Mains conduit fill question (N. America)

You can go through the math and see that there is no need to include the neutral in a total current derating of a conduit. The maximum power transfer without exceeding a current X is when all three phases carry the maximum current, X. In this case, the neutral carries zero current. But what about unbalanced loads? The simplest case is one phase carrying the maximum X and the others zero. In this case the total current is 2X, less than the maximum 3X. If two phases carry the maximum and the other zero, the neutral also carries X and the total is 3X, equal to that from the max load. Any other combination also produces a total current of 3X or less.

There is an exception. Very nonlinear loads like power supplies in computers. They usually draw only during the voltage peak and the neutral current often does not cancel completely. I've heard stories of how the neutral has burned out when supplying a data center full of computers with

3 phase.
Reply to
Michael Moroney
Loading thread data ...

Per NEC all conductors must be included in the fill calculation.

?-)

Reply to
josephkk

Not required to be that large by the NEC but may be required to be so by local code. Personnally i consider it to be proper design.

?-)

Reply to
josephkk

Often true, but always part of conduit fill calculations.

?-)

Reply to
josephkk

Thank you Ed. Jamie should pay attention to Article 250.

?-)

Reply to
josephkk

Guess you've never had to deal with those passing out citations to violations.. They don't care how you, the violator interpret the rules.

I've seen the section that was used to make the citation valid, it was clear and to the point!. You can read all the other side articles you want and hope the one that counts does not get viewed, while your defending yourself. Our lawyers can tell you all about how that one plays out.

I can say with out a doubt, you would never get hired at one of our locations if you claimed to hold an E1 and practice like that.

I'll say no more on the subject, I've said too much already. It's obvious you gamble, and if you do hold a E1, go a head and sign off on those questionable jobs. Just hope no one tells you to start opening up race ways for inspection.

Jamie

Reply to
Jamie

Cite it. We can all read it together.

Reply to
gfretwell

That is what I said in the first sentence of my reply.

-- bud--

Reply to
bud--

In the NEC ground wires are covered by 250.122 [there is a post from gfretwell at alt.engineering.electrical, not crosposted to sci.electronics.design, on this]

Ground wires need to be big enough to produce a fault current that will rapidly open the overcurrent protection. The NEC allows, as circuit amp rating increases, a much smaller ground wire than the circuit amp rating would indicate. For instance for a 400A circuit a #3 copper ground wire can be used - rated around 100A.

If the small ground wire produced dead bodies the code would have been changed.

-- bud--

Reply to
bud--

3/4 Inch EMT is acceptable per:
formatting link
Reply to
geraldnewton84

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.