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.
the 800 Amp rule
- Thread starter ggonza
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curt swartz
Electrical Contractor - San Jose, CA
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- San Jose, CA
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- Electrical Contractor
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.
- Location
- 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.
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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- Chapel Hill, NC
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- Retired Electrical Contractor
Curt where are you reading that info. I don't do tranies so I do not see what you are stating in article 240.4.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Me neither.
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- New Jersey
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- Journeyman Electrician
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- Chapel Hill, NC
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- Retired Electrical Contractor
I read the op and somehow I had 240.4(B) in my head and totally missed the 240.21(C) reference
i did too...
jumper
Senior Member
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Me three. oops.
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- Chapel Hill, NC
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- Retired Electrical Contractor
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Pierre C Belarge
Senior Member
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- Westchester County, New York
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- Location
- New Jersey
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- Journeyman Electrician
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.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
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.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
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...
K8MHZ
Senior Member
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- Electrician
I wanna be Curly.
Where's Larry?
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