I was doing some research for a recent project and I came across this old post can

[dsjr114]

Can someone explain or show me the calculation to get a neutral calculated at 147,200 from a 196800va service

In reference to the Mike Holt NEC 2008 Commercial Calculation example under Section 11.13 video and as a general question...

Service load was 196,800VA @ 240V (820A). Breaker must be 1000A. Conductors are sized to the breaker with say 3 sets of 400 kcmil.

Neutral was calculated as 147,200VA @ 240V per 220.61 (613A) and adjusted by the demand factor of 70% for loads in excess of 200A yielding 489A. 489A divided by 3 raceways yield 163A, which requires a 2/0 neutral conductor per raceway.


However, 250.24(C)(2) says you can also size the neutral to the largest ungrounded service-entrance conductor per Table 250.66 but no smaller than 1/0 when paralleling. In this case, 400 kcmil phase conductors only requires a 1/0. (This method was used in the Section 11.14 example in the video and similarly here:

Which method do we use: 220.61 or 250.24(C)(2)? Method 1 (220.61) seems like it could vary based on load, which could change over the life of the building. Method 2 (250.24(C)(2)) seems to be undersized based on calculated neutral loads per Method 1.

Which method do we use: 220.61 or 250.24(C)(2)? Method 1 (220.61) seems like it could vary based on load, which could change

over the life of the building. Method 2 (250.24(C)(2)) seems to be undersized based on calculated neutral loads per Method 1.

 

[Smart $]

250.24 establishes a minimum size based on ungrounded conductor size. 220.61 establishes a minimum size based on calculated load. The larger of the two is the minimum size required.

 

[bob]

This site has a good explaination of your question.

http://www.ecmag.com/?fa=article&articleID=8664



http://www.google.com/search?sourceid=navclient&aq=hts&oq=&ie=UTF-8&rlz=1T4GZAZ_enUS396US396&q=250.24(C)(2)

 

[jumper]

This site has a good explaination of your question.

http://www.ecmag.com/?fa=article&articleID=8664

I seem to be having a senior moment. That link says this:

Section 250.24(C)(1) refers to Table 250.66 for sizing the neutral conductor when it is used as an EGC. For example, if the ungrounded (phase) conductors are 2/0 THWN copper conductors, and the grounded (neutral) conductor is to be used as an EGC, it is required to be a 4 AWG copper conductor per Table 250.66

How can a neutral be used as an EGC?

 

[bob]

I belive it relates to service conductors and the equipment connected such as meter socket and main.

 

[jumper]

I belive it relates to service conductors and the equipment connected such as meter socket and main.

Okay thanks. Similar to using a neutral for bonding a range or dryer with an existing 3 wire circuit.... I think.

I gotta reread it in the morning when there is a half chance my brain will work.