Zig Zag transformer connection

[mcelweec@gmail.com]

I have to connect a zig zag transformer to a a 4000amp 480v bus located within 10ft. The connections to the zig zag are max #8 conductor. Do I have to follow the tap rules & size my cables to 400 amp?. If so how can I reduce them to #8 to fit the lugs without creating a violation.

 

[jim dungar]

The zig-zag transformer creates a neutral point and as such would only need to be fed by conductors sized to handle the neutral current hence the #8 lug size.
Yes you need to follow the rules, in 240.21(B) for conductor protection as well as any for grounding transformers in 450.

 

[jaggedben]

I have to connect a zig zag transformer to a a 4000amp 480v bus located within 10ft. The connections to the zig zag are max #8 conductor. Do I have to follow the tap rules & size my cables to 400 amp?. If so how can I reduce them to #8 to fit the lugs without creating a violation.

Can we ask why you are asking about taps? Because you can't add a breaker directly to the bus? This just wasn't stated explicitly.

Unfortunately if you need to do a tap on that bus to it looks like you're going to need to use 400A conductors to a fused disconnect and then fuse reducers (multiple sets?) to get down to a fuse size that protects #8 and/or the transformer.

Other possible ideas to persue, depending on your situation and application:
- Find another downstream load feeder to tap, instead of the 4000A bus.
- Do a tap for an existing load that is larger, then use the freed-up overcurrent slot on the switchboard bus for a smaller breaker more appropriate to your transformer. (Assuming your talking about a tap because your dealing with a full switchboard buss.)

 

[mcelweec@gmail.com]

would this violate 450.5(a)(1) since there is a switch & fuse protection built into the transformer.


(1) Connections. The transformer shall be directly connected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system.

 

[jim dungar]

would this violate 450.5(a)(1) since there is a switch & fuse protection built into the transformer.


(1) Connections. The transformer shall be directly connected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system.

I agree. IMO the intent is to prevent the loss of the neutral point occurring independent of opening the main OCPD.

 

[mcelweec@gmail.com]

In this case is it ok to tap the tap ?