Electrician Talk banner
1 - 7 of 7 Posts

·
Registered
Joined
·
1,398 Posts
Discussion Starter · #1 ·
I have to wire a transformer tomorrow for a practical evaluation in school.

It is a small dry type (25kva I think) that is being fed from a 480 panel and feeds a 120/208 panel. I would appreciate if you guys could tell me if I am missing anything as far as procedure. The transformer will have FMC going to it on both sides.

Primary side:

determine feeder ampacity according to 215.2(B)1 (kva/480).

determine wire size according to 310.15(b)16

determine FMC size according to table C.3

determine OCPD size according to 450.3(b)

I plan on using bonding bushings on each end of the primary side flex and terminating the EGC on the bushings as well as the ground points in the transformer and in the 480 panel.

secondary side:

I will size the conductors and the flex the same as above.

I will size the system bonding jumper according to table 250.66 as directed to by 250.30(a)1

I will size the GEC according to table 250.66 as directed by 250.30(a)3.

I am unsure as to what needs to be done about the size of the main disconnect in the 120/208 panel (3 feet away) and the grounding and bonding there also.

I know thats alot but I take my education and my raises seriously!
 

·
Registered
Joined
·
421 Posts
Bonding bushings are only needed on the "load" side..Separately derived service. which is un fused conductors in metal conduit.
 

·
Registered
Joined
·
2,559 Posts
butcher733 said:
I have to wire a transformer tomorrow for a practical evaluation in school. It is a small dry type (25kva I think) that is being fed from a 480 panel and feeds a 120/208 panel. I would appreciate if you guys could tell me if I am missing anything as far as procedure. The transformer will have FMC going to it on both sides. Primary side: determine feeder ampacity according to 215.2(B)1 (kva/480). determine wire size according to 310.15(b)16 determine FMC size according to table C.3 determine OCPD size according to 450.3(b) I plan on using bonding bushings on each end of the primary side flex and terminating the EGC on the bushings as well as the ground points in the transformer and in the 480 panel. secondary side: I will size the conductors and the flex the same as above. I will size the system bonding jumper according to table 250.66 as directed to by 250.30(a)1 I will size the GEC according to table 250.66 as directed by 250.30(a)3. I am unsure as to what needs to be done about the size of the main disconnect in the 120/208 panel (3 feet away) and the grounding and bonding there also. I know thats alot but I take my education and my raises seriously!
. Do your bonding ( neutral x0 ) to ground ( XFMR chassis ) in the transformer . Jeep neutral and ground separate in the panel . Of you don't do this , you'll have to bond in the panel . It's one or the other , but not both .
 

·
Registered
Joined
·
8,054 Posts
butcher733 said:
I don't understand what your getting at. Are you saying I don't need them on the primary side but I do need them on the secondary? Code reference?
250.92. But I think they're only required for service conductors or any conductor over 250 volts to ground. Also, I'm pretty sure you only need them at impaired connections i.e. reducing washers and concentric/eccentric knockouts

The reason they might be required on the secondary side is because you have a new "service" (system) now and the secondary conductors could be called service conductors now. I could be way off but that's how I rationalize it.
 

·
fertilizer distrubuter
Joined
·
1,250 Posts
I have to wire a transformer tomorrow for a practical evaluation in school.

It is a small dry type (25kva I think) that is being fed from a 480 panel and feeds a 120/208 panel. I would appreciate if you guys could tell me if I am missing anything as far as procedure. The transformer will have FMC going to it on both sides.

Primary side:

determine feeder ampacity according to 215.2(B)1 (kva/480).
Not necessary it is under 600 volts. Size your feeder to your ocpd in accordance with 240.4.

[/QUOTE] determine wire size according to 310.15(b)16

determine FMC size according to table C.3

determine OCPD size according to 450.3(b)

I plan on using bonding bushings on each end of the primary side flex and terminating the EGC on the bushings as well as the ground points in the transformer and in the 480 panel. [/QUOTE]

You only need bond bushings if your primary side consists of concentric knockouts. Make sure you remove paint.

No bond bushing on secondary at all.

250.97

Don't forget you need an ebj to the tranny frame from the supply.

[/QUOTE] secondary side:

I will size the conductors and the flex the same as above. [/QUOTE]

You know they must be bigger right?

I will size the system bonding jumper according to table 250.66 as directed to by 250.30(a)1

I will size the GEC according to table 250.66 as directed by 250.30(a)3.[/QUOTE] 250.30(a)5 [/QUOTE]

I am unsure as to what needs to be done about the size of the main disconnect in the 120/208 panel (3 feet away) and the grounding and bonding there also. [/QUOTE]

You said you were installing a system bonding jumper, that is like your main bonding jumper but for a separate system. Your gec can terminate there as well.
You do not need a main 450.3(b) [/QUOTE]

I know thats alot but I take my education and my raises seriously![/QUOTE]

You are pretty on top of it read this it should get you squared away. http://ecmweb.com/bonding-amp-grounding/basics-bonding-and-grounding-transformers
 
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top