Wire/starter sizing for a 125 HP, NEMA Design B motor, 3 Phase, 460 V, FLA= 143 Amps, LRA=908 Amps

[offett]

Greetings to all,

I am looking to size a starter/wires for my 125 HP blower motor. It is 125 HP, 3 Phase, 460 Volts. The Full load amperage is 143 amps, however the "Locked Rotor Amps"=908 Amps. For now, lets assume I have a distance of 100FT back to my MCC. I understand that my wire size should be capable of carrying no less than 115% of rated continuous amperage or FLA. However, since my Locked Rotor or "start up" Amperage is relatively high~908 Amps, wouldn't I have to take this into account? 908 Amps is obviously much larger than 115% of my FLA (Full Load Amperage).

Also, I am looking for a suitable starter to place in the MCC panel. I was looking at thermal magnetic breaker type, non-reversible, NEMA COMBINATION style starter. Again, I am at the same conundrum...The continuous rating is 270 for NEMA 5. My research is showing me NEMA 5 will work. However, how can I be certain that my contactors can handle the start up amperage? Is this data available via the OEM or can I trust that the OEM will adhere to the NEMA Standards Publication ICS 2-2000 for NEMA Size 5 starters?

Thanks,

P.S. Sorry to be so verbose.

 

[dpc]

There is no conundrum. NEMA-rated motor contactors are very robust. A size 5 starter will be adequate. They are designed to start motors across the line and I would not be concerned about any NEMA 5 contactor made by a US company you have heard of before being able to start this motor. You really don't have to worry about motor starting current with NEMA starters unless the locked rotor amps/FLA is extremely high, and yours is not.

As for the motor conductors, they need to be at least 125% of the motor full load amps LISTED IN THE NEC TABLE (156 A) - not your nameplate FLA unless higher the the NEC FLA, and yours is not. See Article 430.

Or maybe hire a EE for a couple of hours.

 

[jraef]

Locked Rotor Amps is factored into the sizing of everything, you do not need to worry about it.

But for conductor sizing it's 125% of the FLA as listed in the tables in the NEC, don't know where you got the 115% thing. I think you are confusing this with Service Factor, a whole different kettle of fish. Read and understand Article 430 of the NEC, it's all laid out for you in there.

Starter sizing in MCCs is per NEMA sizes (for the most part). Size 4 goes to 100HP, size 5 goes to 200HP. You need to use Size 5. Don't make it harder than it needs to be.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

LOL, glad we agree Dave...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

We're in violent agreement!

 

[offett]

Thanks for your responses..I have been doing low voltage controls engineering for the past five years and this higher polyphase voltage project I am managing has me a little excited/nervous! I have electrical contractors assisting me,but i wanted to do a rough sketch of how i would design the system and then compare to their actual design.

 

[electricpete]

After you select cable to meet 125%, one other thing you might check is whether the selected cable will give satisfactory voltage at motor terminals during start and run.

=====================================
(2B)+(2B)' ?

 

[RobsVette]

I'll add my 2 cents in here as well.

Bear in mind that a motor starter is very simple device that consists of 2 things.
1) A contactor to connection the line to the load - in this case the motor
2) A time and current sensitive overload relay that will open the contactor if tripped.

If your case, even though your locked rotor current is very high, you will not automatically trip the overload relays upon motor starting because the current is only near the locked rotor limit when the speed of the rotor is 0. As the rotor picks up speed the current reduces until you reach your operating speed (1750 on a 2 pole motor or 3550 on a 4 pole motor)at which point you will be at your FLA (or possibly less depending on the motor load.)

A size 2 starter should do you just fine. Just remeber to properly set your overcurrent protection once the motor is installed and you should be good to go.

 

[dpc]

RobsVette,

A NEMA Size 2 starter would be a little small. Assume you meant Size 5.

 

[davidbeach]

The relationship of poles and speed is also backward.

 

[jraef]

Well, since we are picking the poor guy apart...

...at which point you will be at your FLA

Technically, once it is up to speed, the LOAD will determine the amp draw. It will only pull FLA if the load demands FLA from the motor because it has increased slip. Otherwise it will pull what is called RLA, Running Load Amps.

But just to be fair, that would/should never be OVER FLA, which is likely what RobsVette meant.

Sometimes we are semantics SOBs around here, I know... but we can't help ourselves. Please forgive us.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

To add a little more fairness, RV did address that in parentheses right after the part quoted.

=====================================
(2B)+(2B)' ?

 

[7anoter4]

Since we are speaking about Locked Rotor Amperage one has to check the voltage drop from the source[transformer]
up to motor terminals. The minimum voltage depends upon a lot of factors[it has to be checked against the torque-speed curve ].
For usually blower load [Load torque =k*speed^2 and the average 1/3 rated load torque] 15% less from rated will be fair.
In the same time one has to check if at all the other motor- already running- terminals the voltage will not
be less than permissible [usually not less than 5% motor rated].