Star-Delta motor cabling 3

[jsearl]

I know I should know this but can anyone just confirm for me please.

I have a motor rated at 37.5 kW and it is to be connected for star-delta starting. The nominal motor current is say, 68 amps. Am I right in saying that the cabling from the MCC for the delta connection can be rated for 39 amps (68/root 3)?

I would be grateful of your help.

Regards

 

[GGOSS]

Hello jsearl,

All cabling in the line circuit must be rated to 68 amps or better. Cabling within the delta loop (between the contactors and the motor) can be rated to 39 amps or better.

Regards,
GGOSS

 

[jsearl]

Dear GGOSS,

Thanks for your time.

Regards
John

 

[Shortstub]

Don't forget the derating factor for having 6 instead of 3-conductors in the conduit or wireway. So the ampacity, ignoring code requirement for increased nominal size, for each of the 6 conductors should equal nominal full-load Amps x 0.577 x 1.25 (Derating Factor = 0.8 for 6-cond) x Service Factor (if warranted). The resultant size is not 100% of full-load Amperes!

 

[GusD]

Hi Jsearl

Info From a previous post related to your question.

The only way to protect your motor is to use a dual function O/L relay, so that you can protect your motor during start (or failed start) and during Delta running.
As per the info below ,one has to be careful when setting the O/Ls for a Delta connected motor.
As a rule ,we set motor overloads as per Table 44 on the CECode or possibly any other code .This rule says that for motors with a Sfactor of 1.15 OLoad is set at 115% of motor nameplate FLAmps or Table 44 and 125% for motors with a 1.25 Sf.This is true for most motors unless ,they have a Star/Delta starter.
The following example indicates how to determine the OLoad setting of 75 HP ,575 volts,3phase(SCIM) controlled with a Wye/Delta starter.The motor is rated for continuous Duty and has a Sfactor of 1.2.

Sorry about the Table info.Is in the CECode.Maybe not much different in the USA or VDE rules in Europe.
Step1= Use Table 44 to determine the FLA of the motor. Table 44?--75 hp,575 v,3Ph SCIM draws 77 Amps.

Step 2==Determine the Phase value of the current.
77Amps/Sroot3(1.73)=44.45 Amps

Step 3==Use rule 28-306(1)to determine the overload setting.44.45x1.25=55.56 Amps

Step 4 == Rule says to comply with the marking requirements outlined in Ru28-306 (2)

If I wasn't thinking about a Star/Delta motor and I set my
OLoads at FLA of 77x 1.25 =96.25 Amps.it would not be hard to burn the motor out,if one wasn't paying attention.

GusD

That should help on deciding what cable to use.


GusD

 

[jbartos]

Suggestion to jsearl (Electrical) Aug 12, 2003 marked ///\\I know I should know this but can anyone just confirm for me please.

I have a motor rated at 37.5 kW and it is to be connected for star-delta starting. The nominal motor current is say, 68 amps. Am I right in saying that the cabling from the MCC for the delta connection can be rated for 39 amps (68/root 3)?
///The Wye-Delta starter can have the open transition and closed transition.
See for example Reference (which is also available on

http://www.squared.com

:
1. Square D Digest No. 172, March 2000,
Page 13-45 Electromechanical Reduced Voltage Starters Class 8630
Essentially, all leads from the motor terminals T1, T2, T3, T4, T5 and T6 to wye-delta starter T1, T2, T3, T4, T5, and T6 terminals may be rated for 39Amps or higher. Normally or as seen in the referenced publication, the three leads to starter are to overload relays and another three leads serve to wye contactors as well as delta contactors. Therefore, there is not the only run, i.e. unique run, from the motor terminals to the delta contactors.\\\

 

[GGOSS]

Hello GusD,

In your post of August 14 you state "The only way to protect your motor is to use a dual function O/L relay". My appology for being a little blunt but the statement is not correct. In both standard & closed transition star/delta starters, correct placement of a standard overload relay will provide protection in both star (during start) and delta (during run).

Regards,
GGOSS

 

[rbulsara]

jsearl:

You are correct!

There are six leads brought from motor to the MCC Y/delta starter. Sizing six motor lead cables to delta current is a norm. Also if your O/L protection relay or sesors are in the delta circuit, which usually is the case, set the O/L trip to 1.25% of 39A =49A.