HELP! Transformer Neutal, MBJ, and GE Conductor Sizing.

[Mattula]

I need your help. I have not sized the Neutral, Main Bonding Jumper, or the Grounding electrode conductor in a while. Please check my work. Im feeling unsure. Ive been out of school for quite a few years now. Thanks for your help.

Transformer:
75KVA
Primary-480v 3Ph 3w - (3#1 & 1#6G)
Secondary- 120/208V 3Ph 4w - ( 4#4/0 & 1#2G)

Neutral: Art.250.66 Table
Largest Ungrounded Conductor - #4/0 Cu Conductor
Neutral Size: #2 Cu Conductor
(This seems too small to me.)

Main Bonding Jumper: #2 Cu
(Uses same table and math as the Neutral)
( This seems too big to me. Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)

Grounding Electrode Conductor:
This one has me a little puzzled.....250.66 Table says to use #2 copper. Article 250.66(A) says you only have to use #6 copper for the connection to the rod. So would you install #2 and bug a #6 to attach to the rod? Or could you use #6 for he entire run? Thanks!

 

[augie47]

I need your help. I have not sized the Neutral, Main Bonding Jumper, or the Grounding electrode conductor in a while. Please check my work. Im feeling unsure. Ive been out of school for quite a few years now. Thanks for your help.

Transformer:
75KVA
Primary-480v 3Ph 3w - (3#1 & 1#6G)
Secondary- 120/208V 3Ph 4w - ( 4#4/0 & 1#2G)

Neutral: Art.250.66 Table
Largest Ungrounded Conductor - #4/0 Cu Conductor
Neutral Size: #2 Cu Conductor
(This seems too small to me.)
!

Should be sized per 220.62 (by load)

Main Bonding Jumper: #2 Cu
(Uses same table and math as the Neutral)
( This seems too big to me. Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)
!

#2 is correct 250.30

Grounding Electrode Conductor:
This one has me a little puzzled.....250.66 Table says to use #2 copper. Article 250.66(A) says you only have to use #6 copper for the connection to the rod. So would you install #2 and bug a #6 to attach to the rod? Or could you use #6 for he entire run? Thanks!

#6 to Ground Rod, but preferred electrode is building steel or water and they would be #2 (250.30(A)(7))

 

[Mattula]

Thank you for your help.

 

[benaround]

Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)



No, run a continuity test between that strap and XO before any wires are connected and

you will not have continuity.

 

[Mattula]

Main Bonding Jumper should be sized per 220.62 (by load)

Can you explain further? I only see 220.61......after that it goes to 220.80. I dont see it?

 

[kwired]

Main Bonding Jumper: #2 Cu
(Uses same table and math as the Neutral)
( This seems too big to me. Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)

anytime a strap or screw is used it is a part of or an accessory to the equipment being bonded it has been tested and listed for the purpose. If using your own bonding jumper then it must be sized by table 250.66

 

[TT009]

Transformer:
75KVA
Primary-480v 3Ph 3w - (3#1 & 1#6G)
Secondary- 120/208V 3Ph 4w - ( 4#4/0 & 1#2G)

Neutral: Art.250.66 Table
Largest Ungrounded Conductor - #4/0 Cu Conductor
Neutral Size: #2 Cu Conductor
(This seems too small to me.)

What is the desired secondary amperage you are trying to get out of your transformer?

 

[TT009]

Main Bonding Jumper should be sized per 220.62 (by load)

Can you explain further? I only see 220.61......after that it goes to 220.80. I dont see it?

Do you have the newest code?

 

[Mattula]

225 amps on the secondary.

NFPA 70 2008 Edition

 

[Mattula]

Sorry the secondary is rated at 208A, but the ocp is 225a. The next size up.

 

[infinity]

Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)

Almost never. For what it's worth this is a common misconception.

 

[TT009]

Like augie said above,

You should be fine with the grounding.

Check your sizing of the neutral, 220.61, depending on your load it will probably be the same as your ungrounded conductors (4/0)

 

[augie47]

Main Bonding Jumper should be sized per 220.62 (by load)

Can you explain further? I only see 220.61......after that it goes to 220.80. I dont see it?

my error 220.61
(not to make excuses, but I'm havimng to use a "spare" computer and the keyboard and I don't like each other.)

 

[infinity]

Main Bonding Jumper: #2 Cu
(Uses same table and math as the Neutral)
( This seems too big to me. Arent most Y transformers bonded when made with that metal strap that connects to the frame from the coils?)

anytime a strap or screw is used it is a part of or an accessory to the equipment being bonded it has been tested and listed for the purpose. If using your own bonding jumper then it must be sized by table 250.66

Had chance to snap a few photo's that pertain to this thread. Here's one of the bonding strap used by the manufacturer to bond the frame to the core of the transformer. The XO connection is nowhere in sight.

 

[augie47]

Rob, it's early, but, when you say "XO connection s nowhere in sight" are you referring only to Picture #1. In your 2nd picture, would the equipment grounding conductor installed from XO through the lug enroute to the conduit not serve as your XO bonding jumper?

 

[infinity]

Bob, it's early, but, when you say "XO connection s nowhere in sight" are you referring only to Picture #1. In your 2nd picture, would the equipment grounding conductor installed from XO through the lug enroute to the conduit not serve as your XO bonding jumper?

I was trying to show that the bonding strap installed by the manufacturer in photo #1 does not connect the XO to the casing of the transformer. I hear this quite often from guys in the filed that the XO is "factory bonded" with that strap. I comes up on the forum every once in a while and was mentioned in this particular thread so I just wanted to point it out.

In answer to your second question yes the lay-in style lug is bonding the
XO to the transformer case. I would add one more thnig about photo #2, the bonding bushings are not required by the NEC.