Sizing a 3 pole 100 amp breaker

[patricknola]

Hey Guys, I'm trying to understand sizing conductors for a 3 pole 100 amp breaker. Does each pole protect 33 1/3 amps or is it a total of 100 amps on any leg. I am wiring a vfd that feeds 3 motors that pump domestic water in a hospital. The panel schedule calls for a 3 pole 100 amp breaker but each motor carry's 17.7 amps at 480 volts.

 

[Skinny72]

Sizing a 3 pole 100 amp breaker

Each leg is protected at a 100 Amps


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[GoldDigger]

Hey Guys, I'm trying to understand sizing conductors for a 3 pole 100 amp breaker. Does each pole protect 33 1/3 amps or is it a total of 100 amps on any leg. I am wiring a vfd that feeds 3 motors that pump domestic water in a hospital. The panel schedule calls for a 3 pole 100 amp breaker but each motor carry's 17.7 amps at 480 volts.

A three pole 100A breaker will only open if it sees an overload (sufficiently more than 100A) on at least one pole.

So 100-100-100 will not trip it, while 150-100-100 will.
So, it is 100A per pole or leg. That clearly refers to current in three wires. The painfully similar wording of 100A per phase is so ambiguous as to be useless.

 

[infinity]

What is the input rating of the VFD?

 

[Smart $]

What is the input rating of the VFD?

Also need to know if the manufacturer's instructions specify a particular short-circuit, ground-fault protection device and rating.

Doesn't matter much under 2011, which is what I believe the OP is subject to, but 2014 added wording specifically for sizing the supply OCPD and it has carried to 2017 with only a slight modification [430.130; 430.131]. I believe the addition of these sections clarifies the intent for prior cycles.

 

[patricknola]

i will check the input rating of the vfd tomm. at jobsite but I was under the impression that the vfd was the ocpd.

 

[topgone]

i will check the input rating of the vfd tomm. at jobsite but I was under the impression that the vfd was the ocpd.

You size your OCPD based on the VFD specs, not the driven motor. The VFD settings will then be based on the driven motor capacities.
OCPD--->VFD--->motor

 

[infinity]

Also the conductors to the VFD are sized according to 125% of the input current of the drive.

 

[Jraef]

The VFD nameplate itself will not show the OCPD requirement in all likelihood, you will need to read that in the instruction manual or technical data. It's important to not guess, because some drives are not listed behind circuit breakers, you may need to use fuses.

 

[kwired]

i will check the input rating of the vfd tomm. at jobsite but I was under the impression that the vfd was the ocpd.

Modern drives essentially are overcurrent protection - for the load side conductors as well as overload protection for a single motor, since you have multiple motors each one will need separate overload protection. You still need overcurrent protection on the supply circuit to the drive, and instructions may specify a certain type and setting.