Neher and McGraft

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.
 
charlie b said:
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.
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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.
 
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.
 
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.
 
white stallion said:
When is the Neher and McGraft required . . . ?
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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?
 
charlie b said:
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?
Click to expand...
We are working on a project where the design engineer used the Neher and McGraft method for sizing the service for a 3000 KVA transformer. They came up with 13 - 750 KCMIL conductors per phase. The size of conductors makes for a difficult installation. We are trying to verify if the equipment is available to be provided with 13 lugs per phase. When we questioned the engineer, he is uneasy having gone this route but was questioned by 3rd party project reviewer to use the Neher and McGraft method. We would normally use 11 - 500 KCMIL conductors on a 4000 amp service. I am trying to verify the need of using the Neher and McGraft method.
 
I haven't looked at applicable NEC sections in years, but at our company, it was generally interpreted to explicitly require a Neher-McGrath calculation for medium-voltage duct banks where mutual heating was an issue, but it was not explicitly required for low voltage circuits. There was a NEC change in the early 80s that added this requirement for low voltage duct banks, but there was such an outcry from electricians that it was quickly rescinded, IIRC. The reality is that the NEC load calculation methods are so conservative that these services and feeders are generally oversized. I don't recall ever getting pushback from the local or State inspectors on this, even in Oregon. Maybe the NEC has changed recently. If so, I'm sure the NEC experts will quickly correct me.
 
I've also been wondering the same question lately. I've always just used the standard tables in 310, but I typically design regular buildings, where the load is diverse, and I've never had a problem with using the standard calculations because the NEC load calcs. are so conservative.

Now I have 5000 amps in a single duct bank, serving a more specific load. However, I also know any attempt to add extra conduits/cables (above and beyond 12 sets) is going to be met with resistance by the client/owner. I'm also wondering about the transformers being used, but everyone here seems to agree there isn't any load calculation requirement for a minimum size transformer.

Wondering what the engineer used for your backfill? Using a concrete with a low value of RHO should allow fewer conduits and/or smaller cable sizes:

 
steve66 said:
I've never had a problem with using the standard calculations because the NEC load calcs. are so conservative.
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The tables in Annex B were made code text in the first draft, a number of cycles ago. The second draft comments that got those tables into Annex B were exactly that....the load calcs are so conservative that this is not a real world issue when loads are calculated per Article 220.

However industrial and other high capacity loads are often calculated under engineering supervision and results in load calculations that are close to the actual real world loading. In those cases the standard ampacity table will result in duct bank failure. In cases like this the Neher-Mcgrath calculations should be used.
 
I once obtained an interpretation from Washington State LNI that for new buildings for which the service load was calculated per the NEC, the 310.16 tables could be used for underground ductbanks. Their reasoning was that the impact of mutual heating was offset by the conservatism of the NEC calculation process.
 
It depends on 2 factors: the earth thermal resistivity and the distance between conduits.If the resistivity is 90 Co.cm/W and the distance center line to center line 7.5'' or 120 Co.cm/W and 12" you'll get 277 A Ampacity per conductor for 13 parallel cables [if the insulation is XLPE or EPR [90oC rated] and 20oC earth temperature].For 75oC insulation [PVC] and earth temperature 25oC even for 90oC.m/W and 12 inch distance[horizontal and vertical] you'll get only 268.5 A for 13*750 kcmils, copper conductor. You need 16" distance, in this case, to get 279.5 A..
 
If you intend to reduce the load factor to 75% -for instance- you may use 500 kcmils [13 cables] if you are positiv for 90 Co.cm/W and 7.5" distance.
 
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