How to size a circuit breaker?

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javajake

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Hello - I have a 7.5HP 3-ph motor PD blower with a on a soil vapor extraction system (pulling air out of the ground to treat contaminants). We're pulling a vacuum of around 4.5 in-Hg with air flow of around 185 scfm. The manufacturer of the treatment system originally assumed 230V service and spec'd a 30A thermo-mag breaker: but we're only running at 212V at the panel. The motor CB tripped after about 2 hours of operation. The manufacturer feels that the issue is that due to the low voltage, the system is requiring more amps.

This raises a question. In this application - basically unpredicable load (vacuum) on the motor, how does one properly select a high enough AMP CB without endagering the motor, or the panel?

Thanks!
- JJ
 
Assuming that the motor has overload protection the breaker would be permitted by 430.52 to be a 60 amp thermal magnetic breaker. I wonder why the breaker tripped, the overload should have tripped first for a high amp condition.
 
Few inputs...

Did you check the amps while it is running, and measure the peak amps?

What is the name plate voltage rating of motor?

Is it connected in delta or star ?

There will be difference in current with reduced voltage,..but definitely not enough to trip a 30 amps breaker.

Cheers.
 
javajake said:
The manufacturer of the treatment system originally assumed 230V service and spec'd a 30A thermo-mag breaker: but we're only running at 212V at the panel.
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It sounds like you have a 208v wye system instead of a 240v Delta. The motor should have current ratings for both voltages, unless it's not rated for 208v, in which case you should boost the voltage to 240.

Remember that power is not free. If you use buck-boost transformers, the input current will be a bit higher than the motor current. Placing these closer to the source is better for voltage-drop reasons.

In any case, the breaker rating can and should be higher than 30a for the 22 to 24 amps indicated in Table 430.150. You should probably have a 40a breaker, but can keep the wire size you have now.

What does the equipment labelling say?
 
Overloads in starter should have tripped. Try a larger breaker. The therm-mag breaker should only trip on instantaneous.
 
don_resqcapt19 said:
Assuming that the motor has overload protection the breaker would be permitted by 430.52 to be a 60 amp thermal magnetic breaker. I wonder why the breaker tripped, the overload should have tripped first for a high amp condition.
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I agree, this sounds like a coordination and design problem. There is something wrong with your breaker sizing and overload heater selection (or setting) or maybe you just don't even have a separate OL relay. Sometimes OEMs think they can get away with having just a circuit breaker as the running overload protection, but although technically not illegal, it is almost impossible to select the correct rating. For example assuming your motor is dual rated for 208 or 230V operation, it would be 25A FLC at 208V. A 30A breaker then is already too small because you can only load it to 80% (24A). But the thermal trip rating is too high to use as an OL device if the FLC was 22A at 230V, because 22 x 1.25 = only 27.5A. You really need a larger breaker so that you don't trip on starting current, i.e. 50A or 60A, then you need a properly selected OL relay.

Post the other info, i.e. the motor nameplate FLA, what type of OL you have (if any) and what the OL heaters are or if adjustable, what the setting is.
 
thanks and info

thanks and info

hi All - thanks for your quick replies! I'm out of the field for the day, but will post additional specs as I get them over the next day or two.

Meanwhile, a couple of pieces of info and comments.

According to our vendor: at 230V the motor is rated at 22A; at 208V the motor is rated at 24V.

We didn't trip on startup, only after about 2.75 hours of operation. After that, we tripped attempting to startup.

It was about 60 degF at night at about 96 degF when the CB tripped for good.

We did an overload test pulling ambient air then putting a metal plate over the inlet and closing the dilution valve. when the system shut down amps were measured at ~33A. this was in the morning. air temp around 80 degF.

Please stay tuned...
 
overload relay question

overload relay question

I'm trying to wrap my arms around this. With an overload relay (identified on the schematic as OL117) in place and set to 28.5A, and the system operating for nearly 3 hours following startup, shouldn't OL117 have failed prior to the CB117 (at 30A) failing?

My assumption (or misconception) was that OL117 was overload protection for the motor while operating and that it was the first line of defense. Can anyone explain how these separate features work within the system?

Thanks,
-JJ
p.s. can i ask the vendor to field modify, or will that jeopardize the UL listing (does UL allow for a field mod like this while retaining panel certification?)?
 
With the OL set for 28.5A, it was a race to see which one tripped first; the OL or the 30A breaker. As I and others said, you were exceeding the allowable current (80%) on that circuit breaker, so the trip values are essentially meaningless. I would also say that if you had a 30A breaker, more than likely they had #10 wire, which would also be too small and likely to overheat. That heat would most likely conduct up to the circuit breaker, more than down to the OLR, so that would also skew the tripping.

All in all, you have an improperly designed circuit. The OEM should pay to correct it.
 
#10 wire is correct

#10 wire is correct

thank you for you reply. i'll ask the question tomorrow.

if anyone else has any additional questions I can put to these folks, please feel free to chime in.

BTW, great message board. I don't have aspirations to be an EE but I am learning more with respect to the questions I need to be asking of my subcontractors.

Greatly appreciated!

-jj
 
Jraef said:
I agree, this sounds like a coordination and design problem. There is something wrong with your breaker sizing and overload heater selection (or setting) or maybe you just don't even have a separate OL relay. Sometimes OEMs think they can get away with having just a circuit breaker as the running overload protection, but although technically not illegal, it is almost impossible to select the correct rating. For example assuming your motor is dual rated for 208 or 230V operation, it would be 25A FLC at 208V. A 30A breaker then is already too small because you can only load it to 80% (24A). But the thermal trip rating is too high to use as an OL device if the FLC was 22A at 230V, because 22 x 1.25 = only 27.5A. You really need a larger breaker so that you don't trip on starting current, i.e. 50A or 60A, then you need a properly selected OL relay.

Post the other info, i.e. the motor nameplate FLA, what type of OL you have (if any) and what the OL heaters are or if adjustable, what the setting is.
Click to expand...
There are a small group of people who even understand what you are talking about. ditto CHECK THE OL SETTINGS!!
 
javajake said:
Hello - I have a 7.5HP 3-ph motor PD blower...The manufacturer of the treatment system originally assumed 230V service and spec'd a 30A thermo-mag breaker: but we're only running at 212V at the panel. The motor CB tripped after about 2 hours of operation. The manufacturer feels that the issue is that due to the low voltage, the system is requiring more amps...how does one properly select a high enough AMP CB without endagering the motor, or the panel?- JJ
Click to expand...

The breaker is only for the short-circuit & ground-fault protection of the circuit, the motor should be protected by overloads. The overload is usually integral with a starter contactor, it is not a breaker.

Make sure the motor is rated for your applied voltage; the manufacture of the motor should qualify this. You may need to increase your voltage by boosting or eliminating voltage drop, if it exists.

Size the breaker as per 430.6(A)(1) & 430.52 at 250% of the Table value in 430.250 (not the nameplate ampere). Given the reason for breaker protection there is no good reason to size it smaller than the maximum allowed, so size it to the max. 60A.

The branch conductors can be sized as per 430.6(A)(1), 430.22 & 310.15 at 125%, so 27.5A = #10cu THHN.

If your motor is not farther than 200' from it's source voltage drop should not be a problem, but check the voltage at the service disconnect and again at the destination while other loads are on to see it there is voltage drop.
 
Actually the breaker is for overload protection of the cable and the O.L.'s are for overload protection of the motor. Your settings should reflect this.
 
zog said:
Actually the breaker is for overload protection of the cable and the O.L.'s are for overload protection of the motor. Your settings should reflect this.
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For a dedicated branch circuit feeding a motor, this is not necessarily true. You are permitted to size the breaker higher than you normally would for a general use branch circuit.

For example, the motor branch circuit is required to be sized for 125% of the motor's FLA. The therm-mag breaker protecting the circuit conductors can be sized up to 250% of the motor's FLA. The therm-mag breaker will not protect the circuit conductors from an overload. The breaker is intended to provide protection for short circuits and ground faults in the conductors and motor.
 
jbt260 said:
For a dedicated branch circuit feeding a motor, this is not necessarily true. You are permitted to size the breaker higher than you normally would for a general use branch circuit.

For example, the motor branch circuit is required to be sized for 125% of the motor's FLA. The therm-mag breaker protecting the circuit conductors can be sized up to 250% of the motor's FLA. The therm-mag breaker will not protect the circuit conductors from an overload. The breaker is intended to provide protection for short circuits and ground faults in the conductors and motor.
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So the conductors are not protected? Excuse my ignorance here but I only work on power systems so this is news to me.
 
zog said:
So the conductors are not protected? Excuse my ignorance here but I only work on power systems so this is news to me.
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Since the conductors are dedicated to the motor, the overload relays in the motor starter protect the upstream conductors from an overload.
 
Jraef said:
I would also say that if you had a 30A breaker, more than likely they had #10 wire, which would also be too small and likely to overheat. That heat would most likely conduct up to the circuit breaker, more than down to the OLR, so that would also skew the tripping.
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The code permits a #10 wire for this motor. Even if we are looking at a 208 volt motor the FLA from Table 430.250 is 24.2 amps and 430.22 requires that the motor circuit conductor have an ampacity of 125% of the table value. This requires a conductor with an ampacity of 30.25 amps. Assuming 75?C terminations, a #10 has an ampacity of 35 amps.
 
jbt260 said:
Since the conductors are dedicated to the motor, the overload relays in the motor starter protect the upstream conductors from an overload.
Click to expand...

Makes sense to me, the only thing dedicated to the stuff I work on is "The south side of the production plant" , or "The hot mill". or "Bldgs 101, 103, and 105"
 
jbt260 said:
Since the conductors are dedicated to the motor, the overload relays in the motor starter protect the upstream conductors from an overload.
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Remember, overloads are phenomena of the load. Shorts and faults are taken care of by the branch circuit device.

The theory is that a direct short on, say, a #12 wire would trip a 50a breaker as quickly as it would a 20a breaker.
 
zog said:
Actually the breaker is for overload protection of the cable and the O.L.'s are for overload protection of the motor. Your settings should reflect this.
Click to expand...

Zog,

I bet you got this but I just want to add my reply to help.

?Overload? is different than ?Overcurrent? in the respect of Definitions. The breaker does provide overcurrent protection just specifically not overload protection. The overload protection for the motor and its branch circuit is provided by the OL?s or the thermo-protection on the motor.

The reason a breaker is sized at 250% is to allow startup (lock rotor), if it is sized too small it may trip. The equipment grounding conductor is sized from the OCP in Table 250.122 but it does not have to be larger than the phase conductor so #10 is fine for EGC on a 60A as well.
 
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