6 feet of parrallel 4/0 feeding the backfed main in the subpanel. The main is a cuttler hammer KD3400F breaker. I did a quick calc, to size the breaker correctly and came up with 270Amps available from the secondary side, with a 250Amp breaker, 4-250kcmil conductors & 1-#2AWG EGC rated at 75 degree C. The available short current at a 4.14% Z was 6.5KA. The existing configuration is a 400 amp main, with 6 feet of parrallel 4/0, and 1-4/0 EGC.
Click to expand...

IMHO you are correct. I calculated 270A also [225,000/(3*277) = 270.75A] for sec and so use a 250A main breaker on each phase. What panel are you using???? A 3 phase 480V p-p and 277 p-N??? Is the N on the secondary tied to ground and is it a SDS???? What is the XFR case tied to???? What is the primary ground (Ufer of what??)
 
Thanks bob, i looked at the table again and it meets the code. I think my ground is definitely oversized though. Would that be a code violation? Its a 4/0 ground for a 400 amp breaker, per TABLE 250.66, I would need a 2AWG correct?
 
acoron01 said:
Thanks bob, i looked at the table again and it meets the code. I think my ground is definitely oversized though. Would that be a code violation? Its a 4/0 ground for a 400 amp breaker, per TABLE 250.66, I would need a 2AWG correct?
Click to expand...

look at what table 450.3(A) says.

There is a column for primary protection for voltages over 600V, and one for secondary voltages 600V or less. That suggests to me that you need to use this table and not table 450.3(B).

It seems unlikely to me that a 13.2kV xfmr would ever be found in a location that would not be considered "supervised", especially given how loosely that term is defined.

As for the EGC size, you can make it as big as you want. However, are you sure it is actually an EGC and not some kind of bonding jumper that might require it be larger than the EGC table would require.
 
petersonra said:
l
It seems unlikely to me that a 13.2kV xfmr would ever be found in a location that would not be considered "supervised", especially given how loosely that term is defined.
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Here is a definition of supervised that while it only specifically applies to article 240 but IMO would likely be what the inspection department would consider supervised.


Supervised Industrial Installation. For the purposes of
Part VIII, the industrial portions of a facility where all of
the following conditions are met:

(1) Conditions of maintenance and engineering supervision
ensure that only qualified persons monitor and service
the system.

(2) The premises wiring system has 2500 kVA or greater of
load used in industrial process(es), manufacturing activities,
or both, as calculated in accordance with Article
220.

(3) The premises has at least one service or feeder that is
more than 150 volts to ground and more than 300 volts
phase-to-phase.

This definition excludes installations in buildings used by
the industrial facility for offices, warehouses, garages, machine
shops, and recreational facilities that are not an integral
part of the industrial plant, substation, or control center.
Click to expand...
 
I'm pretty sure it is an egc, we replaced an older transformer with an out of date transformer (westinghouse). And instead of pulling a new ground, the line crew/maintenance crew knew it would be overprotected,and left the existing ground there.
 
iwire said:
Here is a definition of supervised that while it only specifically applies to article 240 but IMO would likely be what the inspection department would consider supervised.
Click to expand...

since the table refers to note 3 in the table itself where it uses the term "supervised location", it seems to me that note 3 to the table would qualify as the definition of supervised location.

3. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the
transformer installation.
Click to expand...
 
petersonra said:
since the table refers to note 3 in the table itself where it uses the term "supervised location", it seems to me that note 3 to the table would qualify as the definition of supervised location.
Click to expand...

Yeah and the result is the same, the other requirements from 240 are commonly meet.

The AHJ has to feel 'conditions of maintenance and supervision ensure that only qualified persons monitor and service the
transformer installation'.

Call it jurisdictional ego, being conservative, etc but I don't think many AHJs will just assume anyplace with a 13.8KV trans is a
supervised location. Letting the fox guard the hen house is not always the best way to go.

I am willing to bet there is a huge range of different policies depending on which area of the country you live.
 
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