Motor OCPD Question

6845 Views 15 Replies 8 Participants Last post by JRaef, May 14, 2014

Huntxtrm

Discussion Starter · May 6, 2014

I keep running into people, who don't understand the sizing of the OCPD and Circuit conductors for motors. I explain that motor conductors and OCPD size differently than general wiring. They as Why? I say because NEC says so(that's the only answer I know). I am thinking that on motors the breaker is purely for short circuit/ground fault protection, and the overloads handle the rest. and on a general circuit the breaker does both. The Point is often made to me, "so if that there #6 wire shorts out, that there 90 amp breaker is gonna trip? Or is that wire gonna fry first?" I know if wont, but I want to be able to explain it better. Is it actually the KAIC rating of the breaker that protects against short circuit? and the Amp rating is the Inverse time portion? Thanks for any insight.:thumbup:

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99cents

· #2 · May 6, 2014

Wow. Back up. First of all, the IC rating is simply the maximum current the device can interrupt without damage to it's envelop or casing.

Generally speaking, an OCPD is sized to protect the conductor and nothing else. Certainly there is a mathematical relationship between the load, the conductor size and the amp size of the OCPD but think of a purely resistive load like a heater. The heater may fail and cause an overcurrent, resulting in the opening of the OCPD. The heater may be junk after the event but the branch circuit conductors are still intact (and, most importantly, the insulation on the conductors).

It gets a little cloudier with loads that have inrush current. Motors should have their own overload protection, leaving the OCPD again to protect the branch circuit conductors. Inrush does not normally last long enough to degrade conductor insulation but the OCPD still needs to protect against extended inrush times or stalled rotor current that may get past the overloads.

Simple enough - the OCPD protects the branch circuit conductors and the overloads protect the motor. Kinda sorta.

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Huntxtrm

Discussion Starter · #3 · May 6, 2014

99cents said:
Wow. Back up. First of all, the IC rating is simply the maximum current the device can interrupt without damage to it's envelop or casing.

Generally speaking, an OCPD is sized to protect the conductor and nothing else. Certainly there is a mathematical relationship between the load, the conductor size and the amp size of the OCPD but think of a purely resistive load like a heater. The heater may fail and cause an overcurrent, resulting in the opening of the OCPD. The heater may be junk after the event but the branch circuit conductors are still intact (and, most importantly, the insulation on the conductors).

It gets a little cloudier with loads that have inrush current. Motors should have their own overload protection, leaving the OCPD again to protect the branch circuit conductors. Inrush does not normally last long enough to degrade conductor insulation but the OCPD still needs to protect against extended inrush times or stalled rotor current that may get past the overloads.

Simple enough - the OCPD protects the branch circuit conductors and the overloads protect the motor. Kinda sorta.

But, on a motor circuit the the breaker is sized to the FLA from the NEC and the Conductors are sized based on the FLA from the NEC. The OCPD is not sized to protect the conductor.

99cents

· #4 · May 6, 2014

Your overload is also sized according to the FLA of the motor.

Huntxtrm

Discussion Starter · #5 · May 6, 2014

99cents said:
Your overload is also sized according to the FLA of the motor.

Yes, I understand that. I size all of this stuff all the time. I just don't know how to tell someone why it is sized so. I know where it comes from in the NEC. I don't use a program to size it, I actually do the calculations. I had one GC (Master Electrician)the other day arguing the thought that I was supposed to size my wire to my OCPD on my motors. He was getting sore at me, cause he thought I was over breakering my wire. He was so doubtful about it, he went to the city, and luckily the inspector was very knowledgeable, and backed me up.

Let me start from scratch, suppose I am him telling you, you cant do that! Your gonna fry my conductors! You have to size your wire to you OCPD! Not 125% of 430.250! Why on earth would you size your breaker at 250% and your wire at 125%? Your gonna burn something up! Sort of what came out of his pie hole! LOL

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1.21gigawatts

· #6 · May 7, 2014

You will have this battle with many people both in the trade and in the engineering field. The first thing to consider is that the NEC sets minimum's and maximums on devices, conductors etc. There are several situations where the conductor will be smaller then the ocpd. 240.4. A thru G covers these.

The conductor is sized at 125%. Of the motor FLA. Motors are considered to be continious loads. It is the overloads that protect the motor and the conductors from exceeding ampacity and overloads. This is why reason we are allowed to use a larger OCPD if applicable. We are not relying on the breaker to protect our wire in these scenarios.

The reason we set the OCPD higher on a motor is so it will allow the motor or motors to start and not trip the breaker or blow the fuse until it goes to full rpm and current evens out. The second part to this is we still need to protect from ground faults and shorts. The breaker is to protect and open under both conditions and do so quick enough that the conductor is not going to see any damage.The Overloads will not open fast enough during a ground fault/short. And yes, you need to size the breaker AIC for the maximum available fault current at its supply.

It is good practice to match the wire to the OCPD. This is helpfull with voltage drop and if a larger motor or equipment may be installed later. But it is certainly not a requirement by the NEC. Job specs may be another story. Tell your buddy to go read the NEC.

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erics37

· #7 · May 7, 2014

On most circuit applications, the conductors are protected for their ampacity based on 240.4 and 310.15.

For some specific applications, including motors, you are allowed to deviate from that norm because of 240.4(G)

Basically with the motor applications you are taking two of the main overcurrent protection activities and dividing them up into two separate devices. The breaker or fuse or MCP or whatever protects against short circuits and ground faults, which can be of pretty significant magnitude. These devices are typically oversized to allow the motor to start without tripping from the inrush, but should still respond very rapidly in the event of a fault.

The lower-magnitude, more sustained overload conditions are protected by the overload device.

So you're still protecting the conductors from faults and overloads, just with two devices instead of one. The conductors themselves are just sized to the load like usual.

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JRaef

· #8 · May 7, 2014

erics37 said:
On most circuit applications, the conductors are protected for their ampacity based on 240.4 and 310.15.

For some specific applications, including motors, you are allowed to deviate from that norm because of 240.4(G)

Basically with the motor applications you are taking two of the main overcurrent protection activities and dividing them up into two separate devices. The breaker or fuse or MCP or whatever protects against short circuits and ground faults, which can be of pretty significant magnitude. These devices are typically oversized to allow the motor to start without tripping from the inrush, but should still respond very rapidly in the event of a fault.

The lower-magnitude, more sustained overload conditions are protected by the overload device.

So you're still protecting the conductors from faults and overloads, just with two devices instead of one. The conductors themselves are just sized to the load like usual.

Nicely put.

Huntxtrm,
If you ever get a chance to look inside of a breaker (and know what you are looking at) you see a magnetic coil and a bimetal thermal device. The mag coil only acts on rapid rises in current, like a line-to-line or line-to-ground short, but acts almost instantly. The bimetal thermal device only reacts slowly to long term increases in current, called an over load, in the circuit.

A motor starter that uses an "MCP" mag-only breaker plus a bimetal overload relay is doing exactly the same thing, it's just that the two devices are separated. Substitute the breaker for a fuse, the fuse is still there for the same purpose. Substitute the bimetal OLR for a melting alloy or solid state type, they still provide the same function. The REASONS they are separated are because of those differences in motor circuits. The OL portion has to protect the MOTOR windings, which are typically much thinner conductors than you can use in the field, so there is less forgiveness in the protection scheme meaning that the trip point must be much more closely calculated, based on what is called the "thermal damage curve" of the motor, not the conductors, not the conductors feeding it. But because we size those conductors generously because of motor starting current, that OL relay will actually protect them better than the thermal element in a circuit breaker.

So "if that there #6 wire shorts out, that there 90A breaker is gonna trip?". Yes, it is, on short circuit (magnetic). But if that there #6 wire tries to carry 85 amps for too long, but NOT based on a short circuit, the breaker will not trip on magnetic, but SOMETHING had better trip on overload! That would be the overload relay.

The other part of your question is something i get frustrated with quite a bit with other EEs believe it or not. Will it protect that #6 wire from "frying"? Well, that depends on WHERE the short happens. If the short happens somewhere else other than on that wire, hopefully yes. But if the short happens ON that wire, NO! This is the part that people have a hard time with, but you have to think that through; the wire has ALREADY shorted out, what's left to protect? The answer is, everything ELSE on that circuit. If that wire shorts, the damage is done, now what you are doing is preventing collateral damage, like a fire.

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erics37

· #9 · May 8, 2014

Get that thar OCPD fixed up right quick

Nuzzie

· #10 · May 8, 2014

Don't you guys have OCPD's with different trip curves for motors?

JRaef

· #11 · May 8, 2014

Nuzzie said:
Don't you guys have OCPD's with different trip curves for motors?

We have everything. We have what we developed here in North America, then we got what everyone else in the world was using because most of our manufacturers were bought up by large European companies like Siemens, Schneider, ABB and Eaton, who imported the IEC products.

But here, the installation code rules apply differently. IEC style Motor Protection Switches, for example, could only be UL listed as "manual motor starters" for a long time, which meant they still had to have ANOTHER OCPD ahead of them, which usually made it pointless to use them. It took years for North American authorities to allow them to be used by themselves. They are now, but there are other restrictions to their use so most electricians here are unaware of how to use them, or not willing to go through all those hoops and make a slight mistake, drawing a "red tag" from the AHJ on inspection. Worse actually is that not all inspectors are up on them, so they reject them out of hand, which is a PITA for us.

The same holds true for what I think you are referring to, MCBs (Miniature Circuit Breakers) that have specific trip curves for motors. Again, those are available, but for years they were NOT listed for what we call "branch circuit protection", meaning they too had to have ANOTHER circuit breaker listed under UL489, or fuse that has a BCP designation, ahead of them anyway. That changed only a few years ago when mfrs released new versions of the IEC style MCBs specifically for North America that have UL489 (or the equivalent CSA) listing. So yes, we have them, but they are mostly just used by OEMs. When we go to a supply house, they don't have them in stock.

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erics37

· #12 · May 10, 2014

JRaef said:
We have everything. We have what we developed here in North America, then we got what everyone else in the world was using because most of our manufacturers were bought up by large European companies like Siemens, Schneider, ABB and Eaton, who imported the IEC products.

But here, the installation code rules apply differently. IEC style Motor Protection Switches, for example, could only be UL listed as "manual motor starters" for a long time, which meant they still had to have ANOTHER OCPD ahead of them, which usually made it pointless to use them. It took years for North American authorities to allow them to be used by themselves. They are now, but there are other restrictions to their use so most electricians here are unaware of how to use them, or not willing to go through all those hoops and make a slight mistake, drawing a "red tag" from the AHJ on inspection. Worse actually is that not all inspectors are up on them, so they reject them out of hand, which is a PITA for us.

The same holds true for what I think you are referring to, MCBs (Miniature Circuit Breakers) that have specific trip curves for motors. Again, those are available, but for years they were NOT listed for what we call "branch circuit protection", meaning they too had to have ANOTHER circuit breaker listed under UL489, or fuse that has a BCP designation, ahead of them anyway. That changed only a few years ago when mfrs released new versions of the IEC style MCBs specifically for North America that have UL489 (or the equivalent CSA) listing. So yes, we have them, but they are mostly just used by OEMs. When we go to a supply house, they don't have them in stock.

I think if Eaton and Siemens merged it would be the best corporation name ever. Eaton-Siemens :laughing::laughing:

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ponyboy

· #13 · May 10, 2014

erics37 said:
I think if Eaton and Siemens merged it would be the best corporation name ever. Eaton-Siemens :laughing::laughing:

Holy crap you have no idea how many times I've thought that same thing

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JRaef

· #14 · May 12, 2014

I put the Eaton-Siemens joke into the form of an email at my office as if it was an industry announcement. Big mistake. I have been scheduled for a webex with HR later this week, I imagine I must now take additional "inclusion" training, but I'm hoping that's all it is.

<slaps forehead> What the hell was I thinking...:bangin:

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ScooterMcGavin

· #15 · May 13, 2014

JRaef said:
I put the Eaton-Siemens joke into the form of an email at my office as if it was an industry announcement. Big mistake. I have been scheduled for a webex with HR later this week, I imagine I must now take additional "inclusion" training, but I'm hoping that's all it is.

<slaps forehead> What the hell was I thinking...:bangin:

Well if you don't show up for our VFD class at least I'll know why.

JRaef

· #16 · May 14, 2014

scameron81 said:
Well if you don't show up for our VFD class at least I'll know why.

LOL...
Yeah, I was supposed to be there when they came out with the MCC van, but I was sick and had to turn around. I did not want to get dizzy on that road out to your place and end up careening off of a cliff.

For the benefit of others, this is a good day on that road:

A not-so-good day on that same road...

You have to bring your A game when going there.

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