Let's talk about motors...

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celtic

celtic

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Let's talk about motors...
I have a few questions and as time goes by and time allows, I'll ask them.

1st Question:

When do you use the nameplate on a motor and when don't you, ie you would then use the tables in 430.

I'm looking at a question that asks:
What is the minimum ampacity of the branch-circuit conductors for a 25-HP, 460V, 3? squirrel-cage motor with a nameplate FLC rating of 32.5A?

The answer is given as 42.5A

How did they arrive at 42.5A and not 40.625A (32.5 x 125%)?
 
motors

motors

The only time that comes to mind that you use the nameplate current is for determining overload protection (430.32)
 
celtic said:
How did they arrive at 42.5A and not 40.625A (32.5 x 125%)?
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Using FLC from NEC table 430.150, its (34 x 1.25) = 42.5A Nameplate is for the heaters.
 
Bea said:
Refer to 430.6(A)(1)
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How does 430.6(A)(1) apply?

I've read it over a few times...still don't get it.
The book also says to use 430.6(A)(1) ...and ultimately T430.250
 
430.6(A)(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.
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In your example you would use the HP rating on the nameplate to find the current value listed in 430.250. As Ramsy stated, the value 34 amps would be multiplied by 125% to find the ampacity of the conductors. So although the nameplate does indicate that motor has a FLC of 32.5 amps you still are required to use the value listed in the table when sizing the motor circuit conductors.
 
infinity said:
In your example you would use the HP rating on the nameplate to find the current value listed in 430.250.

So although the nameplate does indicate that motor has a FLC of 32.5 amps you still are required to use the value listed in the table when sizing the motor circuit conductors.
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Why?

Does a squirrel-cage motor meet this?

Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors,the values given in ...T450.250
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The motor nameplate did have V and HP:
Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in......T450.250
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Not trying to a PITA, but I ain't getting it.
 
celtic said:
Why?

The motor nameplate did have V and HP:

Not trying to a PITA, but I ain't getting it.
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Not trying to be smart, but, as to why, I can only say because the Code says so.

Trying again. For the vast majority of the motors you encounter (exceptions noted by Stew and Infinity),. the NEC states that you should use tables 430.247-430.250 for your calculations in determining everything except overcurrent protection. I can only assume that this is done as those numbers will suffice in the event the motor is changed out to one with a slighly different load characteristic The nameplate current is used for overcurrent protection as it is directed more specifically to your exact motor.
The other numbers provide SC-GF protection in a generic sense.
HP might be the only thing know at the time the wiring is installed, so following the Tables will provide a safe installation. Once the exact motor is installed, the overload elements can be tailored to that specific motor.

That help ?
 
augie47 said:
Not trying to be smart, but,
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Ok, so we can all agree that no one is a PITA or a smart-donkey ;)

augie47 said:
as to why, I can only say because the Code says so.
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Can you show me WHERE the code says so?


I see this:
430.6(A)(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors....
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In the OP, there is no mention of speed or torque...only that it is a squirrel cage.
Does a squirrel cage meet have "low speeds (less than 1200 RPM) or high torques, and for multispeed motors,"?
 
celtic said:
In the OP, there is no mention of speed or torque...only that it is a squirrel cage.
Does a squirrel cage meet have "low speeds (less than 1200 RPM) or high torques, and for multi speed motors,"?
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Your looking at it backward.

If it was built as a low speed motor

If it was a built as a high torque motor

If it was built as a multi speed motor

You would use the name plate current.

It is was not one of those motors ("Other than") you must use the Table current values.
 
iwire said:
Your looking at it backward.
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What else is new? :rolleyes:


iwire said:
If it was built as a low speed motor
If it was a built as a high torque motor
If it was built as a multi speed motor
You would use the name plate current.

It is was not one of those motors ("Other than") you must use the Table current values.
Click to expand...

So how does one know the squrriel-cage motor is built as you indicate?
 
celtic said:
So how does one know the squirrel-cage motor is built as you indicate?
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The run of the mill squirrel cage motor is not 'high torque' it is not 'multi speed' and not is not 'low speed'.

I don't know what RPM is considered low speed but IMO it is a safe bet it's under 1000.

If it was multi speed you would know as it would have multiple conductors running to it.

I suspect if it was high torque you would see that on the the tag.

So the typical fan and pump motors we run into must be sized per the table.
 
So the "keyword" to correctly answer this question lies in knowing that a squirrel-cage motor falls under 450.6(A)(1) ...and therfore T450.250
 
Pierre C Belarge said:
Celtic
Look just below the title of the table and you will see the inclusion of Squirrel Cage included in that part of the table, as well as others.
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That's when I came to the "keyword" conclusion.


I'm going to have a lot of these questions in the near future as motors are NOT really my strong suit.

Thanks for the help folks!
 
What if the squirrel cage motor has a nameplate current rating of over the listing value in the code book (I cant say I have ever seen that)? then what?
 
NC_Electrician said:
I was always taught to go by the nameplate, always, then size/protect according to that.

Just my $0.02 worth:D
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Size what?
CB..fuse..conductors???
 
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