Locked Rotor Amperage Clarification
- Thread starter RRoc
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mopowr steve
Senior Member
- Location
- NW Ohio
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- Electrical contractor
It would be the 9 amps x 1.25 , so the 15 amp breaker should work. If for some reason it will not you have an allowance in the code to use larger breaker for motor start up reasons but you’ll have to look up the code article.
Sounds good on the 15Amp breaker.
I called the Mixer Manufacturer and they said they usually put a 25 Amp Circuit breaker.
The Tech said Per the NEC its 9Amp * 250% which is about 22.5Amps or rounding up to 25Amp.
I believe the 125% is for Time Delay Fuses and the 250% is for Circuit breakers.
None the less I still think 25Amp might be overkill.
I called the Mixer Manufacturer and they said they usually put a 25 Amp Circuit breaker.
The Tech said Per the NEC its 9Amp * 250% which is about 22.5Amps or rounding up to 25Amp.
I believe the 125% is for Time Delay Fuses and the 250% is for Circuit breakers.
None the less I still think 25Amp might be overkill.
Normally NEC allows rounding up. The oversize rule is a maximum though so 20 A max.
Inrush depends on how much the losses are in the windings instead of core losses. Very high efficiency motors really make this severe. There is a theoretical limit of 24 times FLA. I’ve measured IEC motors hitting 22x FLA. thermal-magnetic breakers are immune because it only lasts 1-2 cycles. At larger sizes (over 100 A) electronic breakers trip so you have to use an instantaneous delay,
Your inrush doesn’t sound right. On fractional HP motors it gets up to 10x FLA. On small sizes it’s usually 6-8x. You don’t usually see motors under 6x FLA under you get over 100 HP or so but even on very large motors the lowest I ever see is 5-6x FLA except very unusual motors that use NEMA Design C of D, Most are Design B or emulate Design B characteristics,
Inrush depends on how much the losses are in the windings instead of core losses. Very high efficiency motors really make this severe. There is a theoretical limit of 24 times FLA. I’ve measured IEC motors hitting 22x FLA. thermal-magnetic breakers are immune because it only lasts 1-2 cycles. At larger sizes (over 100 A) electronic breakers trip so you have to use an instantaneous delay,
Your inrush doesn’t sound right. On fractional HP motors it gets up to 10x FLA. On small sizes it’s usually 6-8x. You don’t usually see motors under 6x FLA under you get over 100 HP or so but even on very large motors the lowest I ever see is 5-6x FLA except very unusual motors that use NEMA Design C of D, Most are Design B or emulate Design B characteristics,
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
It's about a 99.99% chance that is a PSC (Permanent Split Capacitor) type motor, as evidenced by the lack of an external capacitor. Not at all uncommon for something in a sealed oil-filled can like those mixers are. PSC motors have lower than typical LRC and thereby, lower than typical starting torque. That's what makes them basically only suitable for variable torque applications like fans and pumps (these mixers are similar to under water fans, same type of load profile).
Breakers are designed to allow for motor starting current, usually equivalent to the LRC, because their instantaneous trips will not kick in until the current is 10x the rating of the breaker. So for a 15A breaker, that's going to be 150A. You will never get close.
Side note, it's a good idea to use a GFCI breaker in case there is an oil leak in the can. And if you are using this mixer to prevent freezing, power a Normally Closed Relay from that breaker that goes to an alarm that is powered from a separate circuit so that if the breaker trips, you know it did, that gives you a warning that your tank may freeze up on you.
Breakers are designed to allow for motor starting current, usually equivalent to the LRC, because their instantaneous trips will not kick in until the current is 10x the rating of the breaker. So for a 15A breaker, that's going to be 150A. You will never get close.
Side note, it's a good idea to use a GFCI breaker in case there is an oil leak in the can. And if you are using this mixer to prevent freezing, power a Normally Closed Relay from that breaker that goes to an alarm that is powered from a separate circuit so that if the breaker trips, you know it did, that gives you a warning that your tank may freeze up on you.
- Location
- Connecticut
- Occupation
- Engineer
It's 175% for Time Delay Fuses
- Location
- Connecticut
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- Engineer
Wouldn't that be 25A max, allowing for the rounding up?
kwired
Electron manager
- Location
- NE Nebraska
Yes, 25 amp is the max NEC will allow, it pretty much has to fail to hold at that level before there may be provisions to increase it from there.
Actual installation conditions will impact what size breaker may actually hold. Is possible a 15 amp will work in many cases, if not a 20 probably does hold in nearly all cases. In the extreme LRC cases, there likely is a short run of branch circuit, short run of feeder/service conductors and a source with a really low impedance. Most other cases the source impedance along with service/feeder/branch circuit resistance that starting current is impedance limited to some extent from what it would be in worst case scenario.
Add: and "locked rotor current" and that peak inrush during starting are not necessarily the same thing.
Locked rotor current I believe is a steady state current level at rated input voltage in the event the rotor is not turning for whatever reason and will hold at that level until something begins to deteriorate from the excess heating that will occur or overcurrent protection finally terminates things.
That starting surge can be even more than locked rotor current but the worst of it only lasts for a few cycles and then current rapidly drops as the motor accelerates, the overcurrent device only needs to withstand the worst of it for a few cycles and often can withstand the rest of the starting current as long as it is dropping as the motor accelerates.
9 A x 250% = 22.5 A. Closest size without going over is 20 A. You can round up most places in NEC so for instance 125% of 9 A is 11.25 A so we round up to the next standard size or 15 A and you can exceed this if you have to but the 250% is a hard limit so you have to round down to 20 A.
- Location
- Connecticut
- Occupation
- Engineer
250% is not a hard limit...you can round up to the next standard size. 25A would be the max size c/b for a 9A motor.
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