cable sizing 2

[Lc85]

Hi,
I have a 7.5kW induction motor which will be driven by an inverter. The rated voltage of the motor is 460V,3ph,60Hz and the supply voltage is 480V,3ph,60Hz.The full load current is 12.5A, the locked rotor current is 80A the full load power factor is 0.82. I would like to know how to size the cable between the inverter and the motor. can anyone help please?

Thanks

 

[7anoter4]

See: NEC Art.430.6(C)
If you know, the maximum operating current marked on the motor then the cable ampacity has to be 125% of this current.
If you don't know you have to take it as 14*1.5=21 A [as per Table 430.250 for 10 HP].
It depends on the way this cable will run [free air, underground and so on] you may choose the conductor cross-sectional area [copper or aluminum] according to NEC art.310 or Annex B.

 

[alanElec]

As per my calculations,the cable size requires is 4c x 2.5 mm 2 armoured/non armoured cable.
I have considered a derating factor for temperature 0.94(55 C).Derating Factor for grouping also needs to be considered if cables are grouped in a tray.Here there is no need of any tray,GI trunking/conduit would suffice.

Actual Current Carrying Capacity of Cable= Derating Factor * Current Capacity of cable(in ground,air) as per cable catalogue*No of runs per phase
=0.94*27
=25.38
Thus the cable can safely carry a load current of 13.6 A.Voltage Drop calculation are required if the length of cable is large.I have attached excel sheet for calculation.

 

[7anoter4]

Alansam,
I think your calculation -based on IEC60364-5-52, I guess-is correct but the cable and environmental data does not fit this standard.
First –for 3 single insulated conductors in a conduit [PVC insulated] the maximum permissible will be 21 A [B1 method -not 27 as in free air].
Second, the conductor resistance is more than calculated [even for 70 dgr.C]-for such a small, cross-section, no skin effect or proximity effect has to be taken into consideration. On the other hand the reactance is also more elevated than the standard proposes.
Moreover, why 55 dgr.C air temperature?

 

[jraef]

The fact that the OP indicated it is 480V 60Hz would make it a 99.99% chance the installation is in North America, with about a 90% chance within that saying it is the USA (there is some 480V in Canada in certain industries). In that case, discussing IEC rules and metric wire sizes is moot.

The first answer referring to the US NEC Article 430 (or the corresponding CEC section on the offhand chance it is Canada) is the only correct response. The other nameplate data is generally superfluous to this task, the NEC requires that the wire be sized based on THEIR rules and charts for MINIMUM sizes based on the HP rating, not the motor nameplate data*. 7.5kW is 10HP, so look up the NEC table 430.250 FLC rating of a 10HP 460V motor and start from there, adjust for other rules regarding voltage drop over distance, type of wire, type of raceway and number of conductors in the raceway. It's all there.

By the way 7anoter4, 430.6(C) is commonly misinterpreted to apply to motors driven by VFDs, but it is not. That paragraph preceded the development of VFDs, they are referring to special motors designed for varying the voltage ONLY, often seen used in vibrating table feeders, because as the voltage is reduced, the slip, and therefore the current draw, increases beyond normal. Hence the requirement for sizing at 150% of the table values. A VFD varies the voltage AND the frequency, so slip is controlled and conductor sizing is the same as if the motor were connected to the line.

*If the motor nameplate data for FLA exceeds the values in the appropriate NEC table, you are to use the higher value, however it rarely does.


"Will work for salami"

 

[7anoter4]

Thank you,jraef.

 

[waross]

Nice answer jraef. Actually there is quite a lot of 480 Volt systems in Canada. As well as legacy systems, some industries have stuck with 480 Volts. 600 Volts is a much smaller market than 480 Volts for motors, VFDs transformers EHT controllers and other equipment.
The applicable Canadian Code rule is:
28-106 Conductors — Individual motors
(1) The conductors of a branch circuit supplying a motor for use on continuous duty service shall have an
ampacity not less than 125% of the full load current rating of the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[alanElec]

It was nice if you guys could specify the exact size of the cable(from the relevant charts) required.I never checked the charts or relevant NEC codes but i hope I was correct when I chose cable that can carry 25 A load current(after considering derating factors if necessary) and the actual load current is 11.8 A.

125 % OF FULL LOAD current is 14.75 A.By that standards I hope my calculation was right.

Also regret for the faulty calculation where I had considered 415 V instead of 480 V.


55 C because ambient temperature in middle east is 55 C.

 

[7anoter4]

In the open post Liamc85 inquires:
"I would like to know how to size the cable between the inverter and the motor. can anyone help please?"If we take the motor data" 460 V 60 Hz" it is obviously North America [and not Middle East!].
By the way, 55 dgr.C it is not an average temperature even in Sahara Desert and could be only if the conduit is totally exposed to sunshine. An average air temperature has to be 40 dgr.C.
We don't know how Liamc85 intends to run the cable: in conduit, underground trench, in underground duct, in cable tray-open or closed solid bottom- and so on. We don't know what kind of conductor: copper or aluminum, we don't know what kind of insulation: pvc or xlpe[epr] and we don't know other limitations like -per NEC or IEC 60364-5-52-maximum permissible temperature at the terminals of connected devices-breakers, converter.
Therefore, we cannot advise the actual cross-section.

 

[Lc85]

Thanks guys,
I have some more info now but I am still not sure which standard to use. The cable will be run in conduit,it is for a plant in Columbia. My issue is I do not know how to calculate the short circuit conditions. Will it be the inverter short circuit current or is it the mcc that is driving the inverter?

Thanks

 

[jraef]

Short circuit does not factor into conductor sizing.

Are you buying cable that is going to use AWG sizing, or metric sizing?


"Will work for salami"

 

[Lc85]

Hi,
It can be either metric or awg

Thanks

 

[alanElec]

In here a derating factor(0.94 at 55C) is always considered for Cable sizing when running cable in air.

Since there was no mention of cable laying,kind of insulation etc I calculated on a higher side,meaning even if you consider all details,the cable size required couldn't probably be more than what I calculated.