motor calculations

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Jon1980

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Texas
I'm on a burnout job and need to re-pull wire to several motors. I was looking at the conductors feeding these motors and they seem to be way over sized. I have three one hundred horse motors that range from 118 to 123 amps all continuous duty each with their own starters. From what I see in the code it states that their is a 1.25 multiplier. 148 amps and 154 amps rounded up, Now my question is why would the over current device (starter) be rated at 185 amps? Am I missing something? I know that we size our wire by the over current device which protects the wire and the motor.
 

jimdavis

Senior Member
Motor circuit installations differ from most other applications due to the large inrush currents associated with startup. Conductors are typically sized based on 125% of FLC (from the NEC tables, not the nameplate), but the OCPD is sized to accommodate inrush. Table 430.52 specifies values of 250% of FLC for inverse time circuit breakers or 175% for dual-element time delay fuses. So it's typical for a motor circuit to have an OCPD which is considerably larger than the conductor ampacity.
 

Jraef

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Motor circuit installations differ from most other applications due to the large inrush currents associated with startup. Conductors are typically sized based on 125% of FLC (from the NEC tables, not the nameplate), but the OCPD is sized to accommodate inrush. Table 430.52 specifies values of 250% of FLC for inverse time circuit breakers or 175% for dual-element time delay fuses. So it's typical for a motor circuit to have an OCPD which is considerably larger than the conductor ampacity.
That's right, so the conductors have to be based on 125% of 124A, the NEC table value for a 100HP 460V motor, putting the amp basis at 155A, adjusted up as necessary for VD, temperature, fill etc. per your site conditions.

And the values in 430.52 are the "not to exceed" values, rounded up to the nearest higher value if needed, or you can use smaller if you want to try it but you run the risk of nuisance tripping. There are also exceptions allowing higher values if you can demonstrate that the table values are too low to allow the motor to start, and if you use a controller that says it is required to have a different value, you must use the OCPD value listed by the mfr.
 

GoldDigger

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But please to remember that although the circuit protection will seem oversized, the overloads which may be in the starter must be sized closely to the full load current.
Oversizing the overloads and then overloading the motor will cause burnout.
 

iceworm

Curmudgeon still using printed IEEE Color Books
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.... I have three one hundred horse motors that range from 118 to 123 amps all continuous duty each with their own starters. ... why would the over current device (starter) be rated at 185 amps? ....

Is this this a Thermal-Magnetic CB in the starter? If so it is small. As noted by jraef and jim, 250% x 124A = 350A (round up)

However, if this is the overload element, then it should be between 125% to 140% (for motor is 1.15sf) or 147A to 172A. So 185A would be bigger than I would expect.

ice
 

topgone

Senior Member
But please to remember that although the circuit protection will seem oversized, the overloads which may be in the starter must be sized closely to the full load current.
Oversizing the overloads and then overloading the motor will cause burnout.

In brief, you cover the overload protection by choosing an overload relay setting that more or less approximates the heating properties of the motor winding "and" a circuit breaker with a short time/ instantaneous pickup to isolate the motor from the power source if locked-rotor/stalling or accidental shorts occur.
 

augie47

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"Now my question is why would the over current device (starter) be rated at 185 amps?"

What are you using to determine this 185 amp rating ?
 

Jon1980

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Location
Texas
"Now my question is why would the over current device (starter) be rated at 185 amps?"

What are you using to determine this 185 amp rating ?

These are adjustable trip breakers in the MCC. Their settings are at 185A. Its an old gear and they have all the motors temporary in so they can run when needed. All this is based on what is currently installed and set. I haven't changed anything.
 

Jon1980

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Location
Texas
But please to remember that although the circuit protection will seem oversized, the overloads which may be in the starter must be sized closely to the full load current.
Oversizing the overloads and then overloading the motor will cause burnout.

By burnout I meant a Gas line which supplied heaters in this water building ruptured and caused a fire. Which in turn burnt up all wires and conduits above six foot to the roof.
 

Jon1980

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Location
Texas
Is this this a Thermal-Magnetic CB in the starter? If so it is small. As noted by jraef and jim, 250% x 124A = 350A (round up)

However, if this is the overload element, then it should be between 125% to 140% (for motor is 1.15sf) or 147A to 172A. So 185A would be bigger than I would expect.

ice


I haven't dug in that deep to this yet, I would have to shutdown the pumps and look at the breaker info. Right now it's more re install the conduits and re-pull all control circuits. One fire at a time. I'll get more info next week.
 

Jraef

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Breakers with adjustable thermal trips are not allowed to be used here in the US in the same way as they are in other parts of the world, that's why you don't see them listed as an option for the SCPD in the NEC 430.52. So most likely, you have an MCC built by a foreign (to us) supplier, in which case the values they pick will not have validity here anyway. So trying to reconcile their settings against the NEC is futile, it was not supposed to be there in the first place. Adjustable thermal-mag breakers cannot be UL489 listed for providing branch circuit protection, they can only be listed as "manual motor starters", thereby requiring ANOTHER OCPD device such as fuses or another UL489 listed breaker ahead of them. That makes them difficult to implement on individual starters, such as in an MCC.

One problem with adjustable trips in a circuit breaker being used as the motor overload protection is that when there is an overload and it trips, there is a tendency here to tweak the dial up, usually under management pressure to get machinery running again immediately, no excuses. Yes, it's short sighted, but unfortunately very common. I would venture to guess that is how yours ended up at 185A, too high for a 100HP motor. It's possible that when the foreign MCC first arrived and a US electrician went to hook it up and set up the trips, he used the sizing rules on T-M breakers in the NEC, which do NOT apply to those adjustable trip breakers. Or they were set up correctly initially, then something tripped and a maintenance guy tweaked it to make the problem go away, justifying the setting using the wrong rules, then went back and tweaked them all using the same justification. Might be worth checking everything else in that MCC just in case.
 
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Maybe, most likely, I am missing something. Older AB MCCs used instantaneous trip breakers ahead of the starters. The few I have dealt with were adjustable and needed to be set above the startup current of the motor.

OP, is this the style of Breaker you are talking about?

After thought: Although 185 wouldn't allow a 100 HP Motor to start.
 

Jraef

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Location
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Electrical Engineer
Maybe, most likely, I am missing something. Older AB MCCs used instantaneous trip breakers ahead of the starters. The few I have dealt with were adjustable and needed to be set above the startup current of the motor.

OP, is this the style of Breaker you are talking about?

After thought: Although 185 wouldn't allow a 100 HP Motor to start.
Yeah, not MCPs as we know them, the mag trips have always been allowed to be adjustable. But companies like Klockner Moeller, ABB, Schneider (Merlin Gerin) and Siemens (Germany) all sell a version of their molded case circuit breakers in which both the magnetic and thermal trips are adjustable which THEY call a Motor Circuit Protector over there, but since we have something else that already means that, we call them Motor Protection Circuit Breakers (MPCB) here. So they will make a combination motor starter out of a contactor + MPCB alone, no separate OL relay. Here, those MPCBs cannot be UL listed under 489, so if they sell them, they are UL508C listed as "Manual Motor Starters" and can only be used behind another OCPD listed for branch circuit protection. It can get confusing to people here.
 

topgone

Senior Member
"Now my question is why would the over current device (starter) be rated at 185 amps?"

What are you using to determine this 185 amp rating ?

I guess the guy is using what Square D recommends, a correction factor to be included in computing for the trip current setting of overload relays to compensate for ambient temperature where the controller is installed.

In this specific example, a 185A OLR setting for a 100 hp motor corresponds to a correction factor of 1.2 (ambient temp = 20 degrees C, reference temp is 40 deg. C). Trip setting = 123A x 1.25 x 1.2 ~ 185A!

However, NEC allows only up to 140% of FLC for motors with service factors not less than 1.15, what @ice said.
 

kwired

Electron manager
Location
NE Nebraska
Is the device in question the short circuit/ground fault protection device or the motor overload protection device?

In most instances 185 would be low for the SC/GF device but is closer to what is generally acceptable for the overload device.

If it is the overload device - it may not be sized correctly, or you may not be determining it's setting/rating correctly. If it is a thermal operating device it may have had ambient temp, number of elements, other heat producing components factored into it's selection and what may be a 185 amp device in one application may need to be derated for another application. Pay close attention to selection of the selection chart with such devices.
 
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