Medium Voltage Conductor Sizing in underground duct

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.

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.

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. 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 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: KVA- 386Amp
2- FDR 2: KVA- 320Amp
3- FDR 3: KVA- 294Amp
4- FDR 4: 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 - Volts.

...
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?
....

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.* log10 ((T2 + 234)/(T1 + 234) /time)) [in excel mode].
T2=250 oC ; T1=105 oC; time=1 sec
I/A= 0. kA/MCM
A=50/0.=737 MCM[750 MCM OK!]
Ampacity- NEC table 310.60(c) 77-395 A
Isett=<6*395= A/ 1 sec[or less]

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.

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? 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?

I see.
MV Switchgear= Medium Voltage Switchgear.

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

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.

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.

...

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?

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 A. 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.

Medium Voltage Power CableMV cable) installation guide

In the power distribution system, the medium-voltage power cables, which are the main lines for transmitting electric energy, often cause failures due to various hidden problems in the construction process.

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Low-voltage power cables have low voltage, simple structure, and simple construction and installation. As long as a little attention is paid when laying, the hidden danger of failure can be greatly reduced. However, due to the concealment, complex structure, high voltage, and cumbersome construction and installation of medium-voltage power cables, once the professional quality and skills of the construction personnel are slightly lacking, the laying, installation, and grounding of medium-voltage power cables in accordance with the drawings and normalized operation cannot be achieved,it will bury hidden troubles for the safe operation of the power distribution system, and the causes of failures are complex and diverse. At present, line faults caused by poor installation quality of cable accessories have accounted for more than 80% of all medium-voltage power cable line faults.In order to ensure the safe operation of the medium-voltage power cable distribution system, this paper points out in detail the matters that should be paid attention to when laying, installing and grounding medium-voltage power cables during construction.

Laying of medium voltage cables

When choosing the laying path of medium-voltage power cables, it is necessary to avoid sections where the cables are likely to suffer from external forces, overcooling, overheating, corrosion, demolition, etc.Large drops in cable lines should be avoided.Except for waterproof cables, cables should avoid long-term In the water.It should be ensured that the cable laying channel is unimpeded, all sundries have been removed, and the drainage is good. The structures passing through have been constructed, and the turning radius of the cable trench and the turning of the civil part of the tunnel is not less than the minimum allowable bending radius for laying cables.

Pay attention to the following aspects when laying medium voltage power cables:

The ambient temperature at the laying site should generally not be lower than 0°C. If the construction period is urgent in winter (the ambient temperature at the laying site is lower than 0°C), the cable can be preheated by powering on or placed in a drying room.When the surface temperature reaches 20-30°C,the laying can be carried out quickly . When the cable has cooled below 0 °C, it shall not be bent again.

Measures should be taken to prevent the cable from being grazed by the ground, trench wall, pipe mouth, and machinery. Once the cable grazes are found, the laying must be stopped immediately. Only when the cause of the grazes is found out and removed, can the laying be carried out again.

Twisting is not allowed, so as not to damage the cable. In order to eliminate the twisting stress of the cable, an anti-twisting device should be installed at the cable pulling head. For coiled cables and short cables that are not coiled, they should be turned in the direction of the circle when laying to prevent the cables from twisting. If twisting has been caused, it should be released along the twisting direction. Do not hit the cable with any tools or objects to prevent cables from damaging.

Use special equipment and tools that meet the requirements (such as pay-off racks and guide wheels) for laying, and choose laying methods such as end traction laying, mechanical conveying laying, and manual-assisted guiding synchronous laying according to site construction conditions.

The maximum lateral pressure, maximum traction force, and minimum allowable bending radius of the cable during and after laying should not exceed the specified value allowed by the cable.

When there are many cables laid in trenches, pipes and tunnels, attention should be paid to ventilation and heat dissipation.When the medium voltage power cable is directly buried, the buried depth of the cable (the distance between the upper surface of the cable and the ground) should not be less than 700mm.The bottom of the ditch must have a good soil layer, free of hard debris, and be paved with 100mm thick fine soil or yellow sand. After the cable is laid, it should be covered with 100mm thick fine soil or yellow sand, and then covered with concrete or bricks, etc. The covering width of the protective cover should exceed 50mm on both sides of the cable, and measures should be taken to prevent the migration of soil moisture.After re-soiling, path signs should be installed on the ground. In cold regions, cables should be laid below the permafrost layer, and if the ground conditions are not suitable for deep burial, protective measures should be taken to avoid damaging cables.The clear distance between direct buried cables and various facilities in parallel or across them shall comply with relevant regulations.

In addition, in order to ensure the safety of medium-voltage power cables during operation, the following aspects should also be paid attention to during laying.The rated voltage of the cable laying should not be less than the working voltage of the system, and the maximum voltage of the system should not exceed 1.2 times the rated voltage of the cable.The long-term maximum working temperature of the laying cable should not exceed 90 ℃.At 5S short-circuit current, the maximum temperature of the conductor should not exceed Exceeding 250 ℃. It is strictly forbidden to overload the cable for a long time.

Installation of medium voltage cables

The installation of medium-voltage power cable terminal accessories and intermediate joints is the weakest link in the construction process of medium-voltage power cables, and the installation quality will directly affect the safe operation of the entire power distribution system.According to incomplete statistics, line faults caused by poor installation quality account for more than 80% of all line faults. In this regard, sufficient attention should be paid during the construction of medium-voltage power cables to ensure high-quality completion of the installation of medium-voltage power cables.

Medium-voltage power cable terminal accessories can generally be divided into two types: cold-shrinkable cable accessories and heat-shrinkable cable accessories. According to the characteristics of cable materials, user requirements and current construction conditions, it is recommended to choose cold shrinkable cable accessories. Although the price is higher, the safety is also higher.The installation process of medium-voltage power cable terminal accessories is as follows: strip the outer sheath of the cable → remove the steel tape armor layer (optional, if there is, check whether the armor material of the single-core cable is a non-magnetic metal tape) → strip the cable Isolation sleeve → Weld copper tape shielding grounding wire (make the wire core marking line) → Strip the insulation shielding layer (10KV cable insulation semi-conductive shielding layer is generally peelable and easy to peel off; 35KV cable insulation semi-conductive shielding layer is not Peel-off type, special care should be taken not to damage the insulation layer when handling) → clean the insulating surface → install semi-conductive tubes → install branch gloves → strip off the insulation layer and conductor shielding layer → install insulating sleeves and terminals. Pay attention to the following aspects during the installation of medium voltage power cable terminal accessories:

The copper strip ground wire should be welded firmly.

Do not damage the conductor when stripping the shielding layer of the conductor, which will cause burrs on the conductor and cause the partial discharge of the cable to exceed the standard.

The wiring terminal should match the cable conductor material. The aluminum core conductor should use a high-quality copper-aluminum transition terminal, and the terminal should be filled with conductive paste. When crimping, start from the end of the terminal and crimp it in place.

The contact surface between the cable insulation and the terminal should be processed perfectly to ensure a good seal, otherwise it will cause many fault points.

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The installation process of the intermediate joint of the medium-voltage power cable is basically the same as that of the terminal accessories, with some differences. The following aspects should be paid attention to in the specific operation:

Accurately measure the length of the two connecting cables at the joint to avoid the two connecting cables being out of reach.

Strictly control the stripped length of each layer at the end of the two connecting cables, and strive to be consistent.

When crimping the straight-through terminal, the straight-through tube should be crimped with the three wire cores of one connecting cable first, and then crimped with the three wire cores of the other connecting cable.

The copper strip ground wire should be firm, and the wire core mark should be clear.

At the joint, the two ends of the copper wire shielding braid of the three cores should be in full contact with the copper tapes of the two joint cables.

Grounding of medium voltage cables

In the past, in the faults of medium-voltage power cables, there were often cases of short-circuit caused by severe heating of a certain phase of the cable. After dissecting the faulty cable, it was found that the insulation core of the cable was intact, and the copper tape had burns that were not penetrated, but the rest of the cable had been burnt and charred, and all signs indicated that it was burned from the outside to the inside, which indicated that there was no problem with the cable conductor itself during operation. It is other reasons that cause the cable to heat up and carbonize. It is confirmed by analysis that when the medium-voltage power cable is running under alternating voltage, the alternating current passing through the conductor will generate an alternating magnetic field, and the magnetic field lines generated by the magnetic field are connected to the conductor, and are also connected to the metal sheath (shielding layer and armoring layer) ), so that an induced voltage is generated in the metal sheath. If the induced voltage is too high, and the medium voltage power cable is not safely grounded during the actual construction process of the medium voltage power cable, a large induced current will be generated in the metal sheath , causing the cable to heat up severely, resulting in a short circuit fault. Therefore, in order to ensure the stable, reliable and safe operation of the system, it is necessary to adopt a correct and safe grounding method according to the induced voltage and circulating current generated in the metal sheath of the medium-voltage power cable, so as to reduce the risk of system line heating and short-circuit failure.

Conclusion:

Considering that the construction process of medium voltage power cables has the characteristics of high voltage, multiple shielding, long lines, harsh site environment, and high construction requirements, this paper introduces in detail the matters that should be paid attention to during laying, installation, and grounding during construction. However, there are many situations in the laying, installation, and grounding methods of medium-voltage power cables, and the buried laying and cable trench laying methods with high concealment are mostly used, resulting in complex and diverse cable faults, which increase the analysis, location, and location of cable fault points. It is difficult to find and repair. Therefore, the professionalism and basic skills of cable construction personnel and management personnel are very important for high-quality completion of cable laying, installation, and grounding. During construction and management, it is necessary to standardize operations and construct according to drawings. Technical points in the laying process to avoid failures.

What is the difference between HV and MV Cables?

One of the ways to specify electrical cables depends on their voltage. According to the voltage, there are low-voltage, medium-voltage, and high-voltage cables. Most residential, commercial, and industrial cables are low-voltage. Medium-voltage and high-voltage cables have a longer list of characteristics that set them apart, which often creates confusion. Read this blog by the popular cable distributor Nassau National Cable to learn what sets HV and MV cables apart.

Defining a Medium Voltage MV Cable

A medium-voltage cable, shortened as an MV cable, has a medium voltage of between 1kV to 100kV. This is an international standard defined by International Electrotechnical Commission (IEC), founded in London and headquartered in Geneva, Switzerland. The manufacturers always produce medium-voltage cables within this range. For instance, at Nassau National Cable, you may buy medium-voltage power cables with 2.4 Kv, 5 Kv/8Kv, 15 Kv, and 25 Kv/ 35 Kv voltages. These are the standard voltages of MV cables in the United States.

Construction and Applications of an MV Cable

MV cables have many applications, including power distribution in industry and mining, mobile substation tools used for repair and maintenance, and many others.

Aside from the standard construction that most cables have, including a conductor, insulation, and a jacket, an MV cable also has separate shieldings for its conductor and insulation. This is not the case with the medium-voltage jumper cables, as they follow the design characteristics that are typical for jumper cable construction.

What is a High-Voltage HV Cable?

For high-voltage cables, the factor that matters in defining them is not just the voltage limits but the specific role that they serve. As the name suggests, HV cables are used to transmit power at high voltage. A typical voltage of a high-voltage transmission line is 345,000 volts.

Construction and Applications of an HV Cable

High-voltage cables are always used in underground transmission power lines. HV cables are rated for direct burial, but otherwise, they are laid in ducts in underground transmission systems. Other than that, HV cables may be used for AC and DC power transmission, as an instrument cable, as a submarine cable, or as a cable for an ignition system.

The structure of a high-voltage cable is more complex than a medium power cable. Aside from conductor shielding and insulation shielding, an HV cable has additional means of cable protection, such as semi-con layers of insulation. The shieldings of an HV cable are always metallic, the insulation is fully-rated, and the jacket is extra-protective. The construction of the cable is equipped to be protected from the high stress of high-voltage applications. The cable may have additional grounding depending on the specifications of a circuit.

Medium Voltage

Recommended for low and medium voltage power of volts. Used primarily in control circuits and for airport lighting. 5KV XLP power cables can be used in conduit, duct and direct buried.  The temperature rating is 90C.

The company is the world’s best Three Cores Medium Voltage Cable supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.