Underground Raceway Calculations (NEC)

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timm333

Senior Member
Location
Minneapolis, MN
I previously did high voltage underground cable calculations based on IEC 60287 using excel spreadsheet. Now I need to do the low voltage underground cable calculations based on NEC; I have few questions. Thanks for help.

1. Is it advisable to completely ignore the tables of NEC, and do calculations based on Neher McGrath method?
2. Is it possible to do the hand calculations of Neher McGrath method?
3. Some part of cable length is direct buried, and some is in duct bank. As there would be more heat loss in duct bank, so can we ignore the direct buried part, and do the calculations by assuming that the whole cable length is in duct bank?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Often there is less heat loss and a higher temperature in the underground ducts requiring a higher ampacity correction factor than for the above ground part of the run. This assumes there are a number of ducts in the bank and there will be mutual heating raising the conductor temperature.
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
I previously did high voltage underground cable calculations based on IEC 60287 using excel spreadsheet. Now I need to do the low voltage underground cable calculations based on NEC; I have few questions. Thanks for help.
Your spreadsheet may still work, with some adjustments. See: NEC 310.15(C) Engineering Supervision
1. Is it advisable to completely ignore the tables of NEC, and do calculations based on Neher McGrath method?
You can't ignore NEC requirements omitted by the critical table limits, or table notes.
2. Is it possible to do the hand calculations of Neher McGrath method?
Yes, as broadly adopted by "Engineering Supervision", but the industry prefers commercial expert systems, verified & validated for this purpose.
3. Some part of cable length is direct buried, and some is in duct bank. As there would be more heat loss in duct bank, so can we ignore the direct buried part, and do the calculations by assuming that the whole cable length is in duct bank?
Before 2020 Edition of NFPA-70, the NEC medium & low-voltage ampacity sections of 310.15 thru 310.60(C)2 are not separated, and difficult to differentiate, but the only direct burial ampacity table for 0-2000 remains Tbl. 310.16, also known as Table 310.15(B)16.

Some NEC sections critical to ampacity that are omitted by the tables include:
* 110.14(C), equip-termination temperature limits, and derating variables per 310.15
* 110.3(B), 40°C max ambient for circuit breakers, and I'm not aware of heat disipation data published for breaker enclosures.
* 220.18(B), Inductive Motor loads that increase ampacity with impedance.
* 220 Demand Factors that allow reductions in multiple nameplate loads for specific equipment, depending on occupancy
* 220 Non-Linear loads that increase ampacity on neutrals
* Annex D, ampacity calculations for multi-phase services
 

timm333

Senior Member
Location
Minneapolis, MN
Thanks. IEC 60287 includes a calculation example which describes the step by step method to calculate the ampacity of underground cables. I have the original Neher McGrath paper which was published in 1957, it has some examples but these examples are not clear. NEC Annex B has tables, but it does not have example. Maybe IEEE-835 has solved examples of Neher McGrath method, but I don't have IEEE-835.

Is there some solved example available on internet which describes more clear calculations of underground cable ampacity using Neher McGrath method?
 
Last edited:

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
You need ask the engineers on this forum to take a look. If not lurking this week, try sending a private message to request their input.

The user names that come to mind may no longer be active:
kingpb
paulengr
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
IEC 60287-2-1 cpt.2.2.7 presents the main part of the cables in duct bank current carrying capacity-ampacity as called in Neher and McGrath theory. The same proceeding it is in this theory in chapter EFECTIVE THERMAL RESISTANCE BETWEEN CABLES,DUCTS,OR PIPES AND AMBIENT EARTH.
I did two Visual Basic 6 programs 20 years ago one according to IEC 60287 and one according to Neher and McGrath. However, I think it can be done in a simple Excel.
 

timm333

Senior Member
Location
Minneapolis, MN
Thanks. A couple of questions regarding NEC.

  • Is NEC sizing of the conduits (based on number of conductors) the same in above ground conduits and underground ducts?
  • Which NEC article tells that how much should be the spacing between the underground power duct and underground control/instrument duct?
  • Which NEC article tells that how much should be the size of manhole and handhole; and how much should be the spacing between two manholes/handholes?
Thanks for help!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Thanks. A couple of questions regarding NEC.

  • Is NEC sizing of the conduits (based on number of conductors) the same in above ground conduits and underground ducts?
  • Which NEC article tells that how much should be the spacing between the underground power duct and underground control/instrument duct?
  • Which NEC article tells that how much should be the size of manhole and handhole; and how much should be the spacing between two manholes/handholes?
Thanks for help!
1) Yes, wire fill is wire fill and applies no matter where the raceway is located.
2) The code does not really specify that.
3) 110.72 covers the workspace required in manholes so that sort of addresses the manhole size. 314.30 addresses handholes for systems under 1000 volts and 314.72 addresses handholes for over 1000 volts. The spacing for this is just like the spacing for above ground pull points. It is a design issue and not a code issue. Designers will often use software to calculate the pull and use that to determine the number of pull points required. That will look at both the pulling tension and the sidewall pressure if you are pulling around bends. You want to design for as few of pull points as you can use and not damage the cables or conductors.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
The minimum distance between two adjacent ducts is 3 inches for the concrete to catch the ducts well.
The duct for communication and instrument cables it has to be of steel and well grounded at a minimum clearance between pvc power duct of 100 mm [art.800].
If it is possible it is recommended to separate communication from instrument cables.
Manhole dimensions are according National Electric Safety Code[C2] Rule.323. Manholes, handholes, and vaults.
Rule 323B presents the recommended clearances[Table 341-1—Clearance between supply and communications facilities in
joint-use manholes and vaults].
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The minimum distance between two adjacent ducts is 3 inches for the concrete to catch the ducts well.
The duct for communication and instrument cables it has to be of steel and well grounded at a minimum clearance between pvc power duct of 100 mm [art.800].
If it is possible it is recommended to separate communication from instrument cables.
Manhole dimensions are according National Electric Safety Code[C2] Rule.323. Manholes, handholes, and vaults.
Rule 323B presents the recommended clearances[Table 341-1—Clearance between supply and communications facilities in
joint-use manholes and vaults].
Can you cite the section in the NEC that says that? I don't find any code required separation for conduits that contain power conductors and other conduits that contain communication or instrument conductors.

Sure it would be a better design to have the separation and as you suggested separate duct banks, but that is a design issue and not a code issue.

Remember that the code does not really care if the communication and instrument circuits actually function as intended, the code only cares that they are safe from a fire and shock perspective.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
You are right there is no such requirement in the NEC. This is only requirements related to the correct functioning of the communication system and the instrumentation in the installation. Probably ANSI [TIA] instructions
 

timm333

Senior Member
Location
Minneapolis, MN
It makes sense for voltages greater than 2000V. But for 0 – 2000V it is little confusing. Table 310.15(B)(16) can be used for cables in underground raceway for 0 – 2000V, then why have they still provided tables B310.15(B)(2)(1) through B310.15(B)(2)(10) for 0 – 2000 V? Thanks.
 
Ducab Cable, has ampacity derating tables for LV power cables. And makes the correction to the cable ampacity straight foreword.
 
I would very much like to see an example calculation of the Neher McGrath calculation. I would very much like to be able to perform this calculation, but when it comes to understanding the many variables involved and how to calculate them, is where i get lost. Software is great but i need to be able to perform all my own design calculations. If you have experience with this calculation and willing to share, thank you in advance.
 

ron

Senior Member
For low voltage, if you happen to fall within one of the prescriptive conditions shown in those details / tables, use Annex B.
I've never seen a hand calculation. We use Ampcalc, which is the cheapest software we were able to find years ago when we started doing Neher McGrath calcs.
If your direct buried condition is less than 10' or 10% of the run, you can likely ignore it, the ductbank will be the worse case.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
I’ll try to copy and transmit one of calculation example from IEEE 835 based on Neher & McGrath theory.
First a.c. resistance calculation
 

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Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Second thermal resistances
 

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The greatest ampacity derating factor applies. This is considered the overall derating factor. Can you share your excel calculation for your MV installation. That would be very interesting.
 
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