Duct bank

[anbm]

Typically, who will specify duct bank for electrical or telecom conduits run? Electrical or Civil engineer?
I knew EC won't install duct bank but duct bank carry conduits for electrical wiring, does EC only layout
the conduits, then other contractor will finish other pieces to complete the duct bank run?

 

[don_resqcapt19]

The typical electrical contractor will install any duct bank that you pay them to install.

 

[gadfly56]

The typical electrical contractor will install any duct bank that you pay them to install.

I suppose it depends on how big the duct bank is versus how big the EC is. My experience is limited to LaGuardia Airport. It was a tad more complicated. Everybody and his brother was in the coordination sessions for that.

 

[Julius Right]

In a power station there are more than 100-300 manholes and tens of miles cable duct banks.

This cable web is initiated by electrical physical design department since they have experience in this domain.

The design starts by the medium voltage cables routing and then with low voltage power cable. There are at least 3 layers of ducts- the medium voltage cables in the lower most and the instrument and telecommunication cables-in steel ducts-are in the upper most.

At the beginning, the number of duct-power mainly-has to be double than known at this stage and 100% reserve.

 

[don_resqcapt19]

Typically, who will specify duct bank for electrical or telecom conduits run? Electrical or Civil engineer?
I knew EC won't install duct bank but duct bank carry conduits for electrical wiring, does EC only layout
the conduits, then other contractor will finish other pieces to complete the duct bank run?

If they are power, the design needs to be by an electrical engineer who understands mutual heating and the required ampacity adjustment of the conductors from that mutual heating. Maybe the civil guy too for any structural issues from things like roads or buildings on top of the ducts.

 

[anbm]

In a power station there are more than 100-300 manholes and tens of miles cable duct banks.

This cable web is initiated by electrical physical design department since they have experience in this domain.

The design starts by the medium voltage cables routing and then with low voltage power cable. There are at least 3 layers of ducts- the medium voltage cables in the lower most and the instrument and telecommunication cables-in steel ducts-are in the upper most.

At the beginning, the number of duct-power mainly-has to be double than known at this stage and 100% reserve.

Is duct bank required for MV cables and IT wiring per code or for better protection? Could not find anywhere in NEC telling us how to size the manholes.

 

[Julius Right]

310.1 Scope. This article covers general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses.

That means the art.310.60 is for ampacity calculation only. It is not referrers to non-power cables. For instrument and communication cables there are other articles as per chapter 700 or 800.

And, of course, using separate steel conduits for instruments and telecommunication offer a protection required for these cables.

 

[don_resqcapt19]

Is duct bank required for MV cables and IT wiring per code or for better protection? Could not find anywhere in NEC telling us how to size the manholes.

There is not a lot of size guidance in the NEC for sizing manholes, but there is some in 110.72.

Both communication cables and power cables are available in constructions that are suitable for directly buried applications, so if using those cables, a duct bank would not be required. However, there are advantages for future additions or replacements where duct banks are installed, especially if spare ducts are installed.

 

[anbm]

There is not a lot of size guidance in the NEC for sizing manholes, but there is some in 110.72.

Both communication cables and power cables are available in constructions that are suitable for directly buried applications, so if using those cables, a duct bank would not be required. However, there are advantages for future additions or replacements where duct banks are installed, especially if spare ducts are installed.

What do you mean spare ducts? spare conduits inside duct bank?

 

[JoeStillman]

Typically, who will specify duct bank for electrical or telecom conduits run? Electrical or Civil engineer?
I knew EC won't install duct bank but duct bank carry conduits for electrical wiring, does EC only layout
the conduits, then other contractor will finish other pieces to complete the duct bank run?

I have worked on lots of jobs with civil engineers. None of them ever designed the duct banks. That's the job of the electrical engineer. You'll be lucky if you can even get the civil engineers to show the route on their site plan.

Seems like every set of civil drawings includes one called the "Utility Plan" that includes all the utilities except gas and electric. Gas and electric are designed by us un-civil MEP engineers.

 

[don_resqcapt19]

What do you mean spare ducts? spare conduits inside duct bank?

Where you are installing a duct back, the "ducts" are conduits .... at least around here they are.

 

[anbm]

I have worked on lots of jobs with civil engineers. None of them ever designed the duct banks. That's the job of the electrical engineer. You'll be lucky if you can even get the civil engineers to show the route on their site plan.

Seems like every set of civil drawings includes one called the "Utility Plan" that includes all the utilities except gas and electric. Gas and electric are designed by us un-civil MEP engineers.

So when the ductbank turn, I assume we will need manhole, will electrical design manhole as well? Any practical guides/white papers to show us how to design or specify the manhole?

 

[anbm]

Where you are installing a duct back, the "ducts" are conduits .... at least around here they are.

OK. Thanks!

 

[don_resqcapt19]

So when the ductbank turn, I assume we will need manhole, will electrical design manhole as well? Any practical guides/white papers to show us how to design or specify the manhole?

That is a design question. You can install 90 degree bends in the duct bank and pull around them. I try to avoid as many manholes as possible.

 

[JoeStillman]

So when the ductbank turn, I assume we will need manhole, will electrical design manhole as well? Any practical guides/white papers to show us how to design or specify the manhole?

Google has what you need.

 

[JEFF MILLAR]

Manholes and concrete encased PVC ducts is a system used for MV and. LV. Power cables. Normally in power plants. And the design of the cable derating amps is complex.
The structural engineer typically designs the concrete mix for the manholes and duct banks. The duct banks have no rebar and a minimum of 3 inch concrete coverage is typical. Plastic type spacers are commercially available to provide 7 1 /2 inches separation between 4 " duct center lines and this gives 3 inch separation between ducts, The ducts are typically 4 inch PVC schedule 40.
Calculating the cable derating amp factors is complicated. And the derating factor is normally greater than 50% of the cable free air rating. Software for this application is recommended. Separation between voltage levels is required. And communication cables requiring greater separation.
LEAST amount of cable derated AMPS is using , CABLE directly buried in the ground and following recommended separation.
Second best application. is the use of PVC schedule 40 ducts, directly buried in the ground. with recommended separation between ducts.

 

[Julius Right]

First 4" duct is not enough for MV three cables per duct. We need, some times, 5" or even 6" pvc schedule 40.
If 4" nominal conduit is 4.5" outside , for 6" it is 6.625".Then for 6" duct the distance between adjacent ducts centre lines is 9.625" [approximate 250 mm].
No correction factors are exposed in NEC for using other distances.
The NEC does not use other distances between embedded ducts and buried conduits [or cables]. Compare, for instance, Detail 2 with Detail 10 of FIGURE 310.60(C)(3) Cable Installation Dimensions for Use with Table 310.60(C)(77) Through Table 310.60(C)(86).

 

[JEFF MILLAR]

15 KV - 3 - 1 / C XLPE , 500 kcmil , you think will not fit in a 4 inch duct. ?

 

[Julius Right]

One cable of 500 mcm 15kV 133% insulation approx. 3.5 inch outside dia.
According to NEC ch.9 Table 1 Percent of Cross Section of Conduit and Tubing for Conductors and Cables is 53% for a single cable. So, sqrt(3.5^2/.53)=4.8"
If you have 3 single core cables [of 1.65" outside dia] the filling percent will be 40%.So sqrt(3*1.65^2/.4)=4.52"
Then in both cases you'll need a 5" diameter conduit.
By the way , the cable outside diameter I calculated using data from UL-1072 [ Table 8.3 Diameters over compressed concentric-lay-stranded, [maximum 0.813"] , insulation form Table 13.1 for 133% 15 kV and jacket thickness as per Table 25.14]

 

[JEFF MILLAR]

My mistake you are correct in your conduit size.