Fused disconnect at motor

[anbm]

Per NEC 430, the breaker in electrical panel serving the motor will be sized at 250% of motor FLA. What's about the fused disconnect at motor location, (downstream of the breaker), will the fuse inside disconnect be sized at 125% of motor FLA or higher? Assuming this fuse is served as circuit conductor and
motor overload protection only?

 

[augie47]

IF the fuses are actually being used for OVERLOAD protection (as opposed to integral protection or protecvtion by the controller), the 125% of NAMEPLATE value would be the "normal" setting. See 430.32 for more specific details.

 

[anbm]

IF the fuses are actually being used for OVERLOAD protection (as opposed to integral protection or protection by the controller), the 125% of NAMEPLATE value would be the "normal" setting. See 430.32 for more specific details.

Will the fuse melt when motor inrush current starts?

 

[Canton]

Will the fuse melt when motor inrush current starts?

No. You have Short Circuit/Ground Fault protection and you have Over current/over load protection. (2) different protections

The breaker in the panel you are referring to would be the SC/GF protection.

What is your exact one-line setup with the Motor? Where is your motor controller, etc.?

 

[anbm]

No. You have Short Circuit/Ground Fault protection and you have Over current/over load protection. (2) different protections

The breaker in the panel you are referring to would be the SC/GF protection.

What is your exact one-line setup with the Motor? Where is your motor controller, etc.?

See attached.

 

[augie47]

Your diagram shows a different scenario than what I envisioned from the original post.
You disconnect appears to be protecting the VFD and would be sized for that.
Mortor overload should be taken care of by the VFD>

 

[jim dungar]

Will the fuse melt when motor inrush current starts?

That depends on the time characteristic of the fuse.

 

[anbm]

That depends on the time characteristic of the fuse.

since the fuse and breaker size are different even they belong to the same circuit, this confused me.

 

[jim dungar]

since the fuse and breaker size are different even they belong to the same circuit, this confused me.

Except for instantaneous ones (non time delay), overcurrent devices follow an inverse time vs current operation curve. You need to compare the complete curve for each device to see if they do not overlap.

Many people think that sizing upstream devices 2X as large as downstream devices is sufficient, but it isn't always. For general motor loads I would be most concerned with the upstream devices being able to carry 6X motor FLA for more than 20secs and 10X for roughly 1sec.

 

[ptonsparky]

Where does 430 say 250% of FLA is required for the breaker?

 

[anbm]

Where does 430 say 250% of FLA is required for the breaker?

430.52(C)(1) and table 430.52

 

[augie47]

430.52(C)(1) and table 430.52

"A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used." (250%)
is a bit different from "required to be 250%"
There are countless installations with GFCSC breakers sized less than 250%.

 

[anbm]

"A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used." (250%)
is a bit different from "required to be 250%"
There are countless installations with GFCSC breakers sized less than 250%.

Agree, I should write max. 250% motor FLA. What I was confused, will the fuse (inside disconnect locates near motor) be sized at max. 175% motor FLA per above table or can be 125% motor FLA.

 

[ptonsparky]

Agree, I should write max. 250% motor FLA. What I was confused, will the fuse (inside disconnect locates near motor) be sized at max. 175% motor FLA per above table or can be 125% motor FLA.

The fused disconnect is there for the VFD. Not the motor.

Look at the specs for fuses ahead of the VFD. You will probably find them quite large and maybe a specific type. It will be listed in the manual.