Motor Protection with Magnetic Only Breaker

[templdl]

Thanks templdl...
We won't be installing a QP, it would be a Cutler Hammer MCP on a separate panel.

As for the voltage drop, we would have a lower drop due to less cable impedance, at the moment it would be less than 3%. We are supplying 240V for a 230V motors. So I think we are good with that.


I have one other problem...:weeping:
This is what happens you get asked to get involved after the fact everything has been installed and vendor mentions motor is 15HP instead of 20HP. Is this reverse engineering?

The transformer feeding the motor is a 30KVA 4.39% with a secondary current of 72.25A. Calculating voltage drop due to motor inrush is.... high. Nema B motor, inrush about 6xFLA... so 48*6 = 288A divided by sec current (72.25A) times impedance %, it gives me a voltage drop of 17.5%. :rotflmao:

Trying to see what to do...

Installing an Eaton MCP in a stand alone enclosure? Feed by what and from what location? What ever possesed whomever the an MCP would bring anything to the party? If you knew anything about an MCP it is a reverse UL component listed device. TTY o purchase one, which you can do, as if it is 150a continuously rated or less it is the same dimensions as the F-frame series C breakers. BUT it is a violation of its UL listing ás well as a NEC violation!
Thinking that were were close to addressing your application issues now you have thrown a wrench back into the gears.

 

[adamscb]

Hi,

Been trying to read up/learn on NEC08 and got stuck with a doubt.

I have a 220V 15HP 3P Motor SF1.15 FLA 48 and is tripping the magnetic breaker (3P 60A Type QP, 10,000SC) when the HPU starts at full load (when HPU is turns on at a set pressure, not from 0PSI). The motor is being fed by this 3P 60A breaker which is connected to a motor contactor and overload (setting 75A, ABB TF96 75A-87A) with a 6AWG 3/C cable... It is being fed 208V

1) The conductor has to be higher then 125% of the motors FLC, correct?

2) Max overload for SF1.15 or greater is 125% (exception of max 140% if motor can't be started), correct?

3) Vendor seems to have set overload too high? Or selected a larger overload device...

4) Breaker, according to 430.52, an instanteneous trip can be selected at a max of 1100% (if nema B motor)... So if the breaker selected is higher than the conductor size ampacity, then what will protect the conductor? Doesn't the breaker trip value need to be lower than the conductors ampacity?

5) Is having a magnetic only breaker and an overload approved by NEC?


Thanks,
Yogi

1) This is correct. If your conductor temperature rating is 75 deg. C or 90 deg. C then your cable is sized properly.

2) This is also correct. Your overloads are oversized however, and I doubt they're the reason why your motor isn't starting. That 125% rating of NEC FLA table value isn't a minimum, it's a maximum.

3) Yes, the overloads are too large. My guess is the overloads kept tripping in the past, so they were up-sized (bad practice)

4) No, your overloads are what protect your cables under normal operating conditions. The breaker trip value (from what I've always seen) is higher than the cable ampacity.

5) Yes, this is legal. The breaker (provided it's an MCP) provides magnetic protection, while the overloads provide thermal protection. You are required to have both, which you have.

If I were to troubleshoot this I'd say your motor is tripping on inrush current. If there is a dial to adjust inrush current levels, I would make it higher. If it's at highest value and it still wont start then you'll need to upsize the circuit breaker.

 

[kwired]

True, it depends on the Hz... But, vendor mentioned on details that were given about the unit, that it was a 15HP motor. One assumes its 15HP, 60Hz... But anyways, there were more problems with the installation than the motor itself.

The nameplate you showed us seems to prove that wrong at least for what you presently have, unless maybe it is not the original motor for the machine? OEM would not purchase a third party 20 hp if 15 is all that is needed, that would cost more.