Motor cable sizing? 3

[ozziemick]

Hi!
Just seeking help in regards to sizing cables to 415v 3ph motor's (DOL) on an industrial site on the basis of voltage drop. I beleive one technique is to use the locked rotor current and the power factor of the motor at the instant of start (locked rotor pf ?) in the voltage drop calculation (info available from motor vendor's I presume). Obviously if just the LRC was used, the cables would be huge, so I'm guessing that the reduced pf at start is what brings the cable size down? These cable sizes seem to be a couple of sizes up from what they may have been if the volt drop calc was done using just the full load amps. What are the main factor's/reason's to consider when sizing these cables: motor terminal volts to enable the motor to start under load; preventing excessive voltage drop on the MCC bus?? Also thanks to those who replied to my "under voltage relay setting" enquiry. I am still checking into that problem.

 

[Chris33]

When your sizing your cable for a motor you take the FLA of the motor, and in practice I usually multiply this by 1.25

Ex 1: If your motors FLA was 13A, I would then multiply this by 1.25 to get 16.25A. Therefore you would go with a 12 gauge wire because it is good for 20A.

This is not taking into account for voltage drop on long runs. When sizing the cable for your motor you don't need to worry about start up for this current draw is an inrush current which lasts for a sec. If you take your FLA multiply it by 1.25 and then take the size cable up from what you get and you will have no problems. If you are wondering why 1.25 well alot of motors have a S.F. of 1.15, which means the motor can draw 15% more current than the FLA on the nameplate and the motor will be fine. So I use 1.25 to be safe. Don't forget the voltage drop if you have long runs.

 

[tabler]

Hi,
It's be good to get more details (motor's power, connection,start and working currents,time of start).If it is enough big motor with delta-connected stator, perhaps is it need to use soft-starting system or star-delta reconnection? I think such solution's realization will have
the lower cost than to make PF compensation for starting moment.Starting reactor may be helpfull. Generally it is need to compare costs of solution

Best regards,tabler

 

[jbartos]

Suggestions: Check the following
1. The motor locked rotor amps
2. All potential voltage drop causing items:
2a. Cable impedance (dependent on the cable length)
2b. Overload relay heaters
2c. Upstream transformer impedance, especially, if the transformer is on smaller side as far as kVAs are concerned
2d. Contact resistances (which are often neglected)
3. Motor controls (full volt start or reduce volt start)
4. Motor shaft load
5. Motor acceleration time
6. Minimum motor starting terminal voltage (some motors need 80% of their rated terminal voltage (pretty much the standard), others may need only 75% or less of their terminal voltage rating (tend to be more expensive))
It is convenient to use software. In some cases, the 1.25 x FLA approach is good enough without any risk (very short cable runs will do in most cases); however, when cables are long, then the cable impedance may be very high, causing large volt drop at the motor start, and the motor may stall. It will not even move or create any noise.

 

[rhatcher]

My advice is to stick with Chris33's advice on sizing and make sure to allow for voltage drop due to distance. You may calculate voltage drop based on 1.25 x FLA.

Motor starting current is not really a consideration with respect to cable sizing if you follow the 1.25 X FLA rule, but for motors with a significant inrush you may want to brace the cables as they will move when the surge comes through.

It is true that for a relatively large motor started on a relatively small system (ie small transformer) that some method of reduced voltage starting may be required. However, this is due to the transformer's inability to supply the motor inrush without excessive voltage drop and is independent of motor cable size.

 

[electricpete]

Your question was about sizing motor cables based on voltage drop.

Most motor systems are spec'd to start their load with 80% voltage at the motor terminals. Usually it's easy to predict that voltage drop won't be a problem. In cases of long cable where there is a question, then the voltage drop of the motor feeder cables needs to be considered in addition to the transformer impedance when determining whether the motor terminal voltage requirement is met. Also may need to do a load flow simulation which accounts for 1 - drop of the bus voltage during starting: 2 - other motors on same bus draw more current due to reduced voltage, which in turn affects the voltage drop of the bus. Excessive voltage drop can result in motor stall during start.

As you point out a motor has very low power-factor during starting. As such the inductance of the cables takes on more significance in determining starting voltage drop. Unfortunately, you don't significantly reduce the inductance of a cable by making it bigger (this only reduces the resistance). Doubling-up the cables (2 small instead of one big) is often much more effective if there is a concern for starting a motor with long cables, since this will approximately halve the inductance.

 

[don01]

hi ozziemick,
where are you installing this cct standards australia discuss cable selection extensively ~ would this be helpful

Regards Don

 

In reference to ozziemickies question related to sizing conductors for Electric Motors, may I offer this advise.

All the answers to your question are in Section 28, Table 37 of the Canadian Electrical Code. (And a similar section for the USA National Electrical Code) both of which must be on the internet or in your public library.

The reference amperage is the full load amperage (FLA). Not, 1.25 times the FLA. This amperage used for sizing the motor overload device.

Table 37 cross references your FLA to the one way distance of the electric motor from the Electric Power Source (EPS) and identifies the conductor size which will maintain the allowable voltage drop specified in Section 28, which I believe without checking, is 5 percent.

Electric Motors will operate with relatively good characteristics with as much as 10 percent voltage deviation above and below nameplate operating voltage. In fact, greater V.D. due to the distance from the EPS is benificial to the stator winding since the VD due to the locked rotor current will act the same as a reduced voltage motor energizer.

By the way, 415 volt nameplate rating is not a conventional operating AC voltage. It should be 460 or 480 volts.

 

[electricpete]

I should expand on me previous comments to include what others have already said. There are two conditions which need to be checked: #1: Need >90% voltage at the motor terminals during running; and #2 - Need >80% voltage at the motor terminals during starting. (assuming those are the voltages you spec'd when you bought the motor/pump combination).

It seems reasonable that LRA should be used for checking the requirement for 80% voltage during starting, FLA should be used for checking the requirement for 90% voltage during running. Less than 90% voltage during run may be acceptable if the motor can be appropriately derated. Less than 80% voltage during start would be unacceptable since motor may not come up to speed.

 

[jbartos]

Suggestion:
It is also important to consider the shaft torque/horsepower at the motor start. Some motor loads use a very small torque during start, e.g. a fan that is missing some blades, other may require almost rated horsepower or in some instances even more than rated horsepower, e.g. motor operated valves that may experience sediments or corrosion. Often, the rated motor load with normal behavior is tacitly assumed.

 

[rhatcher]

Both Chris33 and electricpete have posted good contributions to this thread (I passed out some stars tonight). The only thing that I may add is that allowable ampacities for a cable are a function of size, insulation type, and installation type.

If you are concerned (or just curious), start by checking the cable type and AWG against the ampacity of the load. Make sure to derate the cable for distance, ambient temperature, and # of conductors (if in conduit) per the NEC.

At this point, keep in mind that the NEC specifies minimum values with respect to cable size per ampacity. Good standard practice is to oversize by some factor. Some engineers choose 25%, some 30%, some 15%. Two good reasons for oversizing that immediately come to mind are to allow for motor service factor (per Chris33) and to allow for voltage variation. It is easy to see that if you allow for the combination of those factors that 15-25% oversizing makes good sense.

Finally (for anders1ca), 415VAC is an older IEC standard voltage. I cannot provide more specific info on that voltage class though since I live in the NEMA world.

 

[don01]

hi guys
just a small note 415 v 3 pahse is standard iso motor sizing where I live. It would interest me to know is ozziemick in australia - it really does sound like it? If so go to AS 3098 cable selection. Down here v drop max for the cable is 5%. If you are causing a drop on the main panel then a starter is required. NOT an option. The cable selection is based ob the FLC of the motor (no service factors used), provided max start time is (from my memory) 6 secs.

The tables in the SAA contain the installation derating factors (eg underground in conduit etc etc) and the mv /a /m factors for each size.

regards Don