Wire Calculation For Motor

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Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Basic wiring question..:ashamed1:

OK, if you have a disconnect feding a motor from a 1p/20a ckt in a 120/208v 3 phase panel how do you know how manywires are needed....hots, neutral/ground?

Do you use the disconnect switch to determine, the panel info etc. In other words do you have enough info listed above to determine that.

If it is a 1p/20a it could only have ONE hot...correct:dunce:.....2pole would be TWO hots etc. etc.?????
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Basic wiring question..:ashamed1:

OK, if you have a disconnect feding a motor from a 1p/20a ckt in a 120/208v 3 phase panel how do you know how manywires are needed....hots, neutral/ground?

Do you use the disconnect switch to determine, the panel info etc. In other words do you have enough info listed above to determine that.

If it is a 1p/20a it could only have ONE hot...correct:dunce:.....2pole would be TWO hots etc. etc.?????

There is invariably one more piece of information, the voltage. But either way it is still always just 2 wires for a single phase motor. If it is a motor controller, starter etc. that may not be true.
 

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
There is invariably one more piece of information, the voltage. But either way it is still always just 2 wires for a single phase motor. If it is a motor controller, starter etc. that may not be true.

So what changes if it is 120/208 or 277/480? Two wire....a hot and neutral???
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
So what changes if it is 120/208 or 277/480? Two wire....a hot and neutral???

It is not, it is, 120 or 208 or 240 or 277 or 480 or other odder voltages. In all cases it is 2 wires for single phase plus the ground. Voltage tells you whether it is two hots or a hot and neutral. Well not really, on a submarine 120 volts is two hots. That is why they hire electricians not plumbers.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Basic wiring question..:ashamed1:

OK, if you have a disconnect feding a motor from a 1p/20a ckt in a 120/208v 3 phase panel how do you know how manywires are needed....hots, neutral/ground?

Do you use the disconnect switch to determine, the panel info etc. In other words do you have enough info listed above to determine that.

If it is a 1p/20a it could only have ONE hot...correct:dunce:.....2pole would be TWO hots etc. etc.?????

You have your own answer in your question, see the highlighted text above. 1p = "1 pole".

If you feed one pole from a 208Y120V panel, that is 120V. But for the circuit to be complete, the 120V load needs a neutral connection. However, you do NOT switch the neutral circuit, you are not allowed to (except under certain specific conditions). So you will have 3 wires going to the load: a hot, through the switch (black wire), an unbroken neutral (white wire) and a green or bare ground wire.
 

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Ok, another side note. So if the same motor was fed from a 277/480V panel with a 3 pole breaker but now the same motor is fed from a 120/208 panel...why is it now a SINGLE POLE breaker. Is it because of the voltage and why??? Sorry for being a PIA
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Ok, another side note. So if the same motor was fed from a 277/480V panel with a 3 pole breaker but now the same motor is fed from a 120/208 panel...why is it now a SINGLE POLE breaker. Is it because of the voltage and why??? Sorry for being a PIA

Not a valid scenario or at least it would not have been fed from 3 poles of a 3 pole breaker. A single phase motor always requires two wires from the source regardless of the voltage. A three phase motor always requires 3 wires from the source regardless of voltage. You really need to take a basic electricity course. You have a curious mind and these questions are easily understood with basic theory. Try your local community college, or even a correspondence course.
 

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Not a valid scenario or at least it would not have been fed from 3 poles of a 3 pole breaker. A single phase motor always requires two wires from the source regardless of the voltage. A three phase motor always requires 3 wires from the source regardless of voltage. You really need to take a basic electricity course. You have a curious mind and these questions are easily understood with basic theory. Try your local community college, or even a correspondence course.


Thanks...I will persue.
 

cadpoint

Senior Member
Location
Durham, NC
In your mind Draw a single line diagram left to right; left terminal is hot, middle is a device, the other wire is neutral to terminal point; apply voltage.
 
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