Motor Feeder Conductor Sizing dilemna
Motor Feeder Conductor Sizing dilemna
(OP)
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.
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.






RE: Motor Feeder Conductor Sizing dilemna
RE: Motor Feeder Conductor Sizing dilemna
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?
RE: Motor Feeder Conductor Sizing dilemna
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?
RE: Motor Feeder Conductor Sizing dilemna
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.
RE: Motor Feeder Conductor Sizing dilemna
#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.