No nameplate on motor

[rodoffshore]

What is the correct way to size motor overloads for a motor that has no nameplate? I understand that overloads are to be sized by the motor nameplate FLA, but cannot find anything in code regarding if nameplate is missing. My assumption would be to use the NEC Table FLA value and multiply by 115%. Thoughts?

 

[jim dungar]

I prefer to follow the directions of the motor overload manufacturer.
Some want you to add a % to the FLA, others have that factor built into their selection tables. Most manufacturers have a table of estimated FLA for situations like yours.

 

[Jraef]

How can you use anything from any chart if you have no idea what the motor is?
About the only thing can think of is to take it to a motor shop that can put it on a dynamometer and test it under a varying load to see what the FLC may be.

 

[rodoffshore]

Let me clarify. The motor is 25 hp rated for 460V. This we know. The problem is even though we know the horsepower rating, to size the overloads per NEC you are to use the nameplate FLA. Question is you know the horsepower rating but have no nameplate FLA. What is the correct procedure. Sorry if I confused anybody.

 

[jumper]

Where did the nameplate go? How will you ever comply with 430.7?

 

[stew]

can you find a brand name anywhere on the unit. sometimes a logo on the conduit box. possibly a brand reference on say a tag inside the conduit box? If you can post a photo I may be able to identify it as to brand.

 

[rodoffshore]

I am not looking to offend anyone for this is a great forum with lots of knowledgeable people who are willing to help others with questions and I appreciate that.
I understand that you want to know where the motor nameplate is. No one knows. They supposedly have looked for it. I have been given the task of sizing the overloads for the motor without a motor nameplate. I am looking for some guidance to comply with NEC. Again thanks.

 

[Dennis Alwon]

Use Table 430.250 I assume - 3 phase.

25HP is rated 34 amps. That is what I would use. In terms of the overload you may need to call the manufacturer.

 

[Jraef]

Use Table 430.250 I assume - 3 phase.

25HP is rated 34 amps. That is what I would use. In terms of the overload you may need to call the manufacturer.

That's what I would do too. Pretty safe "guesstimate" but recognize that this is all it can be at this point. Worst case: you have guesstimated too high and you smoke the motor when it overloads. But if it never overloads, you may never know.

 

[jim dungar]

When motor full current is not known.
Square D says to use FLA=30.8A
Cutler Hammer says 30.4A for 3600RPM and 32.4 for 1800RPM.

Neither one recommends using an adjustment factor.

 

[Besoeker]

Use Table 430.250 I assume - 3 phase.

25HP is rated 34 amps. That is what I would use. In terms of the overload you may need to call the manufacturer.

If there is no nameplate, would you know the who the manufacturer is though?
Here in UK, motors of the same rating look pretty similar. In fact, a bit of "badge engineering" is done, particularly at the low power, high volume end.

 

[stew]

I would use 34 amps as well. The chances of "smoking " this motor using that info is probably not totally out of the realm of possibility but unlikely. If this motor has any kind of service factor rating beyond 1.0 the likelyhood of an overlaod and burnout are even more unlikely. The other thing to do is after you get this install done,put an amprobe on the unit and see what the motor does under full load conditions. At that time also check for over heating.

 

[rodoffshore]

Thanks for everyones input. I think until motor mfr is known will just use the NEC Table FLA value for the overload rating.

 

[weressl]

I am not looking to offend anyone for this is a great forum with lots of knowledgeable people who are willing to help others with questions and I appreciate that.
I understand that you want to know where the motor nameplate is. No one knows. They supposedly have looked for it. I have been given the task of sizing the overloads for the motor without a motor nameplate. I am looking for some guidance to comply with NEC. Again thanks.

Eggzactly! Having a nameplate is also a compliance issue. What you are saying in effect is that you are looking for SELECTIVE compliance with the NEC and that's OK as long as you are aware that this is what you are doing. It's not neccessarily that people criticize what you're doing but trying to help you to get where you should be and be aware if you are choose to do so otherwise.

 

[don_resqcapt19]

The use of the table will most likely result in an oversized overload element. Most motors have a FLA that is below the table value.

There is no code compliant way to use this motor.

I think I would run the motor at its normal load, measure the current and use that current to select the overloads. If it trips, and the motor is not too hot (subjective, unless you have a way of measuring the winding temperature) I would increase the size of the overloads a size.

 

[Besoeker]

The use of the table will most likely result in an oversized overload element. Most motors have a FLA that is below the table value.

I'm sure you're right. Even taking a fairly poor power factor and quite low efficiency, I get 31A as the calculated full load current.

 

[jim dungar]

The NEC specifically points out that its FLA values are to be used for sizing conductors and not for sizing motor running protection. I believe it could be considered irresponsible to use these values in this manner and then even worse to add a fudge factor of 15%, as many do.

The manufacturers of motor protective equipment provide tables for estimating FLA based on motor HP, why is there such a reluctance to use them?

 

[rodoffshore]

Problem solved!!! They are going to replace the motor with a new motor. Again thanks for everyones input. I agreed with everyone's comments, as they were valid and informative.

 

[don_resqcapt19]

...
The manufacturers of motor protective equipment provide tables for estimating FLA based on motor HP, why is there such a reluctance to use them?

Jim,
What code section would permit the use of those estimated values in selecting the overload protective device?