I am working at a plant trying to install u/g from the MCC room to the motors in the field. We have approx. 70 motors, the smallest are 1HP the largert are 250HP. ( sorry but this will take a little explaning ) The MCC room feeds motors in 2 seperate buildings, in the first building the longest motor is around 200'. The second building conduit runs through the first building and an additional 200' for a total of 400'. All the motors are 480V. The Problem----- My eng. are telling me that in an u/g duct bank I can only install 15 motors. Before we have to start derating. ( alot ) The next duct bank has to be 5' away from the first (that is center line of D.B.) and can still only have 15 motors. ( can you see my problem??) I will have to have five duct banks breaking out from the MCC room. This is not possible. Has anyone had problems with this, is their any exceptions to this???? Please help Also what makes a duct bank 2 conduits or 10 ???? Thanks for your time.
Underground conduit Calc.
- Thread starter gilly
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jtester
Senior Member
- Location
- Las Cruces N.M.
Gilly
Before we even get into derating, how do you comply with 225.30 which generally allows only one set of conductors to feed a second building? Have you complied with 225.31 and 225.32?
Jim T
Before we even get into derating, how do you comply with 225.30 which generally allows only one set of conductors to feed a second building? Have you complied with 225.31 and 225.32?
Jim T
Jim,
As far as the code articles 225.31, 225.32, we are ok. In reguards to 225.30 we have only one feed to the mcc room. Everything in these buildings comes out of that room. Hope this helps.
Bob,
Each conduit has only one motor. So we have 3 current carrying conductors and one ground per pipe. When we group motors we do derate according to 310.15(B)(2)(a) We have around 70 conduits.
Thanks
As far as the code articles 225.31, 225.32, we are ok. In reguards to 225.30 we have only one feed to the mcc room. Everything in these buildings comes out of that room. Hope this helps.
Bob,
Each conduit has only one motor. So we have 3 current carrying conductors and one ground per pipe. When we group motors we do derate according to 310.15(B)(2)(a) We have around 70 conduits.
Thanks
jtester
Senior Member
- Location
- Las Cruces N.M.
gilly said:
Gilly, it's not the MCC room that worries me, it is the other two buildings that you are running multiple circuits to. More than one circuit is not necessarily a code violation, but it deserves significant consideration. As applies to 225.31, each circuit running to the other buildings needs a disconnect, they must comply with 225.31 and 225.32 at each of the downstream buildings, and the real kicker is that 225.33 limits you to a maximum of 6 as a general rule. I don't see how you can run 70 conduits to 2 separate downstream buildings, and use only 6 disconnects.
Jim T
Jim,
We are feeding motors with different ckts not the building. This is an industrial plant, where qualified personel are the only people to work on the equipment. All mcc bucket are able to be locked out individually. I also have 225.32 (Exception #1) that will apply. I am just wondering if any other electrical contractors have had an eng firm limit them to the number of conduits in a duct bank. I have some guys that have worked industrial jobs for over 20 years, and have never had a problem with number of conduits in a duct bank.
We are feeding motors with different ckts not the building. This is an industrial plant, where qualified personel are the only people to work on the equipment. All mcc bucket are able to be locked out individually. I also have 225.32 (Exception #1) that will apply. I am just wondering if any other electrical contractors have had an eng firm limit them to the number of conduits in a duct bank. I have some guys that have worked industrial jobs for over 20 years, and have never had a problem with number of conduits in a duct bank.
rcwilson
Senior Member
- Location
- Redmond, WA
Jim - Is it possible that this industrial facility falls under Exception No.1 of 225.32? The disconnecting means for the other buildings might be the MCC Main Disconnect.
Gilly - With that many circuits in a duct bank, there may be significant mutual heating if the duct banks are not spaced apart.
When we design large duct banks for multiple motor feeds, we end up increasing the motor cable size to accommodate the heating effects of adjacent cables, sometimes tripling the size. That creates issues of terminating a #4 awg on a 5 HP motor or 20 A breaker. Also, since the duct banks are concentrated at the MCC or switchgear and then spread out to the loads, the larger cable is only needed for that first portion of the run, but the oversized cable runs the full distance. So maybe spreading the duct banks is a better solution.
Code doesn't strictly require derating of underground conductors. It looks like we could just use Table 310-16 without derating for underground conduits. But if my ampacity calculation program based on Nehr-McGrath equations says the wires will be in a 70C ambient due to mutual heating of the duct bank, a 0.41 derate is needed to account for the high ambient temperature in the duct bank.
Your engineers' "maximum of 15 motors" may be a rule of thumb based on some predetermined calculations or experience. Or maybe they modeled all of the underground duct banks and have determined that spacing the duct banks is the best way to go since it minimizes the conductor size.
My practical experience tells me we are over-designing duct banks and that the real world load diversity factor keeps the ducts' temperatures in line.
Gilly - With that many circuits in a duct bank, there may be significant mutual heating if the duct banks are not spaced apart.
When we design large duct banks for multiple motor feeds, we end up increasing the motor cable size to accommodate the heating effects of adjacent cables, sometimes tripling the size. That creates issues of terminating a #4 awg on a 5 HP motor or 20 A breaker. Also, since the duct banks are concentrated at the MCC or switchgear and then spread out to the loads, the larger cable is only needed for that first portion of the run, but the oversized cable runs the full distance. So maybe spreading the duct banks is a better solution.
Code doesn't strictly require derating of underground conductors. It looks like we could just use Table 310-16 without derating for underground conduits. But if my ampacity calculation program based on Nehr-McGrath equations says the wires will be in a 70C ambient due to mutual heating of the duct bank, a 0.41 derate is needed to account for the high ambient temperature in the duct bank.
Your engineers' "maximum of 15 motors" may be a rule of thumb based on some predetermined calculations or experience. Or maybe they modeled all of the underground duct banks and have determined that spacing the duct banks is the best way to go since it minimizes the conductor size.
My practical experience tells me we are over-designing duct banks and that the real world load diversity factor keeps the ducts' temperatures in line.
jtester
Senior Member
- Location
- Las Cruces N.M.
You seem to be OK with the multiple circuits, the reason I brought it up, is that it is one of the most common violations that I see.
I think if you use 310.16 you must adjust for ambient temperature. If you use Exhibit 310.9 in the Handbook as an example, you must determine the ambient temperature of the earth around the center conduit in that duct bank. It can't be much lower than the steady state temperature of the surrounding conduits. This becomes a real mess.
Your other option is Neher-McGrath. It will also recognize a higher ambient. Darned if you do, darned if you don't.
I agree with Bob that diversity keeps us out of trouble, but not necessarily in compliance with the NEC.
Jim T
I think if you use 310.16 you must adjust for ambient temperature. If you use Exhibit 310.9 in the Handbook as an example, you must determine the ambient temperature of the earth around the center conduit in that duct bank. It can't be much lower than the steady state temperature of the surrounding conduits. This becomes a real mess.
Your other option is Neher-McGrath. It will also recognize a higher ambient. Darned if you do, darned if you don't.
I agree with Bob that diversity keeps us out of trouble, but not necessarily in compliance with the NEC.
Jim T
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
gilly said:
The issue is that the heat generated by the current carrying conductors in each of the conduits has an additive effect on the temperature within the duct bank, as well as on surrounding conduits or other duct banks. Thats why the separation needs to be what they are saying.
There are materials that can be used in the back fill around the duct bank that can help with heat dissipation, and you can add more concrete. You can also try not burying the duct bank as deep. Hopefully your engineer is using a computer program to model this, because it gets very complex. Also, for derating purposes, make sure he/she is starting with the 90 deg C rating.
Your only choice may be to use some distributed MCCs closer to the loads, or can you run outdoor cable tray.
Bob Wilson,
Where can I get a hold of that ampacity calcuations program?? Has anyone ever had a problem with duct banks over heating? Is this a big problem?
kingpb,
We are filling around the conduits with concrete, and trying to stay up as high as possible. I have always thought of underground conduit to motors as a huge money saver. Material cost less and underground can be installed around 30-50 feet an hour. With what we have to go through here it would probably be cheaper to run over head with tray or rigid. Now days with the cost of wire to up size to all the motors does not make much since either.
Thanks All
Where can I get a hold of that ampacity calcuations program?? Has anyone ever had a problem with duct banks over heating? Is this a big problem?
kingpb,
We are filling around the conduits with concrete, and trying to stay up as high as possible. I have always thought of underground conduit to motors as a huge money saver. Material cost less and underground can be installed around 30-50 feet an hour. With what we have to go through here it would probably be cheaper to run over head with tray or rigid. Now days with the cost of wire to up size to all the motors does not make much since either.
Thanks All
Last edited:
- Location
- Massachusetts
Just curious why...
Considering the expense of all these individual circuits, particularly once you apply the derating issues why you would not run one large feeder to each building and place remote MCCs at each building.
Control would be as simple as a LAN.
Not criticizing, I just don't understand the rational, I certainly could be missing something.
Considering the expense of all these individual circuits, particularly once you apply the derating issues why you would not run one large feeder to each building and place remote MCCs at each building.
Control would be as simple as a LAN.
Not criticizing, I just don't understand the rational, I certainly could be missing something.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
iwire said:
My thought too. I am a big fan of running as little conduit as possible.
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