Okay, let's break down the conduit spacing and burial depth requirements for your 13.2kV campus distribution system. This involves understanding the applicability of the NEC vs. NESC, and the specific requirements within each code.1. Spacing Between 5" MV Conduits in Duct Bank



NEC vs. NESC

This is a critical first step. The applicability of the NEC (National Electrical Code) or the NESC (National Electrical Safety Code) depends on the ownership and location of the system.

NEC (NFPA 70)

Generally applies to customer-owned electrical systems within buildings and on private property. Think of factories, office buildings, campuses, etc. after the utility hands off the power. If the distribution system on your campus is owned and maintained by the campus itself, the NEC likely applies.

NESC (IEEE C2)

Generally applies to utility-owned electrical systems, including overhead and underground distribution lines, and infrastructure on utility property. This focuses on safety for utility workers and the public. If the distribution system is owned and maintained by a utility, the NESC applies.

In your case, since it's a campus distribution system (assumed to be owned by the campus), the NEC likely applies. However, always confirm with local authorities (AHJ - Authority Having Jurisdiction) as interpretations and local amendments can vary. NEC Requirements (Assuming NEC Applies)



Figure 310.60(C)(3)

You are correct to look at this figure. This applies to duct bank installations for cables rated over 2000 volts.

Spacing is NOT explicitly defined by the NEC

Figure 310.60(C)(3) does not give specific spacing dimensions. The NEC is more performance-based than prescriptive in this area. It focuses on ampacity (current-carrying capacity) and ensuring heat can dissipate.

Ampacity Derating and Heat Dissipation

The primary concern is heat buildup. Closely spaced conduits in a duct bank will have reduced ampacity due to mutual heating. Therefore, you must perform ampacity calculations according to NEC 310.15 to determine the appropriate conductor size and derating factors.

Engineering Judgement and Industry Practices

Since the NEC doesn't give a specific spacing dimension, you will need to use engineering judgment, considering:

Conductor size and type

Larger conductors generate more heat.

Soil thermal resistivity

Poor soil conductivity will exacerbate heat buildup.

Load factor

Higher load factors mean more sustained heat generation.

Ambient temperature

Higher ambient temperatures reduce the cable's ability to dissipate heat.

Typical industry practices

Consult with experienced electrical engineers and review typical duct bank designs for similar installations in your region. Common spacings are often 7.5" - 12" center-to-center, but this must be verified with ampacity calculations.

Software and Calculations

Use electrical design software (e.g., ETAP, SKM) to perform ampacity calculations and simulate the thermal performance of the duct bank. This will help you determine the minimum acceptable spacing.

NESC Requirements (If NESC Applies)

The NESC provides more specific requirements regarding duct bank construction and spacing. Consult NESC Section 320 for specific requirements for underground conduit systems. NESC generally requires sufficient spacing to allow for proper heat dissipation and to prevent damage to the cables. Again, you need to consult the specific section in the NESC.

2. MV Duct Bank Burial Depth Requirements



NEC 310.60(C)(3) and 300.50



NEC 310.60(C)(3) does reference 300.
50. This is the primary guide for burial depth. 300.50(D) Depth of Burial

This section provides a minimum burial depth based on voltage and application. For 13.2kV systems:

Raceways or Duct Banks

Typically requires a minimum of 24 inches to the top of the raceway/duct bank.

30" Maximum Depth (Note in Figure 310.60(C)(3))

This note is specifically related to the ampacity adjustment factors for the configurations shown in the figure. It means that the ampacity values used in the figure are based on burial depths up to 30 inches. It doesn't mean you can't bury deeper, but it does affect how you apply the ampacity correction factors.

Why 36" Depth is Often Specified



Increased Protection

A deeper burial depth provides greater physical protection for the duct bank against accidental dig-ins, heavy vehicles, and other potential damage.

Frost Line

In colder climates, a deeper burial depth ensures the duct bank is below the frost line, preventing damage from freezing and thawing cycles.

Standard Practice

Many engineers and utilities have adopted 36 inches (or even deeper) as a standard practice for MV duct banks to provide a margin of safety and long-term reliability.

Local Requirements

Some local jurisdictions (municipalities, utilities) may have their own specific requirements that exceed the NEC minimums.

Is 36" Burial Depth Allowed? Yes, generally allowed. The NEC specifies minimums. You can always exceed the minimum requirements as long as you meet all other applicable codes and standards. NEC 310.60(B)(2)(b) Derating Factor for Deeper Burial



Yes, potentially. If you bury deeper than the depths used to establish the ampacity values in Figure 310.60(C)(3) (i.e., deeper than 30"), you may need to apply an additional ampacity adjustment factor. Heat Dissipation

Deeper burial depths can reduce the soil temperature around the duct bank in some cases. This could increase the cable's ampacity. However, it also increases the thermal resistance to the surface, potentially reducing heat dissipation.

Engineering Analysis is Crucial

You need to perform a detailed thermal analysis to determine the actual impact of the deeper burial depth on the cable's ampacity. Factors to consider include: Soil thermal resistivity at the specific depth. Groundwater conditions. Ambient ground temperature. Cable insulation type and temperature rating.

Conservative Approach

If you are unsure, it is always safer to apply an additional derating factor to account for the potential reduction in ampacity due to reduced heat dissipation.

In Summary and Recommendations


Confirm Code Applicability (NEC vs. NESC) with your AHJ.2. Perform Detailed Ampacity Calculations

Use electrical design software and consider all relevant factors (conductor size, soil thermal resistivity, load factor, ambient temperature, conduit spacing).3.

Document Engineering Justification

Clearly document your engineering calculations and rationale for the chosen conduit spacing and burial depth.4.

Consider Local Requirements

Check for any local amendments or utility requirements that may exceed the NEC minimums.5.

Err on the Side of Safety

If you are unsure, use a more conservative design with wider conduit spacing and deeper burial depths. This will improve the long-term reliability of the system.6.

Consult with Experienced Engineers: Engage with experienced electrical engineers who have expertise in MV duct bank design and installations.By following these steps, you can ensure a safe, reliable, and code-compliant MV duct bank installation for your campus distribution system. Flag for review