CABLE SIZE SELECTION

Guest/Salim

30kW is the power (size) of the motor. The wire size is determined by the current (amps) the motor uses to produce this power. Since power is a product of voltage times current, the voltage and current for this motor have to be known to give an answer. Otherwise, our answers would be nearly meaningless.

However, we can say that there need to be six current-carrying conductors between the motor and the starter, so the size of the wire to the motor needs to be adjusted for the number of conductors in use.

--John M.

Hi Salim,

The cable size is dependant on current requirements, volt drop, installation methods and cable insulation. Being a star delta starter the current in the motor cables is calculated at about 58% of what would have been used with a single 3 core installation. You should also take into consideration what size fuse/circuit breaker you will be using, for example if you have a 30kW motor and FLC of 60A, and then use 100A fuses to cope with starting current, you should size your cable for 100A, than derate as needed to comply with the local authorities standards.

Cheers,

Trevor.

trevor,

In the USA, for motors the cable size is determined by the current the cable has to carry (25% more than the motor current), not the fuse or circuit breaker size. This is one of the exceptions to the general rule that the cable is sized to carry the current the fuse or circuit breaker is rated for. This allows for the starting current of the motor, while protection of the motor and cable is provided by the running overcurrent protective devices ("overloads") that are used.

--Regards, JMM

Hi John,

I would agree that 25% is a good value for motor cable current rating, and generally on DOL we would most likely get a similar result. But "down 'ere" the primary purpose of fuses and circuit breakers is to protect the cable while the overload (which ever flavour you use) is to protect the motor from 'burn-out'. I have come across a lot on installations that only use motor thermistors as the overload protection but still have fuses for cable protection.

Cheers,

Trevor.

Trevor,

The National Electrical Code (USA) has a long history of "tweaking" and "fine-tuning", in the motors area (article 430). When I first started, I read this section at least 4 times before it began to make sense. I wouldn't be at all surprised if the codes you deal with "down under" are very different from this one, regarding motors. However, I think we would agree that the conductors from the starter to the motor are sized using 58% of the motor's nameplate current, while those supplying the starter must be sized using 100% of the nameplate current. In the NEC, this size is 125% of the current. The section on running overload protection states that this is also to provide overcurrent protection for the motor circuit conductors. However, short-circuit and ground-fault protection are to be supplied by the fuses or circuit breaker. Depending on the time delay characteristics of the fuses or circuit breaker (and the starting characteristics of the motor), the selected size can be as low as 110% and can range all the way up to 1100% of the motor current!

Since most motors will start fairly quickly, the cable stops carrying the very high starting current before the cable can heat to the point of danger. Therefore, in the NEC, the cable size is not required to be as large as the circuit breaker or fuses would otherwise require. (The fuses or circuit breakers must be sized to carry this high starting current without tripping or "blowing").

No matter how we do this, the NEC way, the "down under" way, or some other, we both need to hear back from the question's "guest" what the motor current is, or else he will never get any answer to his question.

Be safe! --JMM

Hi John,

I am glad to hear that the NEC is as understandable as any other standard, and then leads to interpretation. I have seen the AS/NZ standards become more grey as time goes on and lead to different interpretations. The 58% comes from √3, which is to do with the current requirements for the star connection.

Cheers,

Trevor.

PLZ. SEND US THE COMPLETE CALCULATION FOR COPPER AND ALUMINIUM CONDUCTOR WITH IES CHART FOR 30 KW MOTOR WITH STAR DELTA STARTER

REGARDS

Vijay

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Mechanical Engineers sizing cables? Whatever next?

The British Standards Institution normally charges for that. BS7671 is available from its website.