120 Volt AC Motor Fusing

Tim Ganz

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Join Date
Dec 2010
Location
Dallas, Texas
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When running a 120 volt ac motor circuit through a contactor should I fuse L1 and L2 or just L1? I have seen it done both ways but which is correct?

l2 is grounded at the 480x120 volt transformer motor hp 1 HP 12.3 amps
 
Technicality: if it is a 120V system, it would be L and N, in which L could be L1 or L2. If you run L1 and L2 to the motor, it will be 240V.

I understand you have your own source of 120V from a transformer, but if you don't stick to naming conventions, you are asking for trouble later when another set of eyes will see the L1-L2 labeling and assume it is 240V.

But other than that, T Gibbs is right. You can only interrupt the grounded (neutral) conductor if you also simultaneously break the hot. So a 2 pole circuit breaker is acceptable, but two fuses is not.
 
Last edited:
Technicality: if it is a 120V system, it would be L and N, in which L could be L1 or L2. If you run L1 and L2 to the motor, it will be 240V.

I understand you have your own source of 120V from a transformer, but if you don't stick to naming conventions, you are asking for trouble later when another set of eyes will see the L1-L2 labeling and assume it is 240V.

But other than that, T Gibbs is right. You can only interrupt the grounded (neutral) conductor if you also simultaneously break the hot. So a 2 pole circuit breaker is acceptable, but two fuses is not.

I agree fully with this, it can be quite dangerous working with input power, especially if you aren't fully sure what you are working on (calling Neutral L2 leads me to believe this) if you are unsure I would have an electrician or experienced maintenance person look at it before connecting power, so you A don't shock anyone who touches the machine and B don't burn down the building
 
I meant L1 and N. With that said I could use a double pole fuse holder as it's linked to open both L1 and N. or I could just use a single pole and open L1.

Which is the best method andy why? Just curious what others do.
 
Here is the NEC article that you would want to look at:
240.22
"No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:
(1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.
(2) Where required by 430.46 or 430.37 for motor overload protection"
NFPA 70 National Electrical Code
 
Why would you want to put an overcurrent device in the "neutral" conductor? If,somehow, that device malfunctioned, and they do, it will cause an unsafe condition. I have seen breakers lose 1 leg many times.
Personally, I would only run the "neutral" through a breaker or fuse if it were in a hazardous location where code requires it, such as a gas station, and then it would be a switched neutral breaker.
 
120 Volt Wire Color

I have a panel with a 480 x 120 3KVA transformer. It powers a 1 HP 120 Volt AC Motor, A 120 Volt AC Laptop Outlet and it also has a 120 x 24 Volt Dc Power Supply.There are 20120 Volt small solenoids in the field but they will be given power through a relay which is switched by 24 Volts DC.


My plan was to use Black, White , Green on the 120 volts AC in this as it's just AC power not AC controls which would be red?

I guess power to the 2 field solenoids could be red?

All the controls are 24 Volts DC in Blue and white with blue stripe.

Is my plan correct?
 
Here are some code articles to reference that relate to running the N conductor through a contactor.

"Article 404 – Switches
Article 404.2 (B) – Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.
Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.
Article 430 – Motors, Motor Circuits, and Controllers
Section VII Motor Controllers
Article 430.85 – One pole of the controller shall be permitted to be placed in a permanently grounded conductor, provided the controller is designed so that the pole in the grounded conductor cannot be opened without simultaneously opening all conductors of the circuit."

Reference:
NFPA 70: National Electrical Code
 
Fuse only on the Phase as this means the motor has no voltage on it anymore.
The contactor in both, to be sure it stops even when the phase is shorted!!! safety rule.
an Estop must also be connected in L and N
 

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