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Old 12-16-04, 05:53 PM
lagunavolts
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all code guru's

if i am running a 3/4 emt conduit, (all branch circuits), to feed convience outlets and 2x4 flourescent lights, how many #12 thhn conductors can i fit into this conduit. All the branch circuits are 20 amp circuits.

my question is about conductor deration. All info is greatly appreciated.

lagunavolts
 
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  #2  
Old 12-16-04, 06:12 PM
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To allow you to still use 20-amp breakers on this #12 wire, you may run up to 9 current-carrying (i.e., not counting grounding wires) conductors through the conduit.
 
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Old 12-17-04, 04:32 PM
lagunavolts
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John, Whats the code on deration? The maximun number of #12 conductors in a 3/4 conduit (emt) is 16. I am really confused about deration of conductors. How do you come up with 9 wires, is this including three neutrals or 9 plus the three neutrals. What does the code book mean by Maximum conductors of 16??

Thank you for your input.
 
  #4  
Old 12-17-04, 04:51 PM
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Neutrals are current carrying conductors. They are counted. Grounds do not carry any current. They are not counted.
 
  #5  
Old 12-17-04, 05:38 PM
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Derating and conduit fill are two independent considerations. Conduit fill counts all the conductors, but derating only counts current-carrying conductors. Since conduit fill limits are higher than derating limits, the conduit fill limits are immaterial. Derating is independent of the type and size of the conduit. The answer to your original question would have been the same no matter what size conduit you had mentioned.

There are multiple factors affecting derating. The two most common are ambient temperature and number of current-carrying conductors. There is a table of derating percentages as a function of the number of current-carrying conductors in the code. Nine current carrying conductors derates the ampacity to 70%, whereas ten derates to 50%. Since we are allowed to derate from the 90-degree column, and the 90-degree column for #12 is 30 amps, 70% of 30 amps still keeps you above the 20-amp breaker size, which is the maximum allowed for #12 copper wire.

The above is a very brief summary and doesn't include factors from the dozen other codes that also have an effect on this stuff.
 
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Old 12-17-04, 07:35 PM
lagunavolts
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wire deration

John,
thank you for your detailed response. I am still having a problem with the arrival of these figures, and short of getting into an engineering debate, I do want to ask about this last question; how do these conductors lose integrity if they are not carrying a continuous load? If i have a three phase panel, and I am feeding lights and plugs, the lights of course are typicaly continous, ( in office space.) the receptcales however, in this example, are convience use devices that do not have a load. therefore not raising the temp. around the wire inside the conduit. How are these deration figures developed?

thanks again

laguna
 
  #7  
Old 12-17-04, 08:00 PM
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They don't lose "integrity" (whatever that means). They just heat up. There's no way the code can cover every particular situation (such as "how many conductors can I run together if one carries 5.3 amps for 27% of the time and another carries 3.2 amps for 58% of the time and ..."), so in order to keep the 700-page code from being a 70,000-page code, some simplifications, generalizations, and approximations are necessary. Unfortunately if you don't exactly fit the generalizations, you may be prohibited from doing something that is not really unsafe for your particular situation. Life is a set of compromises.
 
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