the 800 Amp rule

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ggonza

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In 240.21 C. It discusses some rules for conductor ocp on transformer secondary's. At the end of the paragraph it states- "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors. 240.4(B) is the section that allows you to go to the next ocpd size up to 800A as long as the conditions stated are met. My problem is that on a one line I'm pulling wire for, there is a 800A switchboard being fed from a transformer secondary with two parrallel sets of 500 kcmil. I think the wire is undersized, but everyone else thinks I'm wrong and that the code in question has nothing to do with my question. Any opinions would be cool.
 

curt swartz

Electrical Contractor - San Jose, CA
Location
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240.4(B) DEVICES RATED 800 AMPERES OR LESS....

that's why the one-line is correct...

You can not apply 240.4(B) to transformer secondary conductors. Transformer secondary conductors must have an ampacity equal or greater that the overcurrent device.
 

don_resqcapt19

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Illinois
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retired electrician
You have to use conductors with an ampacity of 800 amps or more for that application. It has been that way for a long time, but many were missing that rule, so a couple of code cycles ago, the added the rule that the original poster is asking about.

Remember that the rule in 240.4(B), does not change the ampacity of the conductor...it only permits you to use an oversized OCPD.
 
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Dennis Alwon

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Me three. oops.
The three stoogies.
ry%3D400
 

ggonza

Member
Thank you, thank you all for replying. I thought I understood it correctly, now I have to try to convince the higher-ups of the mistake. It will be like pulling teeth.
 

infinity

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Thank you, thank you all for replying. I thought I understood it correctly, now I have to try to convince the higher-ups of the mistake. It will be like pulling teeth.

For what it's worth I see this mistake quite often in the field. Starts with the engineer making the mistake and ends with the electrician installing the mistake.
 

charlie b

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There is another option that has not yet been mentioned, an option that allows you to use the conductors you describe to serve an 800 amp board. What you do is to set the overcurrent trip on the board's main breaker to 760 or lower. This, of course, presumes that the main breaker has the capability to make that adjustment.
 
There is another option that has not yet been mentioned, an option that allows you to use the conductors you describe to serve an 800 amp board. What you do is to set the overcurrent trip on the board's main breaker to 760 or lower. This, of course, presumes that the main breaker has the capability to make that adjustment.

OR
Add and additional service panel/disco for a house panel. As long as the load does not exceed the conductor rating, 230.90 Ex 3 permits this...
 
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