30 KVA Dry Type Transformer Secondary Tap

Status
Not open for further replies.
K

ken44

Senior Member
Location
Austin, TX
I am ceiling mounting this 30 KVA transformer and due to space restrictions in our Mechanical/Electrical room, it is being located about 100' away from the Transformer in a Copier room. The Panel Board is 100 Amps with a connected load of 71 amps. What are the NEC rules concerning the secondary conductors from the Transformer to the Panel Board.
 
ken44 said:
. . . this 30 KVA transformer . . . is being located about 100' away from the Transformer. . . .
Click to expand...
;) :grin:

I think you meant the panel is 100 feet from the transformer. You will need to put an overcurrent device closer to the transformer. An enclosed circuit breaker or a fused disconnect would do the trick. But they have to be closer to eye level, not located with the transformer above the ceiling.
 
Do you mean that the transformer is being ceiling mounted 100' from where it receives it's supply? If this is what you mean than look at section 240.21 like benaround said.
 
30 KVA Dry Type Transformer Secondary Tap

Charlie, you were correct, the panel is going to be located 100' away from the transformer. It is my intention to place an OCPD in an accessible location inside the Mechanical/Electrical room to protect the feeders between the transformer and the panel. However, I am not able to locate anything in the 2005 NEC that deals with my specific situation. There seems to be a loophole for high bay buildings or outside of a building but I do not see anything specific for this situation and its the first time I have ran into something like this.
 
Inside transformer taps of over 25' are a problem. . The restrictions of 240.21(B)(4) limit the application greatly. . Remember that primary OC is not length restricted because it's located on the line end of the conductors. . Sometimes the best solution is to drop secondary protection completely and go with primary only. . Check out T450.3(B). . You always keep the option of bringing the load end of the secondary conductors to an OCPD anywhere you want, if the secondary OCPD isn't required.

Will primary only OC work in this ap ?
 
dnem said:
Sometimes the best solution is to drop secondary protection completely and go with primary only. . Check out T450.3(B). . You always keep the option of bringing the load end of the secondary conductors to an OCPD anywhere you want, if the secondary OCPD isn't required.

Will primary only OC work in this ap ?
Click to expand...

look at 240.21 (C) (1)
 
240.21(C)(1) does not apply in this situation.

You will most likely need to look at .21(C)(2) or (6). That is my guess based on the info you have provided. Don't forget about 408.36(B) [the exception to .36(B) is very restrictive].
 
Pierre C Belarge said:
240.21(C)(1) does not apply in this situation.
[/COLOR][/I]].
Click to expand...


pierre

i was responding to davids post about dropping secondary protection and going with primary protection only
 
dnem said:
Inside transformer taps of over 25' are a problem. . The restrictions of 240.21(B)(4) limit the application greatly. . Remember that primary OC is not length restricted because it's located on the line end of the conductors. . Sometimes the best solution is to drop secondary protection completely and go with primary only. . Check out T450.3(B). . You always keep the option of bringing the load end of the secondary conductors to an OCPD anywhere you want, if the secondary OCPD isn't required.

Will primary only OC work in this ap ?
Click to expand...
David,
You have to be careful here. There are two articles that must be complied with. Article 450 for the transformer protection and Article 240 for the protection of the secondary conductors. It is a rare installation where the primary only protection for the transformer can also protect the secondary conductors. In most cases the secondary conductor protection must be within 25' wire feet from the secondary terminals.
 
don_resqcapt19 said:
David,
You have to be careful here. There are two articles that must be complied with. Article 450 for the transformer protection and Article 240 for the protection of the secondary conductors. It is a rare installation where the primary only protection for the transformer can also protect the secondary conductors. In most cases the secondary conductor protection must be within 25' wire feet from the secondary terminals.
Click to expand...


I agree with this statement, not that it means much if I do. :wink:
 
don_resqcapt19 said:
David,
You have to be careful here. There are two articles that must be complied with. Article 450 for the transformer protection and Article 240 for the protection of the secondary conductors. It is a rare installation where the primary only protection for the transformer can also protect the secondary conductors. In most cases the secondary conductor protection must be within 25' wire feet from the secondary terminals.
Click to expand...

If the primary OCPD value is set according to T450.3(B) and the transformer is one listed in 240.21(C)(1), then primary OC only is OK. . Why would you categorize this as a rare installation ?
 
Pierre C Belarge said:
240.21(C)(1) does not apply in this situation.

You will most likely need to look at .21(C)(2) or (6). That is my guess based on the info you have provided. Don't forget about 408.36(B) [the exception to .36(B) is very restrictive].
Click to expand...

408.36 says, "at any point on the supply side of the panelboard." . You have no restriction of distance between panel and transformer so the "about 100 feet" would be fine. . The secondary panel would need a main but that's not a problem. . The problem is the distance between panel and transformer.
 
dnem said:
408.36 says, "at any point on the supply side of the panelboard." . You have no restriction of distance between panel and transformer so the "about 100 feet" would be fine. . The secondary panel would need a main but that's not a problem. . The problem is the distance between panel and transformer.
Click to expand...


408.36 states the protection to be on the secondary side.
240.21(C) sets the limits as to the distance this protection will be installed to.


Take a close look at the type of systems 240.21(C)(1) sets as a parameter. These systems are not your typical installation for premises wiring.
 
dnem said:
If the primary OCPD value is set according to T450.3(B) and the transformer is one listed in 240.21(C)(1), then primary OC only is OK. . Why would you categorize this as a rare installation ?
Click to expand...
Because delta-delta and two wire to two wire transformer installations are not very common. There are a lot more delta-wye and two wire to three wire transformer installations.
 
The transformer is a 480 volt primary to 208Y/120 volt secondary. The panel will have a 100 amp main. I have read all the codes that were referenced and it looks to me like it would be in my best interest to provide an OCPD for the secondary conductors.
 
don_resqcapt19 said:
Because delta-delta and two wire to two wire transformer installations are not very common. There are a lot more delta-wye and two wire to three wire transformer installations.
Click to expand...

That's what happens when I read thru the code article too quickly. . I didn't pick up on the restriction to a delta primary. . I also didn't pick up on the single voltage secondary restriction that would exclude a centertapped delta secondary.

When you exclude all wye primaries and center tapped delta secondaries that would mean that primary only protection would be very rare.

I do my inspections from the approved plans and don't get in on the decision making side very often. . Now I know why I never see primary only protection.
 
Status
Not open for further replies.
Top