Motor feeder sizing and load calcs

[mic]

I am trying to interpret Table 430-22(b) and have several questions.

1. Does this table apply to sizing a mixed service (motors, heaters, lighting) and the feeder?

2. In the table is the "Minute Rating" the manufacturers rating on the motor or is it the maximum operating time of the motor in the application?

3. If I have a continuous rated motor in an application that is limited to less than 15 minutes operating time do I use 125% of the full load current (as per 430-22(a)) or 140% (as per the table)?

4. This is the silly question (assuming the others are not), if the answer to 3. above is 140% then why is the percetage higher than for continuous duty? Wouldn't there be more heating in the continuous duty situation?

Thanks in advance
C

One additional question: if I have a 5 Hp motor that only runs for a max of 3 seconds at at time and approx 5 minutes rest afterward, do I need to include this motor in the load calc? (Please explain why or why not)

 

[rbalex]

Re: Motor feeder sizing and load calcs

I am trying to interpret Table 430-22(b) and have several questions.
(I assume you are referring to the 1999 NEC)

1. Does this table apply to sizing a mixed service (motors, heaters, lighting) and the feeder?
Indirectly. Read in context, (Part B, starting with Section 430.21) Table 430.22(b) applies only to circuit conductors for a single motor. However, also see 430.24 and its Exception 1

2. In the table is the "Minute Rating" the manufacturers rating on the motor or is it the maximum operating time of the motor in the application?
It is a UL rating based on NEMA ?definite purpose? motors. Such motors are generally ?recognized components,? designed for use with specific equipment.

3. If I have a continuous rated motor in an application that is limited to less than 15 minutes operating time do I use 125% of the full load current (as per 430-22(a)) or 140% (as per the table)?
Read the note at the end of the Table. Unless the operation is clearly one of the other duty cycles (Short-time, intermittent, periodic or varying ? see their definitions in Art 100, [Edit Add] under "Duty.") the service is assumed to be continuous or the AHJ must grant special permission per the main text of Section 430.22.

4. This is the silly question (assuming the others are not), if the answer to 3. above is 140% then why is the percetage higher than for continuous duty? Wouldn't there be more heating in the continuous duty situation?
For other than ?short time? duty, the other duty cycles assume frequent motor starts which imposes greater thermal stress on the conductors.

Thanks in advance
C

One additional question: if I have a 5 Hp motor that only runs for a max of 3 seconds at at time and approx 5 minutes rest afterward, do I need to include this motor in the load calc? (Please explain why or why not)
Yes. See the answers to 1& 4.

[ May 09, 2005, 12:05 PM: Message edited by: rbalex ]

 

[mc5w]

Re: Motor feeder sizing and load calcs

You mean table 430.22(E). The minute rating of a motor is the manufacturer's rating, not how long it is actually used. Obviously, a 30 minute rated motor cannot be used for nore than 30 minutes at full load. A continuous duty motor can also be deliberately overloaded for a limited period of time.

Whether a continuous duty rated motor has to be wired with 140% or 200% of nameplate conductors depends more on how much it is overloaded when running. I would also make the resistance heater rating of the motor controller and disconnect switch 140% or 200% or motor nameplate current - repetitive overloads have a tendency to spot weld contacts turning a motor controller or circuit breaker into something that isn't. Examples of loads that require 200% conductors are rubbish compactors, anything that is subject to jamming, and the hoist chain motor for a roller coaster. These loads have a tendency to load the motor at 200% of the horsepower rating.

Some air compressors need conductors that are rated at 140% of motor nameplate current because the motor is deliberately overloaded during a part of the operating cycle. Do not expect the motor to last very long in this application if you use the compressor a lot.

How long a motor will last in this kind of application depends on how long it is overloaded versus the rest period.

Also, gearboxes need to be oversized for this application lest you have lubrication failure ( metal to metal contact ) during the torque peaks which will grind up an inadequately sized gearbox.

 

[mc5w]

Re: Motor feeder sizing and load calcs

I did not really state that the definitions in Article 100 for continuous duty, intermittent duty, periodic duty, and varying duty do not include applications where a motor is deliberately overloaded on a periodic or occasional basis.