Motor OCPD choice. NEC Arts 240 - 430

[Andres Arias]

Good evening.

I have an inquire in regards on how to choose the OCPD for a motor, given that sometimes I see that it is chosen to the next higher standard and sometimes to the next lower standard OCPD in Art 240. See below the following question (3.1K):

As per Table 430.250 for this motor the FLC is 99A. Then in Table 430.52, the factor to apply to this FLC is 1.75, which turns out in 173.25A. Then we go to Art 240 to choose the OCPD, which as far I understand for motors, should be chosen to the next lower standard OCPD (as opposed when choosing an OCPD for wires which is the next higher, contingent upon the current is less or larger than 800 A), that is 150A. However in the solution provided by the textbook (Study guide for PE exam), the OCPD chosen is 175A. See below the solution of the book:



When dealing with motors, how to choose the OCPD in Art 240?

Thank you beforehand for your collaboration.

 

[david luchini]

Motor OCPD is selected per 430.52. 430.52 allows the next standard size up.

Art 240 doesn't come into play except for determining what a standard size is.

 

[TwoBlocked]

Andres, you may be thinking of the overload setting, which is normally part of the controller/contactor. If I remember right, that may not be set higher than the nameplate FLA, (with exceptions). The OCPD can be put to the next higher standard setting, based on the FLC tabulated in the NEC according to the HP and voltage. The OL for the controller cannot be set higher than the FLA, based on the name[plate.

Yep, kinda grandular, but there it is. There is a difference between a motor's "Full Load Current", and a motor's "Full Load Amps."

 

[Eddie702]

NEC is used to select OC protection. Motor nameplate is used to select OL setting. OL are usually selected at 115% of motor name plate as a minimum. Some motors cane be selected at 125% of name plate

 

[augie47]

Exception #1 to 430.52(C)
Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.

 

[don_resqcapt19]

Andres, you may be thinking of the overload setting, which is normally part of the controller/contactor. If I remember right, that may not be set higher than the nameplate FLA, (with exceptions). The OCPD can be put to the next higher standard setting, based on the FLC tabulated in the NEC according to the HP and voltage. The OL for the controller cannot be set higher than the FLA, based on the name[plate.

Yep, kinda grandular, but there it is. There is a difference between a motor's "Full Load Current", and a motor's "Full Load Amps."

While many instructors have taught that, the code only uses full load current, but uses two different full load currents. It uses the FLC from the tables in Article 430 for conductor and branch circuit short-circuit and ground-fault protection, and uses the FLC from the nameplate for overload protection.

When I teach, I teach code full load current and nameplate full load current.

 

[Andres Arias]

Thank you all for the answers, it looks like we have to size down to next standard OCPD when using Art 430.32a1 (continuous duty motors - separate overload device); and size up to the next standard OCPD when applying Table 430.52 and rule 430.52C1.

Notice this other example from NCEES PE exam question and its solution. In this case the table 430.52 is not applied, it´s only applied the service factor and correcting the FLC with 1.15 factor. Then Art 430.32a1 is used (OCPD chosen sizing down to the next standard as per 240)

 

[kwired]

With motors there is two types of protection.

Short circuit/ground fault protection. Usually is well above the motor rated current as it needs to be able to hold during motor starting, doesn't really hurt anything to round this up to next standard size as it is only intended to operate for short circuits and ground faults at high current levels but will operate in short time.

Motor overload protection. Is going to be selected to be usually at 115-125% motor rated current, but can be less when you encounter a motor with a 1.0 service factor. This is normally a slower reaction time to overcurrent than the short circuit/ground fault protection is, but is intended to actually take on similar heating characteristics as the motor itself since that it basically what it is protecting the motor from.

 

[TwoBlocked]

NEC is used to select OC protection. Motor nameplate is used to select OL setting. OL are usually selected at 115% of motor name plate as a minimum. Some motors cane be selected at 125% of name plate.

I should have said not to exceed the overload setting on the overload device. My understanding (I would have to find where I got it...) is that the setting on the OL device takes into account the 115% multiplier. So if the FLA on the nameplate says 10.0 amps, and you set the OL to 10.0 amps, it will trip at 11.5 amps.

 

[kwired]

Andres, you may be thinking of the overload setting, which is normally part of the controller/contactor. If I remember right, that may not be set higher than the nameplate FLA, (with exceptions). The OCPD can be put to the next higher standard setting, based on the FLC tabulated in the NEC according to the HP and voltage. The OL for the controller cannot be set higher than the FLA, based on the name[plate.

Yep, kinda grandular, but there it is. There is a difference between a motor's "Full Load Current", and a motor's "Full Load Amps."

Overload "setting" or thermal element selection charts already have 125% factored into them.

Should you need lesser value for a 1.0 SF motor you need to adjust your full load current before making selection.

 

[don_resqcapt19]

I should have said not to exceed the overload setting on the overload device. My understanding (I would have to find where I got it...) is that the setting on the OL device takes into account the 115% multiplier. So if the FLA on the nameplate says 10.0 amps, and you set the OL to 10.0 amps, it will trip at 11.5 amps.

Some work that way, and others you set to the value you want the overload to trip at. You have to read the instructions for the actual overload device you are using.

 

[Jraef]

Some work that way, and others you set to the value you want the overload to trip at. You have to read the instructions for the actual overload device you are using.

This.
I once had to help a team at my company develop a solid state OL relay system, and there was a lot of debate over this issue, referred to as the "pick-up point" of the overload curve, the point at which the relay STARTS the process of tripping. So I looked at all of the OL devices on the market at the time, coming to the conclusion that there was zero agreement among manufacturers on where that pick-up point should be and how to deal with differences in the field.

I CAN say however that VERY FEW (as in maybe two?) had to use the motor FLA and apply your own value to it. MOST of them already had a pick-up point value built in. For IEC OL relays, it was generally 115-117% across the board, probably due to some IEC regulation. For NEMA devices though, it was 100% (as mentioned above), but most often 115%, 120% even 125%, then you were given instructions on how to make adjustments to compensate for different conditions.

So the ONLY safe answer is "RTFM". This is actually very important because if you, as in that test example, automatically ADD a 1.15 multiplier, and you are using one of the brands that ALREADY used 1.25, then you end up with a pick-up point that is 115% of 125%, so 144% of the motor FLA, and the motor burns up! Or in the case of a motor withy a 1.15 Service Factor, where it says 125%, you can end up with the pick-up point being 157% of the motor FLA! Motor shops LOVE that people do this...

* Just in case you don't know, RTFM is short for "Read The Factory Manual" (although "Factory" can be interchanged for another word that starts with F...)

 

[don_resqcapt19]

I am old enough that most of the overloads I did, you went to table provided by the manufacturer and selected the part number of the physical "heater" element that you installed in the overload relay. Even there you had to make sure you were using the correct table for not only the type of motor, but the NEMA size of the starter.

 

[Jraef]

I am old enough that most of the overloads I did, you went to table provided by the manufacturer and selected the part number of the physical "heater" element that you installed in the overload relay. Even there you had to make sure you were using the correct table for not only the type of motor, but the NEMA size of the starter.

Right. The table selection process itself had those adjustment factors baked into them.