Sizing Breaker on MCA

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IMO no because as the name implies that is the Minimum circuit ampacity. There are probably contrary opinions.
 
Yes if the inrush current lasts for too long and trips the breaker. This could be exacerbated by voltage drop due to long wire runs or transformers that are overloaded. I have no experience in tracking installs done with overcurrent done at MCA instead of max breaker, so I'm speculating here.... I'd like the breaker to be a bit larger than MCA, but not necessarily as large as the max breaker listed on the nameplate.

This allowance for motors and HVAC breaker is in the code for a reason. How commonly things trip if the breaker is sized per FLA+25% or MCA I do not know.
 
suemarkp said:
Yes if the inrush current lasts for too long and trips the breaker. This could be exacerbated by voltage drop due to long wire runs or transformers that are overloaded. I have no experience in tracking installs done with overcurrent done at MCA instead of max breaker, so I'm speculating here.... I'd like the breaker to be a bit larger than MCA, but not necessarily as large as the max breaker listed on the nameplate.

This allowance for motors and HVAC breaker is in the code for a reason. How commonly things trip if the breaker is sized per FLA+25% or MCA I do not know.
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Do you think the tripping will be on thermal or magnetic elements? Thermal might hold (at least wishfully thinking).

FWIW MCA is FLA x 125%.
 
Thermal. MCA is FLA of largest motor x 125% plus everything else in it (electronics, controls, other motors). Depending on what all is in there, and when they run, the 25% extra may not be enough.

I like it when the MCA is 16 or 26 -- just over a standard breaker size, so you have to move up.
 
suemarkp said:
Thermal. MCA is FLA of largest motor x 125% plus everything else in it (electronics, controls, other motors). Depending on what all is in there, and when they run, the 25% extra may not be enough.

I like it when the MCA is 16 or 26 -- just over a standard breaker size, so you have to move up.
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Alright.
 
mbrooke said:
Do you think the tripping will be on thermal or magnetic elements? Thermal might hold (at least wishfully thinking).

FWIW MCA is FLA x 125%.
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If it trips instantly when motor is starting - magnetic trip. If it has had time to attempt to accelerate before it trips most likely tripped on thermal function.
 
kwired said:
If it trips instantly when motor is starting - magnetic trip. If it has had time to attempt to accelerate before it trips most likely tripped on thermal function.
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True, though as the motor ages it might take some time accelerate?
 
mbrooke said:
True, though as the motor ages it might take some time accelerate?
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Well with a brfeaker sized at MCA level could be either trip feature I guess, though magnetic is still likely going to be "instantaneous" as in a cycle or two. Thermal trip is always going to take enough time that it is noticeable delay to humans.

Even if rotor is locked it will draw more in the first cycle or so than the steady locked rotor current is, and that is where if it doesn't trip on magnetic initially that it would trip on thermal sometime later.
 
kwired said:
Well with a brfeaker sized at MCA level could be either trip feature I guess, though magnetic is still likely going to be "instantaneous" as in a cycle or two. Thermal trip is always going to take enough time that it is noticeable delay to humans.

Even if rotor is locked it will draw more in the first cycle or so than the steady locked rotor current is, and that is where if it doesn't trip on magnetic initially that it would trip on thermal sometime later.
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Yes. I have to admit this whole motor sizing rule always makes me uneasy. I always wish we could just size the wire and breaker to the MCA, but, there is good reason I guess as to why the code lets you do it there way.
 
The load charistics on the motor also matter. A power saw is easy to start, an the motor only draws significant current when it is cutting. Compressors and pumps are working right away, so they have more initial load. But those loads are soft. A motor that is getting some apparatus moving from a stop can be under a lot of load right at locked rotor (e.g. electric car or locomotive, especially on a hill).
 
General purpose breakers and motors spell big trouble. If you have several (3+) of the same size motor as your largest motors, it’s usually OK to use an MCA type rule. But when it’s individuals you will have trouble.

Inrush is up to 300% of LRA. On decent size motors this is 6xFLA. As you get close to fractional HP LRA increases as high as 10xFLA. Instantaneous for class C (highest) UL 489 breakers is up to 20x LT rating. So surge can be 3x10 or 30 times FLA but even on “good size” motors it’s up to 18xFLA. So using 125%x20 gets you to 25x FLA but with more typical 6-10x Breaker rating (class B) you get 7.5-12.5x, not nearly enough. The “knee” on hard starts will trip breakers sized under NEC max (250%). Motor specific breakers typically don’t have “instantaneous” table.
 
paulengr said:
General purpose breakers and motors spell big trouble. If you have several (3+) of the same size motor as your largest motors, it’s usually OK to use an MCA type rule. But when it’s individuals you will have trouble.

Inrush is up to 300% of LRA. On decent size motors this is 6xFLA. As you get close to fractional HP LRA increases as high as 10xFLA. Instantaneous for class C (highest) UL 489 breakers is up to 20x LT rating. So surge can be 3x10 or 30 times FLA but even on “good size” motors it’s up to 18xFLA. So using 125%x20 gets you to 25x FLA but with more typical 6-10x Breaker rating (class B) you get 7.5-12.5x, not nearly enough. The “knee” on hard starts will trip breakers sized under NEC max (250%). Motor specific breakers typically don’t have “instantaneous” table.
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I'm saving this to my notes :)
 
I kind of messed this up. Motor specific breakers have either a current limiting function or a fairly slow “instantaneous” trip. If instantaneous is delayed 1-2 cycles they avoid the high inrush of high efficiency motors.

The next “trap” is the thermal curve. A motor pulls near/at FLA until it reaches about 85% speed before you see a significant drop in current. So if you get the breaker “tight” (right at 125% FLA, no “one size up”), it will nuisance trip on all but the softest of starting. No magic here. Plot the curves if you have the capability or look at the Cooper/Bussmann book. If you set it up for Code maximum (250% FLA) it won’t nuisance trip but it also won’t protect the wiring going to the motor anymore. It’s short circuits only.

So if you have a “textbook” case of MCA with3+ similar sized motors it will often work. You may notice goofy things like it works fine on motors built and intended for the Noeth American market but trips on “World” motors (IECs rerated for sale in North America). True thermal-magnetic breakers fair better than electronic trip units. But if you have only 1-2 large motors and the rest of the load is of no consequence make sure to check NEC max breaker settings. If you are under that threshold you will nuisance trip. If you can just size it up bigger with bigger feeders. If you can’t consider fused disconnect (current limiting type) or if it’s electronic trip or available can you add 2-3 cycle delay to “instantaneous”, or see if a current limiting breaker is available. Anything to trim inrush on motors that essentially violate the NEC “maximum” 17x FLA with adjustable trip breakers. That is by the way another option. Just be aware of your 3 cycle design limit for short circuit on MCCs, panel boards too since that’s the reason for the default “10x” trip in general purpose molded case breakers. Soft starts and drives are another option I forgot but it’s expensive so not necessarily the first option depending on conditions.

I work for a motor shop. We see this kind of thing frequently on new startups where the engineers follow the basic Code rules. The MCA rule sounds very simple and it is but it is also very easy to misapply it.

A somewhat “safer” design rule is to add up all the HP. Multiply the largest motor by 5 and add that to the total. Assume 1 HP = 1 kVA and add your other loads as kVA. Now the engineers are all screaming “units”. Well yes but this gets you about 0.85 as a derating factor (power factor and efficiency) with a slight cushion and the 5x largest motor covers the 6x LRA (counted 1x in the initial total). Then size feeder, breaker, and transformer for this. It will give you a slight cushion for more loads later. Still have to watch out for that nasty 3xLRA from energy efficient and IEC motors.
 
paulengr said:
I kind of messed this up. Motor specific breakers have either a current limiting function or a fairly slow “instantaneous” trip. If instantaneous is delayed 1-2 cycles they avoid the high inrush of high efficiency motors.

The next “trap” is the thermal curve. A motor pulls near/at FLA until it reaches about 85% speed before you see a significant drop in current. So if you get the breaker “tight” (right at 125% FLA, no “one size up”), it will nuisance trip on all but the softest of starting. No magic here. Plot the curves if you have the capability or look at the Cooper/Bussmann book. If you set it up for Code maximum (250% FLA) it won’t nuisance trip but it also won’t protect the wiring going to the motor anymore. It’s short circuits only.

So if you have a “textbook” case of MCA with3+ similar sized motors it will often work. You may notice goofy things like it works fine on motors built and intended for the Noeth American market but trips on “World” motors (IECs rerated for sale in North America). True thermal-magnetic breakers fair better than electronic trip units. But if you have only 1-2 large motors and the rest of the load is of no consequence make sure to check NEC max breaker settings. If you are under that threshold you will nuisance trip. If you can just size it up bigger with bigger feeders. If you can’t consider fused disconnect (current limiting type) or if it’s electronic trip or available can you add 2-3 cycle delay to “instantaneous”, or see if a current limiting breaker is available. Anything to trim inrush on motors that essentially violate the NEC “maximum” 17x FLA with adjustable trip breakers. That is by the way another option. Just be aware of your 3 cycle design limit for short circuit on MCCs, panel boards too since that’s the reason for the default “10x” trip in general purpose molded case breakers. Soft starts and drives are another option I forgot but it’s expensive so not necessarily the first option depending on conditions.

I work for a motor shop. We see this kind of thing frequently on new startups where the engineers follow the basic Code rules. The MCA rule sounds very simple and it is but it is also very easy to misapply it.

A somewhat “safer” design rule is to add up all the HP. Multiply the largest motor by 5 and add that to the total. Assume 1 HP = 1 kVA and add your other loads as kVA. Now the engineers are all screaming “units”. Well yes but this gets you about 0.85 as a derating factor (power factor and efficiency) with a slight cushion and the 5x largest motor covers the 6x LRA (counted 1x in the initial total). Then size feeder, breaker, and transformer for this. It will give you a slight cushion for more loads later. Still have to watch out for that nasty 3xLRA from energy efficient and IEC motors.
Click to expand...
Did you mean LRA there?
 
mbrooke said:
Yes. I have to admit this whole motor sizing rule always makes me uneasy. I always wish we could just size the wire and breaker to the MCA, but, there is good reason I guess as to why the code lets you do it there way.
Click to expand...
There is two types of protection involved. The branch circuit device is usually only providing short circuit and ground fault protection. The motor controller usually is also providing overload protection. Overload protection protects both the motor and the conductors.
 
paulengr said:
I kind of messed this up. Motor specific breakers have either a current limiting function or a fairly slow “instantaneous” trip. If instantaneous is delayed 1-2 cycles they avoid the high inrush of high efficiency motors.

The next “trap” is the thermal curve. A motor pulls near/at FLA until it reaches about 85% speed before you see a significant drop in current. So if you get the breaker “tight” (right at 125% FLA, no “one size up”), it will nuisance trip on all but the softest of starting. No magic here. Plot the curves if you have the capability or look at the Cooper/Bussmann book. If you set it up for Code maximum (250% FLA) it won’t nuisance trip but it also won’t protect the wiring going to the motor anymore. It’s short circuits only.

So if you have a “textbook” case of MCA with3+ similar sized motors it will often work. You may notice goofy things like it works fine on motors built and intended for the Noeth American market but trips on “World” motors (IECs rerated for sale in North America). True thermal-magnetic breakers fair better than electronic trip units. But if you have only 1-2 large motors and the rest of the load is of no consequence make sure to check NEC max breaker settings. If you are under that threshold you will nuisance trip. If you can just size it up bigger with bigger feeders. If you can’t consider fused disconnect (current limiting type) or if it’s electronic trip or available can you add 2-3 cycle delay to “instantaneous”, or see if a current limiting breaker is available. Anything to trim inrush on motors that essentially violate the NEC “maximum” 17x FLA with adjustable trip breakers. That is by the way another option. Just be aware of your 3 cycle design limit for short circuit on MCCs, panel boards too since that’s the reason for the default “10x” trip in general purpose molded case breakers. Soft starts and drives are another option I forgot but it’s expensive so not necessarily the first option depending on conditions.

I work for a motor shop. We see this kind of thing frequently on new startups where the engineers follow the basic Code rules. The MCA rule sounds very simple and it is but it is also very easy to misapply it.

A somewhat “safer” design rule is to add up all the HP. Multiply the largest motor by 5 and add that to the total. Assume 1 HP = 1 kVA and add your other loads as kVA. Now the engineers are all screaming “units”. Well yes but this gets you about 0.85 as a derating factor (power factor and efficiency) with a slight cushion and the 5x largest motor covers the 6x LRA (counted 1x in the initial total). Then size feeder, breaker, and transformer for this. It will give you a slight cushion for more loads later. Still have to watch out for that nasty 3xLRA from energy efficient and IEC motors.
Click to expand...


Why do IEC motors have worse inrush and LRA?
 
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