225kva transformer feeders

Status
Not open for further replies.

eager2learn

Senior Member
Location
Mennifee,Ca
what size conductors do i need for a 225kva three phase transformer on the primary side = 480 @ 270 amps - secondary = 208y/120 @ 625 amps - i feel like im doing something wrong
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
You haven't given enough information to provide an answer. The primary and secondary conductor sizes of your 225kVA transformer will be based on the primary and secondary OCPD sizes, not the rated primary and secondary currents.
 

eager2learn

Senior Member
Location
Mennifee,Ca
the owner wants to run 200 amps to all three phases on the line side using existing conductors which are 3/0 - on the secondary side he wants to run 2 panels that are rated at 225 amps each. i am aware that on both the primary and secondary side i will also have to add 125% to the ocpd - i guess the real question that i have is, can i run the 200 amps on the 3/0 conductors or do i have to bump them up to 4/0 - as far as the secondary side the owner wants us to run parallels to each panel individually - i hope that makes sense - he also has existing conductors which are 1/0 - i have to total the load but i am more less concerned with how well the transformer will be effeciecnt.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
the owner wants to run 200 amps to all three phases on the line side using existing conductors which are 3/0 - on the secondary side he wants to run 2 panels that are rated at 225 amps each. i am aware that on both the primary and secondary side i will also have to add 125% to the ocpd - i guess the real question that i have is, can i run the 200 amps on the 3/0 conductors or do i have to bump them up to 4/0 - as far as the secondary side the owner wants us to run parallels to each panel individually - i hope that makes sense - he also has existing conductors which are 1/0 - i have to total the load but i am more less concerned with how well the transformer will be effeciecnt.

Does he want to run one set to each panel? That would make them not parallel.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
(Since Bob is apparently no longer on line, I will add my 2 cents)
1st.. what he said.. unless the conductors begin and end at the same point they are not parallel conductors. If you begin at the transformer and end and different panels they are simply transformer secondary conductors which fall under 240.21(C).
2nd. You mentioned adding 125% to the OCPD, The OCPD must be sized to protect the conductors,,there is no "adder" in this case. If you have 3/0 conductors, the OCPD would be 200 amps (normally). This install has no valid reason to go higher.
3rd. IMO, it is possible that the 200 amp OCP might not handle the inrush on the
transformer so that may be a basic flaw in the plan.
4th on the secondary side, if you are using existing 1/0 and are running 1 set of wires to each panel. the panel main could not exceed 150 amps. If you are in fact paralleling to a panel, then it would depend on your method.
 

iwire

Moderator
Staff member
Location
Massachusetts
what size conductors do i need for a 225kva three phase transformer on the primary side = 480 @ 270 amps - secondary = 208y/120 @ 625 amps - i feel like im doing something wrong

You are not NEC required to size the breakers to transformer ratings, the NEC would allow you to use 15 amp breakers on both sides of this transformer and 14 AWG.

It would not work but it would be allowed, the thing to keep in mind is the NEC is not an instruction book for good design.

You need to start with the load you anticipate connecting and work backward from there.
 

eager2learn

Senior Member
Location
Mennifee,Ca
I have 3/0 feeding the disconnect on the line side of the cartridge fuses which will be rated @ 175 amps - on the secondary side of the fuses feeding the primary side of the x-frmr i have 1/0 - i then have 1/0 also feeding the two seperate panels on the load side of the secondary - the total load between each panel as of right now is expected to be approximately 40 amps - the panels will be utilized for lighting and a few general circuits. the owner just bought this factory and is trying to utilize all materials with in at no additional expense.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I have 3/0 feeding the disconnect on the line side of the cartridge fuses which will be rated @ 175 amps - on the secondary side of the fuses feeding the primary side of the x-frmr i have 1/0 - i then have 1/0 also feeding the two seperate panels on the load side of the secondary - the total load between each panel as of right now is expected to be approximately 40 amps - the panels will be utilized for lighting and a few general circuits. the owner just bought this factory and is trying to utilize all materials with in at no additional expense.

You have multiple NEC violations. The 1/0 conductors from the 175A fuses to the xfmr primary are not properly protected per 240.4.

And each set of 1/0 conductors from the transformer secondary to the 225A MCB in each panel is not properly protected per 240.21(C).

As Gus pointed out in post #5, the panels would need 150A (or smaller) MCBs to use the 1/0 secondary conductors. The primary fuses would also need to be 150A to use the 1/0 primary conductors, but again, there is the concern of the inrush current that Gus mentioned.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have a difficult time following, but if your fuses are protecting a 1/0 cable, they need to be 150 amp.
Likewise is 1/0 is feeding your secondary panels they need to comply wit 240.21(C) with your panel mains being 150 amp max.
 

erickench

Senior Member
Location
Brooklyn, NY
Sounds like to me your conductors are all undersized to begin with. Why don't you size the conductors according to the rated current and set your OCPD according to NEC table 450.3(B).
 

eager2learn

Senior Member
Location
Mennifee,Ca
so if have a 3 - 200 amp fuses at the switchgear which feeds the primary disconnect with the 3/0 conductors which are rated at 225 amps, can these conductors feed the disconnect which will have another set of 3 - 200 amp fuses installed with 3/0 conductors feeding the primary side of the transformer be suitable. in addition, the two panels located on the secondary side of the transformer are within 7 feet of the transformer, so there is no need for a secondary disconnect. the 1/0 is rated at 170 amps at the total load for both panels is approximately 80 amps, can these conductors be utilized or do i have to also provide 3/0 to both panels. the panels are load centers with no main breaker just branch breakers. it all seems pretty tacky to me, i'm just trying to help out the owner with what he has. im getting ready for the install in two days and dont want to waste too much time on the job trying to figure this out. i know a lot of this may be redundant, but all the information given would be greatly appreciated. i have taken a look at the codes you have all mentioned earlier and im having a hard time. thanks
 

eager2learn

Senior Member
Location
Mennifee,Ca
so that would mean that i would have to install 300 kcmil on the primary side according to the 75 degree column for the 270 amp current rating - can i take these conductors to the panels on the secondary side and i would also have to install ocpd on the secondary side before the panels. there within 7 feet of the transformer and the ampacity of the conductors are not less than the primary ocpd.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
so if have a 3 - 200 amp fuses at the switchgear which feeds the primary disconnect with the 3/0 conductors which are rated at 225 amps, can these conductors feed the disconnect which will have another set of 3 - 200 amp fuses installed with 3/0 conductors feeding the primary side of the transformer be suitable.

The 3/0 has an ampacity of 200A. I don't see why you would need another set of 200A fuses, but yes, the 3/0 from the second set of fuses to the xfmr primary is compliant for the 200A OCP.

in addition, the two panels located on the secondary side of the transformer are within 7 feet of the transformer, so there is no need for a secondary disconnect. the 1/0 is rated at 170 amps at the total load for both panels is approximately 80 amps, can these conductors be utilized or do i have to also provide 3/0 to both panels.

The 1/0 has an ampacity of 150A. Per 240.21C, the secondary conductors must be protected by an OCPD not exceeding the ampacity of the conductors, at their termination. You'd need to have a 150A OCPD at the end of the secondary conductors.


the panels are load centers with no main breaker just branch breakers. it all seems pretty tacky to me, i'm just trying to help out the owner with what he has. im getting ready for the install in two days and dont want to waste too much time on the job trying to figure this out. i know a lot of this may be redundant, but all the information given would be greatly appreciated. i have taken a look at the codes you have all mentioned earlier and im having a hard time. thanks

Panelboards are required to be protected against overcurrent by an OCPD not greater than the rating of the panelboard by 408.36. If the two load centers do not have MCBs, then a separate C/B of fused disconnect must be installed in the secondary conductors before the panels.

so that would mean that i would have to install 300 kcmil on the primary side according to the 75 degree column for the 270 amp current rating

No, you are not required to install either primary or secondary conductors for the "rated transformer current." Your conductors are required to be large enough to carry the load current, and are required to be protected against overcurrent with an OCPD.

can i take these conductors to the panels on the secondary side and i would also have to install ocpd on the secondary side before the panels. there within 7 feet of the transformer and the ampacity of the conductors are not less than the primary ocpd.

Yes, you need on OCPD on the secondary side, both to protect the conductors per 240.21(C) and to protect the panel per 408.36. The primary OCPD cannot be used to protect the secondary conductors (except for the specific conditions listed in 240.21(C)(1).) You do not meet the conditions in 240.21(C)(1), so the secondary conductors must be protected by secondary OCPDs.
 
Status
Not open for further replies.
Top