Service Calcutation

[Dennis Miller]

I want to feed 2000 amp MCC gear off an existing 2000 amp breaker, Fed from a Y-transformer.

Can I use 4 runs of 750 MCM, or do I need to derate based on the neutral being a 4th current carrying conductor, and go the 5 runs?

 

[charlie b]

At best, a 750 MCM conductor is only good for 475 amps. Four of them are good for 1900 amps. That will not be adequate.

In answer to your specific question, you usually don't have to derate by virtue of the neutral counting as a fourth current-carrying conductor. It would only count if there were a significant amount of harmonics present.

 

[GH_Vegas]

Charlie_b is correct. 750MCM THWN is rated at 475A for THWN insulation. THWN-2 insulation is rated at 90 degC rise and is thus rated for 535 amps. But all your terminal connections will need to be rated at 90 also. Additionally, the ambient temperature for both these cases is 30-39 degC. If you use 40 degC you must derate the cables for the elevated ambient, as is done in Las Vegas for example.

 

[Dennis Miller]

I want to feed 2000 amp MCC gear off an existing 2000 amp breaker, Fed from a Y-transformer.

Can I use 4 runs of 750 MCM, or do I need to derate based on the neutral being a 4th current carrying conductor, and go the 5 runs?

Can I use 4 runs of 750 MCM, or do I need to derate based on the neutral being a 4th current carrying conductor, and go the 5 runs?[/QUOTE]

Ok, with that I would need six runs of 750MCM, because the inspector here does consider the neutral as a current carrying conductor and expects me to derate to the 80% factor. He as so wants the temperature factor taken into consideration as well.

So with six parallel runs of 750MCM cu, what would you suggest for the size of the Grounding Electrode?

Is there some reference, or exception to the neutral not being considered a current carrying conductor in the NEC?

Thanks'
Dennis

 

[GH_Vegas]

Neutral not a Current Carryin Conductor

Neutral not a Current Carryin Conductor

NEC code section 310.15(B)(4)(a) specifies that the neutral shall not be required to be counted. However, part (c) of that same section does say if a 'major' portion of the load is non-linear, then the neutral will need to be counted.

 

[Dennis Miller]

NEC code section 310.15(B)(4)(a) specifies that the neutral shall not be required to be counted. However, part (c) of that same section does say if a 'major' portion of the load is non-linear, then the neutral will need to be counted.

Thank you for your help. I will have very little unbalanced load to consider.

 

[charlie b]

I am taking this out of sequence.

Is there some reference, or exception to the neutral not being considered a current carrying conductor in the NEC?

Yes, and GH_Vegas gave that answer.

Ok, with that I would need six runs of 750MCM, because the inspector here does consider the neutral as a current carrying conductor and expects me to derate to the 80% factor.

The Inspector does not have the privilege of ?expecting? more than the code requires.

That said, you only need five sets of conductors (which by the way, you would need anyway, as I said in my first comment). Presuming you have a conductor type that has a 90C insulation system, you get to derate the 80% from the 90C column of Table 310.16. 80% of 535 is 428 amps. Five sets of 428 amp conductors gives you a total ampacity of 2140 amps. That is more than you need.

He as so wants the temperature factor taken into consideration as well.

What temperature would that be? Where will this project be installed, and what is the average temperature in that area? If you derate for 35C (95F), and use the 96% derating factor (again, from the 90C column), you can still get by with five sets of 750 MCM copper.

So with six parallel runs of 750MCM cu, what would you suggest for the size of the Grounding Electrode?

The grounding electrode (i.e., ground rod, UFER ground, etc.) is not based on the size of the service conductors. But the Grounding Electrode Conductor is. See Table 250.66. Your installation maxes out that table, and so the GEC need not be bigger than 3/0 copper.

 

[charlie b]

Thank you for your help. I will have very little unbalanced load to consider.

You may need to be more careful in your choice of words, or else you are going to have trouble communicating with the Inspector. "Unbalanced load" is not the issue. Rather, the issue is "non-linear load."

Will your project have a slew of computer systems, fluorescent lighting, and uninterruptible power supplies (anyone want to add to this list?)?