service conductor size based on distance

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Telaid

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What is the correct method, (NEC section) to determine the proper size of my service conductors? Im installing an 800 amp main fed from service laterals from a pole 175 feet away. Im running 2 4" conduits with 3 500kcmil each and i want to make sure the size is ok.
 
The NEC is not a design manual. It doesn’t tell you how to choose or size anything.

It does, however, give limiting values for a variety of components. Table 310.16, for example, will tell you that a 500 MCM copper conductor has an ampacity of 380 amps, not 400. So if the load is greater than twice 380, then you can’t use a pair of parallel 500’s.

You need to start with a service load calculation, per Article 220.

Edited to add (after noticing that Chris got his post in before mine) that 230.42 gives limits, and not a design process. It also tells you to calculate the load per 220.
 
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You're the electrical engineer, show us your calculations and we will review them. There are no requirements in the NEC for limiting voltage drop of service conductors however you are the design professional and should probably take that into consideration.

So, what do yout think? Will parallel 500 kcmil's 175' long properly serve the calculated loads at or about the nominal voltage of the system??? You tell us...
 
I agree that the NEC is not a design manual, I posted the reference to 230.42 to show the section of the NEC that deals with the minimum size and rating of the service entrance conductors.

Chris
 
I'm always confused by the statement that service conductors only need to be sized for the computed load (230.42), but then 230.90 states that the OCP must not exceed the ampacity of the conductors.

So, 230.42 allow the conductors to be sized for the load instead of what, exactly?
 
LarryFine said:
I'm always confused by the statement that service conductors only need to be sized for the computed load (230.42), but then 230.90 states that the OCP must not exceed the ampacity of the conductors.

So, 230.42 allow the conductors to be sized for the load instead of what, exactly?
Click to expand...

Larry, if you computed the load to be 120 amps. You would need a #1 awg cu. minimum (service conductor) and the breaker would be 125 amp max.Based on wire size.
Rick
 
LarryFine said:
So, 230.42 allow the conductors to be sized for the load instead of what, exactly?
Click to expand...
I'm not rightly sure myself.

What has always put a big question in my mind is why is 230.42(A) calls for a calculation outside of what has already been calculated in Article 220, Parts III, IV, or V? Why do I not see this called-for calculation in any of the Annex D examples?

If one takes the text literally, 230.42(A) requires a completely different calculation than Article 220 Parts III, IV, or V, using loads determined in Article 220, Part II. And the result of that calculation would mean the service entrance conductors would have to be sized to handle the current as if all determined loads were conducting at full power, i.e. everything ON.

I tried to point this out one other time and was scoffed in doing so... so here we go again :grin:
 
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Thanks guys,

My confusion came from the continual circular references between all the sections for sizing the conductors, aticle 3XX says goto 220 which says back to 3xx and so on. Gets a bit confusing.
 
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