Nec 1996
- Thread starter bcsmith
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Article 450 has had very few changes to it. I believe table 450.3 is still the same.I am looking to get information on the code requirerments for installing an 112.5 KVA transformer. The transformer was installed in 1996, thus I need to obtain code info from 1996. I believe the original installation was sized improperly.
What specific questions do you have?
cadpoint
Senior Member
- Location
- Durham, NC
Wasn't it in the 90's that the NEC let an odd situation out for a cycle.
Wasn't that in respects to 310.16 or another table were wire rating and derating was oddly changed ? I'm sorry again od to the memory!
Wasn't that in respects to 310.16 or another table were wire rating and derating was oddly changed ? I'm sorry again od to the memory!
1971 to 1975.
- Location
- Massachusetts
Assuming the unit has secondary protection (it should) I come up with an NEC maximum primary overcurrent protection value of 338 amps or a 350 breaker size.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Almost, but not quite.
The NEC does not specify a OCPD specific size. Basically for 600V, it says the primary OCPD must not exceed 125% of the primary amps unless you have secondary protection. If you have secondary protection of not more than 125% secondary amps, you are allowed, but not required, to have your primary OCPD larger than 125%.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I have 350 for the secondary OCPD, and I agree with 175 for primary. But is this a 240 delta secondary, or is it one of those wierd high leg thingys that I have never (fortunately) had to deal with? :-? :wink:
K8MHZ
Senior Member
- Occupation
- Electrician
Check voltage from each leg to ground. Carefully....
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Doesn't matter when sizing a 3-pole circuit breaker.
- Location
- Massachusetts
Never mind.
Last edited:
- Location
- Massachusetts
I have to try again. :smile:
I get this
PRI
122,500va / (480x1.732)= 135a
135a x 2.5 = 337 amps
next higher standard = 350a
I get the same for the secondary.
I get this
PRI
122,500va / (480x1.732)= 135a
135a x 2.5 = 337 amps
next higher standard = 350a
I get the same for the secondary.
Last edited:
e57
Senior Member
- Location
- San Francisco, CA
Bob wheres the 2.5 come from???? Or is it one of those reversed math tricks - like 80% of 125% is the same....
Y*1.25=Z
Z*.8=Y
- Location
- Massachusetts
Table 450.3(B) under 'Currents of 9 Amperes or More' and to the right of 'Primary and secondary protection'
e57
Senior Member
- Location
- San Francisco, CA
Ah the 250% (why is it there isn't a head shaking emoticon?) The numbers do come up pretty simular though?
kbsparky
Senior Member
- Location
- Delmarva, USA
I have an extra copy or 2 of the 1996 NEC lying around. You want to buy one?
?..Primary OC protection should be 168.75 or 175 breaker size..?
?..Primary OC protection should be 168.75 or 175 breaker size..?
With Primary and Secondary protection FOR THE Transformer as the Equipment.
Primary protection has no Note 1 for Primary, ie.. ?next higher standard?
Secondary protection does. [240.6 Standard Ampere Ratings.]
Would not Primary OCP be stated at be 150 for the 168.75 or 300 for the 338 calculations.?
(112KVA x 1000)/831,3ph = 135.3 Amp transformer Primary, 169.1 @125% or 338.3 @ 250% if secondary provided.
At this point I Select Wire per 240.4 Protection of Conductors. #1, 75C @ 150A to 300kcmil for the 300A
Is this not correct?
?..Primary OC protection should be 168.75 or 175 breaker size..?
With Primary and Secondary protection FOR THE Transformer as the Equipment.
Primary protection has no Note 1 for Primary, ie.. ?next higher standard?
Secondary protection does. [240.6 Standard Ampere Ratings.]
Would not Primary OCP be stated at be 150 for the 168.75 or 300 for the 338 calculations.?
(112KVA x 1000)/831,3ph = 135.3 Amp transformer Primary, 169.1 @125% or 338.3 @ 250% if secondary provided.
At this point I Select Wire per 240.4 Protection of Conductors. #1, 75C @ 150A to 300kcmil for the 300A
Is this not correct?
- Location
- Massachusetts
In my opinion there is no correct answer, a lot of it is design decisions.
Your answer will work but it is not the maximum the NEC allows.
In this case the NEC will allow a 350 amp breaker on both the primary and secondary size.
Cold Fusion
Senior Member
- Location
- way north
bc -I believe the original installation was sized improperly.
Just curious - what is it about the original installation that makes you think it is improperly sized?
cf
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