Medium Voltage Conductor Sizing in underground duct

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Alangi1361

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Alright, I am getting confused on NEC table 310.60(c) 79.
I have a MV switchgear with 4 feeders connected to it rated 600A.
We have the 4 conductors, 3 phase, + 2 additional in an underground duct bank.

Based on the NEC table mentioned above, if I want to have 6 conduits in underground bank( detail 3), and if I size conductors to be 750 (@4.16 KV, copper, type MV-105), the current ampacity will go down to 375A.
So, if with future expansion, I draw 450A from the conductor, I will have overcurrent situation. Now, if my CB is rated for 600 A, it will not detect the OC fault.

Would somebody help me with this dilemma?

Thanks.
 
Alangi1361 said:
Alright, I am getting confused on NEC table 310.60(c) 79.
I have a MV switchgear with 4 feeders connected to it rated 600A.
We have the 4 conductors, 3 phase, + 2 additional in an underground duct bank.

Based on the NEC table mentioned above, if I want to have 6 conduits in underground bank( detail 3), and if I size conductors to be 750 (@4.16 KV, copper, type MV-105), the current ampacity will go down to 375A.
So, if with future expansion, I draw 450A from the conductor, I will have overcurrent situation. Now, if my CB is rated for 600 A, it will not detect the OC fault.

Would somebody help me with this dilemma?

Thanks.
Click to expand...

From your post, you have 4 600 amp feeders. Each feeder consists of 4 - 750 cu conductors, 1 per phase, connected to a 600 amp OC device.
Rating of the conductor in conduit is only 375 amps. I do not have the original ampacity. If my understanding is correct ,you can not use a 600 amp breaker with a 375 amp rated conductor. Would you clear up what you mean by this " Now, if my CB is rated for 600 A, it will not detect the OC fault."
You will not have an OC fault.
 
Terminology is important here.

MV switchgear - means your not using a MV motor contactor.
Switchgear would mean a breaker, but they have a minimum rating of 1200A.
If the device is rated 600A, then it sounds like a loadbreak fused switch; which is actually MV load interrupter type switchgear.

So, you will need to select new MV fuses that properly protect the circuit.
 
My question is why are you using table 79? Are you really using a cable that has the three phase conductors, the neutral wire (if used at all), and a ground wire, all contained within an overall jacket? For MV applications, I always use single conductors. That puts me into table 77, and gives you an ampacity of 395. But here again, that is well below the desired 600 amps. You are going to have to use at least 2 parallel conductors per phase. One way is to plan on a total of 8 conduits for your 4 feeders, which means you need a second ductbank. But it also means you can drop down to 500 MCM conductors. Another way is to put both sets of parallel conductors in the same conduit. That should require derating the conductors, but we don't have a mechanism for calculating the amount of derating. Specifically, the 80% derating for 6 current-carrying conductors shown in table 310.15(B)(3)(a) does not apply to systems over 2000 volts. So you would need to have a formal calculation performed under engineering supervision, if you wish to use this option.
 
I have 3#750MCM (that is what I specified in early stage design) for each feeder.

The loads connected to each are:

1- FDR 1: 2776 KVA- 386Amp
2- FDR 2: 2300 KVA- 320Amp
3- FDR 3: 2117 KVA- 294Amp
4- FDR 4: 1385 KVA- 193Amp

These feeders are feeding commercial buildings, and we are sizing 600A for future expansion.

When I was sizing the conductors, I looked at the catalog of a manufacturer and got the 3#750MCM.

I was fine :) until, I went to plan check and the plan checker picked on the sizing.But, he did not know the answer himself.

My concern is, if I have the system with this sizing and circuit breakers with 600A rating, and my conductor can take up to 375A(for example),
what will happen if we are drawing 450A, the conductor can not take it and starts getting damaged, and our dear circuit breaker does not break the circuit since it is set for 600A?

Our conduits are concrete encased, so maybe we can increase the spacing between conduits?

By the way, table 310.60(c)(79) has the ampacity for 2001-5000 Volts.
 
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Alangi1361 said:
...
My concern is, if I have the system with this sizing and circuit breakers with 600A rating, and my conductor can take up to 375A(for example),
what will happen if we are drawing 450A, the conductor can not take it and starts getting damaged, and our dear circuit breaker does not break the circuit since it is set for 600A?
....
Click to expand...
As kingpb noted, "Terminology is important here."

Read 240.100 and 240.101, then come back and ask questions using the terminology in those sections as it applies to your installation.
 
As Smart $ said: NEC 240.100 Feeders and Branch Circuits. (B) Feeders over 600 Volts. (C)Conductor Protection.
The conductor has to be protected [only] for available short-circuit current.
240.101 Additional Requirements for Feeders. (A) Rating or Setting of Overcurrent Protective Devices:
?The long-time trip element setting of a breaker... shall not exceed six times the ampacity of the conductor?.
So, you have to know the ?available? short circuit current.
Let?s say Ssc=360 MVA. The Isc=360/sqrt(3)/4.16=50 kA.
See:
Table 240.92(B) Tap Conductor Short-Circuit Current
Ratings. Copper conductor. EPR insulation.
I/A=Sqrt(0.0297* log10 ((T2 + 234)/(T1 + 234) /time)) [in excel mode].
T2=250 oC ; T1=105 oC; time=1 sec
I/A= 0.067771 kA/MCM
A=50/0.067771=737 MCM[750 MCM OK!]
Ampacity- NEC table 310.60(c) 77-395 A
Isett=<6*395=2370 A/ 1 sec[or less]
 
Julius Right said:
As Smart $ said: NEC 240.100 Feeders and Branch Circuits. (B) Feeders over 600 Volts. (C)Conductor Protection. The conductor has to be protected [only] for available short-circuit current.
Click to expand...
I have to disagree here. It is true that paragraph (C) speaks only about short circuit current. But the opening sentence of 240.100 says that the conductors must be protected against overcurrent. It goes on to say that if you don't put the overcurrent protection at the point of the conductor's supply (in this case, it is at the point of supply), then an engineer must design the system, and must take into account the time-current characteristics of the OCPD. Since a major portion of the time-current curves deals with overcurrent conditions, I have to conclude that we don't get to only protect against short circuit conditions.

 
Alangi
http://www.houwire.com/products/technical/article310_77.html

Using your posted information, you must realize that your design is not going to work using only 1 conductor per phase. Look at the
link I provided and check the ampacities using Detail 3. The max capacity for 750 kcm is 395 amps. If you are going to used the 600 amp
amp breaker, then your phases need to be a min of 2- 500 kcm per phase. This does not include any derating for the number of conductors.
Will your conduit size allow for additional phase conductors? Does your load include the 1.25 factor for continuous loads?
 
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I see.
MV Switchgear= Medium Voltage Switchgear.

The utility conductors are entering the switchgear, the bus rating is 1200A.

I am using SEL-751A relays, with both overcurrent protection and ground fault protection thru core-balance CTs.

Let me clear my concern here. Is 3#750MCM fine for conductors here? Does it comply with NEC?

Thanks every body.
 
bob said:
...Does your load include the 1.25 factor for continuous loads?
Click to expand...
Hmmm, that applies to the OCPD rating [215.3] but not the ampacity of feeder conductors over 600V. See 215.2(B) for the specifics.
 
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Bob,

Thank you. That was what I was looking for.
Using 2 conductors is the only option I was thinking about, but it adds up to cost, and Also, I am not sure if it brings other downsides as well as cost.

The conduit is 5", and the manufacturer recommended conduit for 3 single conductors was 4".

So, if I have to split each feeder into two, how many conduits can I put in a duct bank?
 
Alangi1361 said:
I see.
MV Switchgear= Medium Voltage Switchgear.

The utility conductors are entering the switchgear, the bus rating is 1200A.

I am using SEL-751A relays, with both overcurrent protection and ground fault protection thru core-balance CTs.

Let me clear my concern here. Is 3#750MCM fine for conductors here? Does it comply with NEC?

Thanks every body.
Click to expand...
Your hitting on one of the problems I see with MV installations Code. Code separates rating requirements for OCPD and conductor ampacity and provides no means of coordination to each other. For 600V we have 240.4(B), but there is no equivalent for over 600V.
 
Alangi1361 said:
...

The conduit is 5", and the manufacturer recommended conduit for 3 single conductors was 4".

So, if I have to split each feeder into two, how many conduits can I put in a duct bank?
Click to expand...
Code provides for up to 3 conductors per conduit and up to six ducts per bank. Exceed either and you have to go under engineering supervision per 310.60(C).
 
:huh:

It is getting complicated!

Maybe I put 2 duct banks, 2 conductors instead of one, one on top and the other in the bottom in my vault.

Interesting part is, my supervisor says the have done such design in the past, (600A ,one conductor) and no plan checker picked on it and it worked fine.:?
 
I'm confused. what are you trying to accomplish? Do you need 600A, or are you limited to a certain duct bank size/configuration?

I am curious as to what "breaker" you have that is "rated" 600A. ANSI gear that is an MV breaker is going to be rated for minimum 1200A. The SEL relay will be set to trip at whatever you desire. So, if you only have 375A for cable, then set relay to trip at anything over 375A.

MV gear is all rated for 100% continuous use. There is no requirement/need for sizing based on 125% continuous and 100% non-continuous.

For duct banks - there are a lot of parameters used to determine the maximum current. Depth and even surface material can make a difference. Computer programs do the analysis best. I would recommend your shop defaulting to a local engineer capable of properly sizing/designing the duct bank.
 
Kingpb,

Correct, all circuit breakers are 1200A. On the secondary of the CT's we have the setting for 600A protection, and yes, we can adjust the relay setting based on what we want.

To my understanding, we have 4 buildings, each with let's say 390 ish load. We are trying to design a medium voltage switchgear, and I need to get the permit/approval of this project from city.
So, I have to make sure the design complies with NEC.

We are the designer and involved with getting the permit from the city.

May I ask what type of software does the analysis for duct banks?

Thanks.
 
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