How to solve these questions?

[xxj.electricxx]

1) 400 amp service supply with 500 MCM THW, What is the minimum size grounding conductor?


2) 40 amp heat element with a 30 amp pump what is the total amp required?

 

[erickench]

Perhaps they mean grounding electrode conductor. I would refer you to NEC Table 250.66. For 500 MCM the minimum size would be no. 1/0 for both copper and aluminum.

 

[infinity]

Where are you finding these questions? The use of MCM has been gone for 10 years or more. And the term grounding conductor is pretty meaningless. Since the question mentioned service I agree with Eric that T250.66 is the most likely applicable table.

 

[xxj.electricxx]

Thanks for the replies!!

 

[charlie b]

The use of MCM has been gone for 10 years or more.

No, it is still being used, mostly by guys like me who still keep a slide rule at their desk and still remember how to use one. :roll::grin:

 

[RichB]

DITTO!!! CharlieB

 

[S'mise]

I think they want equiptment grounding. 250.122t says #3cu
they dont say what kind of pump but I;m thinking AC. Largest used at once would be the heat. 40amp

One of those cases where you take a guess and see if it matches the answers available. Better yet, ask the teacher what are are they asking?

PS;(I sometimes think kcmil? Now, whats that in MCM? I'm glad we havent switched to metric)

 

[jbrown]

As one of the older fogey's, I also still use MCM instead of Kcmil. Its shorter, easier to say and write, and everybody knows what your talking about. Per Table 250.66, 1/0 cu or 3/0 al is the correct answer for 500 MCM copper. We can figure the service is copper because you would need 600 MCM aluminum to carry 400A (Table 310.15(B)(6))

Does your book give 77.5A as an answer for #2?

ps: I still refer to a typical basement pull string lamp base as a porc-pull base...