Motor Feeder Conductor Sizing dilemna

[nightfox1925]

Good day to all. I did a cable sizing calculation to determine the appropriate power conductor size for a 3phase, 480VAC motor feeder conductor based from

a)Ampacity (to consider conductor de-ration)

b)Steady State Voltage Drop (based from an allowable 3% Vdrop)

c)Motor Starting Voltage Drop (based from a modest 10% conductor Vdrop)

d)Short circuit thermal withstand capability (based from ICEA conductor thermal equation with the available 480VAC 3phase fault at a portion of the cable calculated with acceptable accuracy and the MCP clearing time specified.

Just when I thought the calculation was appropriate, I was directed not to consider item d) since they do not normally include it on LV conductor sizing. Is this code wise appropriate? The CEC code also take reference to this ICEA thermal equation as well…is this supposed to be an optional check and depends on client’s preference or is this one of those situations where a compromise has to be taken?

I was always under the impression that the conductor should be able to withstand the momentary fault within the OCPD clearing time.

 

[davidbeach]

If you do item d) base on the fault current available at the far end of the circuit - for withstand of through faults - you will find that nearly all circuits of any appreciable length of smaller wire will no problem meeting that test. Short runs of multiple conductors can get you into that range or short runs of small wire with high available fault currents. But if you subdivide your system in an appropriate manner - no running 20A circuits from 4000A boards - you'll rarely find a problem.

 

[nightfox1925]

Thank for the prompt response david. I agree with you but to some extent. I have calculated the incoming fault to be 10.46kA (500kVA, Z=5.75% assuming infinite source) and if I take a fault at the cable at around 5 meters away from the starter, I am getting around 12kA (source kA + motor other motors' contribution). A no. 8AWG may have suffice but it is only giving me around 9.2kA.

Adequately sizing the conductor at this point is needed and 6AWG will give me 14kA of withstand capability before before damaging temperatures prematurely degrade the conductor's insulation. is this one point that I may ahve to reconsider it as defined in 9.2 of IEEE 242 2001?

 

[wareagle]

I ran a calculation making some assumptions and no motor contribution. Using 40 ft of #6 cu the fault was about 6500
amps. The 10.5 KA you are using as the fault at the service will be considerably less if you include the utility impedance. I would not be concerned. Just for information what is the time for the breaker to clear the fault and what is the withstand rating of #6 using that time?

 

[nightfox1925]

Thanks wareagle. The MCCB is 1 cycle (0.0167 sec). I calculated a 3 phase fault along the 6AWG cable 5m away from the motor starter. The MCC where the motor is connected is fed from a 500KVA, 13.8-480V, Z=5.75% power transformer. The total other MCC motor lumped load is 327.63kVA with an assume lumped Xm = 0.167 (based from 6 x FLA). The conductor R is 0.00824 ohms, conductor X is 0.00122 ohms for a 16ft length. The 10HP motor is 8.28kVA at LF=0.8

using a 500kVA (0.50MVA) power base, a fault on the MCC would yield

P.U. XT = j0.0575
P.U. Xm = j0.234
P.U. X10HP = j9.26
P.U. Rcond = 0.01788 from (0.515 ohms per 1000ft)
P.U. Xcond = j0.0026 from (0.0764) ohms per 1000 ft)

IFmcc = 500,000 / (sqrt(3)*480*0.4593)
= 13.0939 kA

IFcond. = 500,000 / (sqrt(3)*480*0.05169)
= 11.634kA at a 5M conductor distance.

If we consider the entire length of the conductor, then we are referring to the S.C. at the motor terminals which will result to total P.U. Z = 0.5723 and result to 1,050Amps only.

 

[wareagle]

The withstand rating chart put out by the ICEA shows that
#6cu at 1 cycle has a rating of about 18000 amps.
This chart is based on the fact that a conductor can withstand 1 amp for 5 seconds for 42.25 circular mils.
So for #6 cu the cmil = 26240.
rating = 26240/42.25 = 621 amps for 5 seconds
rating for 1 sec = 621 x 5 = 3105 amps for 1 second
rating for 1 cycle = 3105/0.0167 seconds = 18592 amps
for #6 with thermoplastic insulation.