13.8kV Feeder Cable Sizing

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joshtrevino

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Location
Beaumont, TX
Hello,

I have an application where I am designing a feeder rated for 4.0MW (197 amps) at 13.8kV from an existing switchgear.
The cubicle that I am utilizing is a 1200 amp fully equipped space for which I have specified a 15kV, 1200 amp breaker and SEL-351 relay.

NEC 215.3.B seems unclear to me about how to size the cables. For 13.8kV (medium voltage) the cables can be sized for load only, correct?

I was originally going to use a 3/C - 2/0 with ground cable, but I am looking on the existing one-line and all the feeders are 3 - 500kCMIL or 6 - 500kCMIL.
 
W

wsbeih

Member
Location
USA
In MV applications, you need size the conductors such that it is adequate for the FLC (load) and the short circuit current (within ~10 cycles).

The Calculations for short circuit withstand are based on the IEEE STD 141-1993, ICEA P-32-382.
 
T

topgone

Senior Member
wsbeih said:
In MV applications, you need size the conductors such that it is adequate for the FLC (load) and the short circuit current (within ~10 cycles).

The Calculations for short circuit withstand are based on the IEEE STD 141-1993, ICEA P-32-382.
Click to expand...

You have missed to include the restrictions regarding allowable voltage drop on the line. If the load being supplied is a bit far than usual, it is the voltage drop limitation that rules over the others, IMO.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Sorry, I am late.

NEC art.430.224 Size of Conductors. Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device(s) is selected to trip.
NEC 430.225 Motor-Circuit Overcurrent Protection.
(1) Type of Overload Device. Each motor shall be protected against dangerous heating due to motor overloads and failure to start by a thermal protector integral with the motor or external current-sensing devices, or both. Protective device settings for each motor circuit shall be determined under engineering supervision.
240.101 Additional Requirements for Feeders.
(A) Rating or Setting of Overcurrent Protective Devices. The continuous ampere rating of a fuse shall not exceed three times the ampacity of the conductors. The long-time trip element setting of a breaker or the minimum trip setting of an electronically actuated fuse shall not exceed six times the ampacity of the conductor.
If the cable ampacity will be 1.25*Imotor rated then the feeder overcurrent protection setting will be 6*1.25=7.5*Imotrated.
That means for 27 sec. the conductor has to withstand 7.5*Imotor
By using the formula from IEEE 242/2001 chapter 9.5.2 Overload capacity:
IE/In=sqrt(((TE-To)/(TN-To)-(Io/In)^2*exp(-t*k))/(1-exp(-t*k))*(230+TN)/(230+TE))*100
If we shall take Io=Imotor rated=200 A and conductor ampacity at 90oC and 20oC ambient in duct [one single triplex] Table 310.60(C)(77) Detail 1 IN=295 A at 1500 A in 27 sec. the conductor temperature will not be more than 57oC.
 
Sahib

Sahib

Senior Member
Location
India
topgone said:
You have missed to include the restrictions regarding allowable voltage drop on the line. If the load being supplied is a bit far than usual, it is the voltage drop limitation that rules over the others, IMO.
Click to expand...
I think the 13.8KV cable to be sized for current carrying capacity only and not voltage drop because it may be for load center use only.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Sahib said:
The industrial practice is to size a MV cable such as 13.8Kv cable for current carrying capacity only because perhaps the load center transformer at which such cable is terminated would compensate for any voltage drop in the cable.
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Seems like losses would still be worth looking at even if VD was compensated for by say an auto tap changer to maintain voltage.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
By the way, the cable [4/0 copper EPR -or XLPE-insulated ] withstand 15 kA [RMS] for 1 sec.
The voltage drop for 10000 ft distance will be less than 3% for 200 A and less than 15% for 1200 A[6*Imotor rated]
 
T

topgone

Senior Member
Julius Right said:
By the way, the cable [4/0 copper EPR -or XLPE-insulated ] withstand 15 kA [RMS] for 1 sec.
The voltage drop for 10000 ft distance will be less than 3% for 200 A and less than 15% for 1200 A[6*Imotor rated]
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Correct. But that's just 10,000 ft or about 5 miles! Please consider a typical distribution line of 15 miles and that voltage drop balloons to about 19%.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
What I think topgone wanted to say 10000 it is about 2 miles [even not 5 miles] and a cable like this could be 15 miles long [24 km??]. For a transformer supply a maximum 12 km it is usual but for motor supply-as Sahib already said-it has to be closer to switchgear [up to 3000 feet usually] in my opinion.
 
Sahib

Sahib

Senior Member
Location
India
Julius Right said:
By the way, the cable [4/0 copper EPR -or XLPE-insulated ] withstand 15 kA [RMS] for 1 sec.
The voltage drop for 10000 ft distance will be less than 3% for 200 A and less than 15% for 1200 A[6*Imotor rated]
Click to expand...
That 15% drop for 1200 A[6*Imotor rated] is well within 35% or so drop limit for motor DOL starter contacts drop out. :)
 
T

topgone

Senior Member
Julius Right said:
What I think topgone wanted to say 10000 it is about 2 miles [even not 5 miles] and a cable like this could be 15 miles long [24 km??]. For a transformer supply a maximum 12 km it is usual but for motor supply-as Sahib already said-it has to be closer to switchgear [up to 3000 feet usually] in my opinion.
Click to expand...

You don't allow the receiving end voltage to go lower than the possible range the transformer tap changer can handle. The regulation range I know of were just about 20%. Unless you pay the premium and have a weirdly-made 50% tap-changer range!:roll:
 
Sahib

Sahib

Senior Member
Location
India
topgone said:
You don't allow the receiving end voltage to go lower than the possible range the transformer tap changer can handle. The regulation range I know of were just about 20%. Unless you pay the premium and have a weirdly-made 50% tap-changer range!:roll:
Click to expand...

Are we discussing about industrial power distribution or POCO power distribution? Which one?
 
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