Neher and McGraft

[white stallion]

When is the Neher and McGraft required to be used for 480V buried circuits? Is it limited to concrete encased ducts only? Does it apply to ducts in direct buried without concrete?

 

[ron]

We do Neher McGrath calculations for all underground ductbanks that do not have a prescriptive configuration in Annex B of the NEC.

 

[charlie b]

The Neher McGrath calculation method can be used for any underground installation. But I believe it is never "required." There are other methods of determining ampacity. Two notes:
1. If you wish to use this method, the calculation must be performed by, and sealed by, a licensed Professional Engineer.
2. My memory of past attempts to use this method tells me that it will generally give you a lower ampacity value than the NEC tables. There is sometimes a question as to whether the tables can be used in a given situation.

 

[ron]

The Neher McGrath calculation method can be used for any underground installation. But I believe it is never "required." There are other methods of determining ampacity. Two notes:
1. If you wish to use this method, the calculation must be performed by, and sealed by, a licensed Professional Engineer.
2. My memory of past attempts to use this method tells me that it will generally give you a lower ampacity value than the NEC tables. There is sometimes a question as to whether the tables can be used in a given situation.

Whether it is "required" is a bit of an interpretation for the designer.
NEC Section 310.16(3) indicates that the typical Table 310.16 can only be used when "Wiring is installed in a 30°C (86°F) ambient temperature." Below ground with mutual heating from adjacent ducts, changes the ambient temperature, causing the need to follow an Engineered solution.

310.14(3) indicates that "no conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved." The Informational Note directs the reader to Annex B. Of course an informational note is not enforceable, but it give guidance regarding the intent of the code section. Where the prescriptive Annex B doesn't apply due to a different configuration, then it sends me to an Engineered solution with a calculation.

 

[Julius Right]

In NEC Informative Annex B Application Information for Ampacity Calculation
B.2 Typical Applications Covered by Tables it is noted:
For additional information concerning the application of these ampacities, see IEEE STD 835, Standard Power Cable Ampacity Tables.
In IEEE STD 835 it is an example how to calculate the ampacity of a cable installed in 3" pvc conduit in the earth.
A.2.1 Example 1: 3-1/c 350 kcmil aluminum, 600 V cables installed in a 3 inch PVC conduit in earth.
It is based on Neher & McGrath theory also.

 

[Julius Right]

However, it is a calculation for one conduit only. If you have more conduits-but the same cables and currents-you may use the formula 44 from Neher &McGrath article.
But, if there are different cables and currents, you may follow IEC 60287-2-1 chapter 2.2.3.1 Unequally loaded cables.

 

[charlie b]

When is the Neher and McGraft required . . . ?

Out of curiosity, may I ask whether you are already using this method, and if so, how? Or is your concern that you may be required to use it for a project, but you don't know how to use it?