Motor FLA is NOT to be Used to Size Motor Conductors !

Because of 3 or 4 posts with errors on this subject, this US National Electrical Code admonition needs repeating:

NEC Handbook Article 430.22 note:
"Motor nameplate full-load current is not to be used to size branch-circuit conductors."

That is plain language in a NEC note that was written to kill the very belief that many now hold. Motor conductors must be no SMALLER than 125% of motor FLA, but in many cases they will have to be much LARGER than 125% of FLA to meet other requirements in the National Electrical Code (to carry the current passed by the motor branch-circuit device).

...the maximum Inverse time breaker you can use with a 124 FLA motor and #1/0 wire is 310A.

Click to expand...

You cannot legally (by the NEC) use a 300 Amp inverse-time breaker to supply a 1/0 wire. Instead you would have to raise your wire size to 350 kcm.

Oh, boy! There are a lot of mis-sized motor conductors out there. They will work okay, everybody fat, dumb, and happy - until someone cuts the wires with a forklift or crane or some other way. Then those too-small #1/0 150-Amp-rated wires will smoke, flame up, burn, and set the building on fire long before the 300 Amp breaker even thinks of tripping. I hope there are good smoke detectors.

A timely reminder.

Its not just motors where the malpractice of sizing wires by the load happens. It happens all the time at PLCs and other instruments. Wires are to be sized according to the circuit protection. If you have a 15 amp breaker you cannot use #16 or #18 wires ANYWHERE in the circuit, including on your PLC outputs. It doesn't matter if the PLC outputs are only rated for 2A ea, they are not a circuit protection device and you cannot downsize wire there. If your circuit protection is 15A you must use #14 or larger wire on your PLC outputs. Only if you downsize the branch circuit protection for your PLC module may you use smaller wire. The sole exception to this is a tap rule that applies only if you have individually fused your PLC outputs, meaning you can use the smaller wire only if it goes straight from the output terminal to a nearby fuse in the same control panel.

Tim,

Good advice as usual. I think using the too-small wires downstream of a breaker is a common mistake in PLC control panels.

I can't believe how common is the motor contductor mis-sizing. I don't know where the mis-information is coming from. There must be an Ugly's book or some other guide out there that is misleading a bunch of people.

You mean to say I can't carry all 16 of my unfused 24V inputs on a 18C 22AWG cable with 300V insulation in the same pipe as my 600V motor leads???

Please take the time to look at section 430.72 (Motor Control Circuits: Overcurrent protection). This is the section that deals with sizing overcurrent protection of motor branch circuits.

I'm starting to think that you may be off your meds.

Lancie1 said:

Because of 3 or 4 posts with errors on this subject, this US National Electrical Code admonition needs repeating:

NEC Handbook Article 430.22 note:
"Motor nameplate full-load current is not to be used to size branch-circuit conductors."

That is plain language in a NEC note that was written to kill the very belief that many now hold. Motor conductors must be no SMALLER than 125% of motor FLA, but in many cases they will have to be much LARGER than 125% of FLA to meet other requirements in the National Electrical Code (to carry the current passed by the motor branch-circuit device).



You cannot legally (by the NEC) use a 300 Amp inverse-time breaker to supply a 1/0 wire. Instead you would have to raise your wire size to 350 kcm.

Oh, boy! There are a lot of mis-sized motor conductors out there. They will work okay, everybody fat, dumb, and happy - until someone cuts the wires with a forklift or crane or some other way. Then those too-small #1/0 150-Amp-rated wires will smoke, flame up, burn, and set the building on fire long before the 300 Amp breaker even thinks of tripping. I hope there are good smoke detectors.

Click to expand...

Here is the math. 124FLA * 1.25 = 155A

Using table 310.15(b)(16) (NEC 2011 handbook) and 90 degree C wire the table calls out a minimum wire size of 1/0 (170A max). So 1/0 90 degree C wire exceeds code. (In free air you could go as low #3 AWG)

Now to verify that he wire you have chosen does not have an issue with the motor branch short circuit protection you need to flip over to artivle 430.72 of the NEC in particular look at table 430.72(B) Note 3 This dictates the the branch circuit protection can be no larger than 300% than the curren specified in table 310.15(b)(16) for 60 degree conductors.

Now flipping back to table 310.15(b)(16) we find the 60 degree ampacity of 1/0 wire is 125.

Now lets pull out the calculator and find 300% of 125 which is 375 Amps. So the larges motor circuit branch short circuit protection device can be 375A

Therefore using 1/0 wire and a 300A breaker for a motor with a nameplate of 124FLA is well within code.


Lancie,
You keep escallating this thing when you do not have the facts behind you. I am starting to see who you really are and I do not like who I am seeing.

Lancie1 said:

Because of 3 or 4 posts with errors on this subject, this US National Electrical Code admonition needs repeating:

NEC Handbook Article 430.22 note:
"Motor nameplate full-load current is not to be used to size branch-circuit conductors."

That is plain language in a NEC note that was written to kill the very belief that many now hold. Motor conductors must be no SMALLER than 125% of motor FLA, but in many cases they will have to be much LARGER than 125% of FLA to meet other requirements in the National Electrical Code (to carry the current passed by the motor branch-circuit device).



You cannot legally (by the NEC) use a 300 Amp inverse-time breaker to supply a 1/0 wire. Instead you would have to raise your wire size to 350 kcm.

Oh, boy! There are a lot of mis-sized motor conductors out there. They will work okay, everybody fat, dumb, and happy - until someone cuts the wires with a forklift or crane or some other way. Then those too-small #1/0 150-Amp-rated wires will smoke, flame up, burn, and set the building on fire long before the 300 Amp breaker even thinks of tripping. I hope there are good smoke detectors.

Click to expand...

The Temperature ratings of the conductor connections have to be applied also, which sets the minimum wire size regardless of insulation type. ( 110.14.C.1.b )

With a load of 155A the breaker is larger than 100A so the 75 deg C column is used to find the minimum wire size, which will be 2/0 ( 175A ), so 2/0 THHN ampacity is 195A minimum size.

430.52.C.1.c OCPD 250% -300% of 124 FLA for starting

300% = 372A 400 MCM THHN

minimum = 2/0 THHN w/200A CB
maximum = 300MCM THHN w/400A CB

that doesn't take ambient temperature factors,
Current-Carrying-Conductors factor or
Available-Short-Circuit-Current factor into account.

Let me Clarify /prove my position in regards to proper conductor sizing.

"Conductors (motor or otherwise) have to be able to carry the current passed by the next upstream short-circuit device. That is a basic code rule and supercedes your erroneous belief in a "125% of FLA" rule that simply does not exist."

The NEC clearly does clearly specify 125% of FLA for the sizing of motor conductors.

Article 430 of the NEC cover motor circuits. (read section 430.1 scope). Conductor sizing is addressed in 430.6 and in part II (430.21 to 430.29)

Article 430.21 General state "The provisions of Articles 250, 300, and 310 shall NOT apply to conductors that form an integral part of equipment, such as motors, motor controllers, motor control centers, or other factory-assembled control equipment"

Article 430.22 Single Motor states "Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current rating, as determined by 430.6(A)(1)"

Article 430.6 states "The size of conductors supplying equipment covered by Article 430 shall be selected from allowable ampacity tables in accordance with 310.15(b) or calculated in accordance with 310.15(c)"

"You cannot legally (by the NEC) use a 300 Amp (no 310 available) inverse-time breaker to supply a 1/0 wire. Instead you would have to raise your wire size to 350 kcm."

The NEC clearly does allow the use of a 300 Amp inverse time breaker to supply a 1/0 wire, in a motor branch circuit. (it must be a motor branch circuit falling under the scope of article 430

Article 430 Part 4 covers Motor branch-circuit short-circuit & ground-fault protection

430.51 General states Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor apparatus, and the motor against over-current due to short circuits or ground faults. These rules add to or amend the provisions of Article 240.

430.52 states that you use table 430.52 to size the motor branch-circuit short circuit protection

Table 430.52 shows that the maximum breaker size for an Inverse Time Breaker is 250%.
(note: Exception No 1 allows you to go to the next larger size breaker if there is not a standard size breaker that is 250%)

Articles 210 and/or 240 do not over-ride section 430 (in motor branch circuits)

Article 210.1 Scope states "This article cover branch circuits EXCEPT for branch circuits that supply only motor loads, which are covered in Article 430

Article 240.3 has a table 240.3 of equipment type and NEC Articles and tell you to use those articles for sizing conductor protection for those equipment types . The table calls out Article 430 for "Motors, motor circuits, & controllers"

"Oh, boy! There are a lot of mis-sized motor conductors out there. They will work okay, everybody fat, dumb, and happy - until someone cuts the wires with a forklift or crane or some other way. Then those too-small #1/0 wires will smoke, burn, flame up, and set the building on fire long before the 300 Amp breaker even thinks of tripping. I hope there are good smoke detectors"

With the 1/0 wires in this example the breaker will trip (or the O/L will trip if it is a low A fault) before the wires melt or vaporize (assuming the Short circuit withstand of the breaker and O/L is greater than the available short circuit current at that breaker, but you may have an even bigger problem with 350MCM in that case)

"NEC Handbook Article 430.22 note:"Motor nameplate full-load current is not to be used to size branch-circuit conductors."
You left out the sentence just before this one in this paragraph which completely changes the meaning. "The ampacity of the motor branch-circuit conductors is based on the full-load current rating provided in Tables 430.248 through 430.250."

Those tables state that you need to use 124A as the FLA. 124 is what we have been using in our example as FLA

"That is plain language in a NEC note that was written to kill the very belief that many now hold. Motor conductors must be no SMALLER than 125% of motor FLA, but in many cases they will have to be much LARGER than 125% of FLA to meet other requirements in the National Electrical Code (to carry the current passed by the motor branch-circuit device).”

This language is not in the NEC. The NEC states the opposite! However, you often need to increase conductor sizes based several reasons such as ambient temperature, number of conductors in a conduit or raceway, under ground feeds, conduits/conductors exposed to sunlight etc. (see section 310.15 for the de-rates). But the size of the breaker is not one of them when you are dealing with Article 430 motor branch circuits.

Over-sizing conductors is not a bad thing it is just not very practical. In my last big job we spent 1.2MUSD on copper (cable & wire)(the cost of wire & cable boils down to copper cost on big mill orders). 350MCM contains 3.3 times more copper than 1/0. If we oversized everything by 330% our cost for copper would have been almost 4.0 MUSD. You would never win any contract if your cost are 3.3 times higher than your competitors.

Okay, I have to be convinced now, however illogical it appears!

I am sorry about my mistakes. I have a failing memory, and often go back to the days before I studied Electrical Engineering, when my thinking was based on common sense and not NEC code. Short-term memory goes first, and the older stuff lingers around. Unfortunately it is not a case of "getting off my meds". There are yet no medicines for restoring memory loss. There is the Aricept drug, which just makes you feel better about losing your memory.

I remember now, but only for a while. This is why I quit work, before I did damage to anyone. I guess I need to stop posting on here too, or at least limit it to harmlesss stuff.

I do remember The Protection Gap. That is the gap in current protection when you follow the Article 430 rules. The 300 Amp breaker protects against short circuits, and that is about all, because it is too big to handle overloads below 300 Amps. The 155 Amp motor overloads handles overload currents above 155 Amps. But there is no protection for a ground fault current of 155 to 300 Amps on the wires between the breaker and the motor overload. But that is okay, it is allowed by the code.

Excellent interpretation of the NEC code bimini3. That should convince anyone in doubt.

another important thing to note, is in a short circuit situation, it is not the 300A that is the limit, but can be in the kilo-amps, and all breakers have a seperate current rating for short circuits and then their regular load trip rating.
I was preparing a CEC version of what bimini3 did, but i think that he did a good job.

Also Lancie1. everyone makes mistakes we are human. i Still have a high respect for you with programming, and i intend to learn more from you in the future.