I am being told that if the motor branch circuit calculation for TW wire comes out to 30.25 amps that I have to use #8 wire. I thought you could drop the .25 amp as per 220.2 B, and use #10. Also the same person tells me that you can use 220.2 B to drop small percentages of Kw when doing oven calculations. Can anyone confirm or correct these?
dropping fractions fron calculations
- Thread starter dok
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- Illinois
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- retired electrician
Re: dropping fractions fron calculations
Article 220 is for load calculations and the rules there do not apply to the wire sizing rules in Article 430. The in 430 says that the conductors shall have an ampacity "not less than". In your example, you must use a conductor with an ampacity of at least 30.25 amps and #10 TW does not have that ampacity.
Don
Article 220 is for load calculations and the rules there do not apply to the wire sizing rules in Article 430. The in 430 says that the conductors shall have an ampacity "not less than". In your example, you must use a conductor with an ampacity of at least 30.25 amps and #10 TW does not have that ampacity.
Don
Re: dropping fractions fron calculations
Aticle 220 is for branch circuit, feeder and service calculations. Why does the calculation for the branch circuit supplying a motor not fall under these rules? Aren't all calculations load calculations that are then used to determine wire size, overload protection, short circuit and ground fault protection, etc?
Aticle 220 is for branch circuit, feeder and service calculations. Why does the calculation for the branch circuit supplying a motor not fall under these rules? Aren't all calculations load calculations that are then used to determine wire size, overload protection, short circuit and ground fault protection, etc?
- Location
- Illinois
- Occupation
- retired electrician
Re: dropping fractions fron calculations
You are not doing a load calculation, you are sizing conductors. That is not the same thing.
Don
You are not doing a load calculation, you are sizing conductors. That is not the same thing.
Don
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- Windsor, CO NEC: 2017
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- Service Manager
Re: dropping fractions fron calculations
Think of this another way: you are sizing conductors for a branch circuit. Has there been zero voltage drop in the system to this point? As the volts go down, the amps go up.
If your volts at the motor have dropped below the nominal, then your motor is going to pull more than 30.25 amps. Why undersize the conductors?
If this is a test question, then the people writing the test aren't going to think that way either.
Think of this another way: you are sizing conductors for a branch circuit. Has there been zero voltage drop in the system to this point? As the volts go down, the amps go up.
If your volts at the motor have dropped below the nominal, then your motor is going to pull more than 30.25 amps. Why undersize the conductors?
If this is a test question, then the people writing the test aren't going to think that way either.
- Location
- Massachusetts
Re: dropping fractions fron calculations
Sometimes the current will drop with the voltage.
We could see a 700 post thread on that subject.
Sometimes the current will drop with the voltage.
P
Physis 2
Guest
Re: dropping fractions fron calculations
In another thread I showed how the right capacitance in series can make the motor do the opposite of what it would do without it. ie., if the voltage drop caused the motor to use .5 more amps, the right capacitance could cause it to use .5 amps less.
And George, Bob doesn't like it when motor current increases due to voltage drop!
In another thread I showed how the right capacitance in series can make the motor do the opposite of what it would do without it. ie., if the voltage drop caused the motor to use .5 more amps, the right capacitance could cause it to use .5 amps less.
And George, Bob doesn't like it when motor current increases due to voltage drop!
- Location
- Massachusetts
Re: dropping fractions fron calculations
This subject was my first big battle when I started posting, I stated that the current always increases when the voltage supplying a motor drops.
Well it was also a learning experience, if I remember right it was Roger that filled me in, although it took some hard work for it to sink in.
If the mechanical output of the motor stays constant and the voltage drops the current must increase. If the mechanical output drops with the voltage the current may drop until the motor stalls.
Originally posted by Physis 2:
And George, Bob doesn't like it when motor current increases due to voltage drop!
This subject was my first big battle when I started posting, I stated that the current always increases when the voltage supplying a motor drops.
Well it was also a learning experience, if I remember right it was Roger that filled me in, although it took some hard work for it to sink in.
If the mechanical output of the motor stays constant and the voltage drops the current must increase. If the mechanical output drops with the voltage the current may drop until the motor stalls.
- Location
- Lockport, IL
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- Retired Electrical Engineer
Re: dropping fractions fron calculations
The motor's output power is determined from whatever mechanical device is being driven by the motor. I can't see how lowering the applied voltage can cause the motor's output power to drop. If there is a separate control mechanism that will sense the applied voltage and alter the mechanical load as voltage rises and falls, then I would accept this statement. But I don't know of an application that requires such a control mechanism.
- Location
- Massachusetts
Re: dropping fractions fron calculations
Charlie a fan is one such application where reduced voltage to the motor will result in less mechanical load.
The fan will spin slower and move less air (or whatever).
Another application could be a centrifugal pump.
Now take a positive displacement pump maintaining a constant pressure (like a hydraulic pump) or a vertical conveyor lifting an unchanging amount of weight and IMO these are the types of loads that will cause the current to increase if the voltage drops at all.
[ November 07, 2005, 04:53 PM: Message edited by: iwire ]
Charlie a fan is one such application where reduced voltage to the motor will result in less mechanical load.
The fan will spin slower and move less air (or whatever).
Another application could be a centrifugal pump.
Now take a positive displacement pump maintaining a constant pressure (like a hydraulic pump) or a vertical conveyor lifting an unchanging amount of weight and IMO these are the types of loads that will cause the current to increase if the voltage drops at all.
[ November 07, 2005, 04:53 PM: Message edited by: iwire ]
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- Massachusetts
Re: dropping fractions fron calculations
Keep in mind I am not taking about throwing a rheostat on a typical motor and chopping the voltage by 50%.
I am thinking more in the range of typical voltage drop. 5% 10% maybe 15%.
I understand at some point the motor will not be able to keep turning and we will end up with LRC.
Keep in mind I am not taking about throwing a rheostat on a typical motor and chopping the voltage by 50%.
I am thinking more in the range of typical voltage drop. 5% 10% maybe 15%.
I understand at some point the motor will not be able to keep turning and we will end up with LRC.
P
Physis 2
Guest
Re: dropping fractions fron calculations
I've spent some time learning more about motors because of this subject and I still have a ways to go.
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