amperage inrush

just curious, is it possible that 2/0 Aluminum feeding a 300 amp service causes sufficient enough of a voltage drop so that HVAC equipment reports of 500% in rush and lights dimming and etc? and I know the utility company always downsize is their wire. everything in the system to the equipment is properly sized. all the terminations are tight from main disconnect on. no bluing or discoloration of any terminals.
 

Jraef

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When the voltage drops, the starting current DROPS, not increases. That's why you use a "reduced voltage starter" to try to help control starting current on motors.

500% starting current is normal and is what needs to be factored in to how the service is selected, of if there is no choice there, then the starting method used for that service in order to prevcent the voltage drop and flicker.
 
just curious, is it possible that 2/0 Aluminum feeding a 300 amp service causes sufficient enough of a voltage drop so that HVAC equipment reports of 500% in rush and lights dimming and etc? and I know the utility company always downsize is their wire. everything in the system to the equipment is properly sized. all the terminations are tight from main disconnect on. no bluing or discoloration of any terminals.


this is a 3phase a/c condensing unit of a split system with a 48.1mca and a 70mco. if what you're saying is true about 500% being normal then why is everything calculated for 125% for motor inrush? it's on a little retail shop and it just seems odd to me that the inrush would exceed the amp rating of the theaters coming into the building. I was thinking that the smaller conductor not being able to supply the required amperage was causing for the voltage to drop and amps to spike.
 

gar

Senior Member
160225-1328 EST

cloudibew:

At thread http://forums.mikeholt.com/showthread.php?t=174880 my post numbered #3 shows two different starting currents for the same unloaded motor resulting from different motor voltages. Note: 10 A corrresponds to 50 mV on the scope.

The first plot is for no added resistance in series with the motor. The second plot has added resistance that lowers the motor voltage and you see the starting current is lower. Once the motor is up to speed and has a fixed mechanical load torque, then motor current can be expected increase as motor voltage decreases.

Also note that on startup that with a lower motor voltage it takes longer to reach full speed because there is a fixed amount of energy required to bring the inertia of the motor load up to speed, and with lower voltage and lower input current the rate of energy input (power) is less so time has to increase.

Exactly what happens between 0 speed and full speed will depend upon how the motor external load varies with speed on startup. Some place there is a transition between current decreasing with decreasing voltage and increasing with decreasing voltage.

.
 

luckylerado

Senior Member
this is a 3phase a/c condensing unit of a split system with a 48.1mca and a 70mco. if what you're saying is true about 500% being normal then why is everything calculated for 125% for motor inrush? it's on a little retail shop and it just seems odd to me that the inrush would exceed the amp rating of the theaters coming into the building. I was thinking that the smaller conductor not being able to supply the required amperage was causing for the voltage to drop and amps to spike.
500% is not normal.
In simple terms; Since Current=Voltage/Resistance, when Voltage goes down so does Current.
 
I'm just trying to wrap my head around it because as many inrush readings as I've taken in the last 15 years of doing service work I've never seen a motor with that much of an in rush. it is pretty much 450 amps on a brand new compressor. however it does settle down to about 25 amps afterwards. and the timing of the lighting is extremely noticeable and has to be raking some sort of havoc on the LED controller that they've just put in within the last year I can't see it not adversely affecting them. I would really love to pull the rubber tape off of the utility taps and see the color of that wire and lugs.
 
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GoldDigger

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Because it is a "3phase a/c condensing unit of a split system with a 48.1mca and a 70mco"

Am I wrong??
Yes.
The MCA is essentially 125% of FLA and the MOCP relates to short circuit protection, not starting current (I am deliberately not using "inrush").
Ideally the starting current will always be less than the mag trip value of a breaker sized at MOCP. Which in turn is upwards of 10 times the nominal value.
Note that we are talking about 500% (a factor of five) not 500A.
It takes either a scope or a specialized meter to capture the peak starting current reading.
 

tsparks1

Member
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Is the 2/0 Alum from the power company? If so they use a different code than we do for free air wiring.


Sent from my iPhone using Tapatalk
 

Jraef

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this is a 3phase a/c condensing unit of a split system with a 48.1mca and a 70mco. if what you're saying is true about 500% being normal then why is everything calculated for 125% for motor inrush?
You are misinterpreting something here. The "125%" is for sizing of conductors, that has nothing to do with "inrush". All induction motors are nothing but a short circuit for the instant that it is first energized and the magnetic fields are not yet established. This is the true definition of "inrush current" because there is no impedance yet, only resistance of the winding conductors, which is very low. So theoretically that can be as high as 2000%+ of FLA, but only for about 1 cycle. After than, the magnetic fields come into being and interact to drop the current to 500-600% of FLA, until the motor reaches about 80% speed. After that, it drops to FLC fairly fast.

it's on a little retail shop and it just seems odd to me that the inrush would exceed the amp rating of the theaters coming into the building. I was thinking that the smaller conductor not being able to supply the required amperage was causing for the voltage to drop and amps to spike.
You don't size conductors for inrush or starting current and not even the transformers unless you think there is going to be a problem. All transformers have what's referred to as a transient load capacity before they show a significant voltage drop, but it's based on kVA for short periods of time. So the higher the starting current and/or the longer it takes to start a motor, the more you run into a problem. That's typically something the PoCo will tell you, either directly by stating the maximum starting kVA, or indirectly by telling you the maximum size of motor you can start without having to use Reduced Voltage Starting. If someone is adding a load that is causing a VD, and in the permitting process, nobody called the PoCo to tell them what they were doing, it might be a violation.

Having the conductors being too small will cause them to heat up and act like resistors to drop the voltage, which is actually a form of Reduced Voltage Starting that is available (albeit using actual resistors, not the conductors). So although not good for the conductors themselves, it is actually probably HELPING the situation a little. Again, NOT the way it should be done, I'm just pointing pout that your thinking the conductors is the CAUSE of the problem is not correct.
 

jim dungar

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Because it is a "3phase a/c condensing unit of a split system with a 48.1mca and a 70mco"

Am I wrong??
Motor overload relays (thermal units or bi-metallic) are traditionally referred to as Class 10, Class 20, or Class 30 or fast trip, normal trip, and slow trip. The trip time of these descriptions is based on 600% of FLA. Therefore a 500% 'inrush' current is not unexpected and should be of little concern to the components in the circuit.

Effects of 500% current on the operation of the system is a design issue, not an installation one.
 
Yes.
The MCA is essentially 125% of FLA and the MOCP relates to short circuit protection, not starting current (I am deliberately not using "inrush").
Ideally the starting current will always be less than the mag trip value of a breaker sized at MOCP. Which in turn is upwards of 10 times the nominal value.
Note that we are talking about 500% (a factor of five) not 500A.
It takes either a scope or a specialized meter to capture the peak starting current reading.
48.1 *5 or 48.1*500% is still the same number my (fluke 377) meter was recording as the. "max" wasnt exatly that high i see my big thumbs stated 450 was closer to 250amps still way high compared to what i have seen even on older motors. but the rest of the explanation makes sense thanks.
 

iwire

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48.1 *5 or 48.1*500% is still the same number my (fluke 377) meter was recording as the. "max" wasnt exatly that high i see my big thumbs stated 450 was closer to 250amps still way high compared to what i have seen even on older motors. but the rest of the explanation makes sense thanks.
Very unlikely your Fluke meter is fast enough to capture the true peak when the contactor closes.

Or you may have enough impedance in the circuit to hold the inrush down.


Take a cruise to table 430.52 and you will see the code allows the over-current device anywhere from 150% to 1,100% of the motors full load amperage. This is to deal with the high inrush.
 

topgone

Senior Member
Very unlikely your Fluke meter is fast enough to capture the true peak when the contactor closes.

Or you may have enough impedance in the circuit to hold the inrush down.


Take a cruise to table 430.52 and you will see the code allows the over-current device anywhere from 150% to 1,100% of the motors full load amperage. This is to deal with the high inrush.
MCA is computed as 125% of the largest current + the sum of the other loads. I did some calcs and for an MCA=48.1A with an MOP = 70, the largest current is 21.9A and the other loads total to 20.72A. You can verify my numbers by using the largest amp and the rest of the load when computing the MOP = 2.25 x 21.9 + 20.72 = 69.9~ 70A!

That being the case, A 500% inrush will be 5 x 21.9 = 109.5A. The inrush he may measure will therefore be = 109.5/(21.9+20.72) = 257% of RLA! Or 227% of the MCA. I think this is what's bothering the OP.
Hope that clears the issue/s.
 

kwired

Electron manager
Location
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Very unlikely your Fluke meter is fast enough to capture the true peak when the contactor closes.

Or you may have enough impedance in the circuit to hold the inrush down.


Take a cruise to table 430.52 and you will see the code allows the over-current device anywhere from 150% to 1,100% of the motors full load amperage. This is to deal with the high inrush.
Which was mentioned early on that the small sized service conductor will increase impedance and hold the inrush down.

Though the surge of current is limited, so is the starting torque and duration needed to reach full speed will also be increased. So in some cases with a large motor you may see less severe voltage sag, but what you do see will last for longer time - usually most noticeable by lights dimming when the large motor starts.
 

iceworm

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... You don't size conductors for inrush or starting current and not even the transformers unless you think there is going to be a problem. All transformers have what's referred to as a transient load capacity before they show a significant voltage drop, but it's based on kVA for short periods of time. ...
I recall this coming up in another thread. I never got a clear understanding. I did a search on IEEE papers, IEEE 100, and even google - nothing except for some heat capacity related issues.

Do you have a reference for this?

If not, do you have some physics explanation?

How about personal test data - test setup, what measured, how measured?

I'm absolutely open to some education this morning.

ice
 

gar

Senior Member
160226-1719 EST

iwire:

Very unlikely your Fluke meter is fast enough to capture the true peak when the contactor closes.
There is no appreciable short peak current when you apply voltage to a motor unless there are shunt capacitors. You can not instantaneously change the current in an inductor.

What occurs in a transformer is different than a motor because of the large air gap in the motor. The possible peak inrush current in a transformer does not violate my statement that current can not change instantaneously in an inductor. You need to study the characteristics of ferromagnetic materials to see why.

I have already presented some motor starting current waveforms I measured and referenced above, and these show no short large peak inrush current.

.
 
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