Motors

[jonny1982]

Still confused about sizing protection for motors.

Does this make sense? A 60 HP motor, 3 phase, 480 volt. This is how it is installed. Circuit breaker is 250A. Wire size is #3 copper. Fuse is 100A. Out of panel to fused disconnect, then into motor.

Wire is not rated for 250A. Is this generally speaking how motors are done?

 

[augie47]

Generally speaking, I would have a problem with the install.
Primarily with the 250 amp breaker.
Ignoring the fuse for a moment, 430.52 would have a "normal" max breaker rating of 250% of the Table 430.250 FLA of 77 amps or 193 amps allowing you to use a 200 amp breaker (larger than 200 is allowable but under under certain conditions spelled out in 430.52).

The #3 wire to the motor is per Code (430.22) , the 100 amp fuse is o.k. although possibly undersized as 150 amp would be allowable.

There could be a difference of opinion as to the proper wire size between the breaker and the fusible disconnect. Were the disconnect non-fuse the #3 would be fine as the breaker would be branch circuit short-circuit/ground fault protection.
With the fusible switch some might allow the #3 as permissible by the motor feeder rule, some might say it should be sized by the breaker.

 

[ptonsparky]

Looks pretty close, although the 250 amp CB is really stretching it. Not sure if you can go next size up in this case. Do you have overcurrent protection besides the 100 amp fuse?

 

[jonny1982]

Looks pretty close, although the 250 amp CB is really stretching it. Not sure if you can go next size up in this case. Do you have overcurrent protection besides the 100 amp fuse?

just the 250 amp cb and the fuses

 

[jonny1982]

Looks pretty close, although the 250 amp CB is really stretching it. Not sure if you can go next size up in this case. Do you have overcurrent protection besides the 100 amp fuse?

Lets put it this way. You have a 25 HP, 3 phase, 480 volt motor. Generally speaking, how would you electrically install it? Please include explanations.

Thanks

 

[Kurta]

Still confused about sizing protection for motors.

Does this make sense? A 60 HP motor, 3 phase, 480 volt. This is how it is installed. Circuit breaker is 250A. Wire size is #3 copper. Fuse is 100A. Out of panel to fused disconnect, then into motor.

Wire is not rated for 250A. Is this generally speaking how motors are done?

I assume your stating the # 3 Cu. is only on the load side of the 100 Amp disconnect?
and the 100 Amp fuse is protecting the load & #3 Cu. Which is fine as # 3 Cu is rated at 100
amps if it's THHN. The Line side conductor needs to be rated at 250 Amps if that is the breaker it's fed by.

 

[jonny1982]

I assume your stating the # 3 Cu. is only on the load side of the 100 Amp disconnect?
and the 100 Amp fuse is protecting the load & #3 Cu. Which is fine as # 3 Cu is rated at 100
amps if it's THHN. The Line side conductor needs to be rated at 250 Amps if that is the breaker it's fed by.

Lets put it this way. You have a 25 HP, 3 phase, 480 volt motor. Generally speaking, how would you electrically install it? Please include explanations.

Thanks

 

[ptonsparky]

Lets put it this way. You have a 25 HP, 3 phase, 480 volt motor. Generally speaking, how would you electrically install it? Please include explanations.

Thanks

Look it up in 430!!

I would start with the 25 HP motor current from the tables in 430. Take that times 1.25. Look for a conductor with an ampacity of at least that under the 75 deg column for wire amp. You are allowed a max CB of 250% of the motor current rating and up to 300% , IIRC, under special conditions. Ignore that. The CB is for short circuit protection of the wires. Motor overload protection is normally sized at 125% of the Nameplate FLA, not the tables in the Code book. Hard starting motors may require larger fuses at start and that is allowed. I personally use a motor starter with overloads. I have seen to many single phased motors when 1 fuse blows and the others don't.

Simplified but that should get you going.

 

[jonny1982]

Look it up in 430!!

I would start with the 25 HP motor current from the tables in 430. Take that times 1.25. Look for a conductor with an ampacity of at least that under the 75 deg column for wire amp. You are allowed a max CB of 250% of the motor current rating and up to 300% , IIRC, under special conditions. Ignore that. The CB is for short circuit protection of the wires. Motor overload protection is normally sized at 125% of the Nameplate FLA, not the tables in the Code book. Hard starting motors may require larger fuses at start and that is allowed. I personally use a motor starter with overloads. I have seen to many single phased motors when 1 fuse blows and the others don't.

Simplified but that should get you going.

So is it ok to use #3 wires on a 250 amp breaker if it has a 100 amp fuse in front of it?

 

[petersonra]

The tap rules would appear to apply to the run between the CB and the fused disconnect. IMO, depending on the details, it could be NEC compliant.

 

[Jraef]

As I see it, what you are apparently not understanding is, why is it that I can use a 250A breaker on conductors for a motor circuit that are not rated to carry 250A? Is that correct? If so, here's the deal-e-o...

BECAUSE it is a SINGLE motor branch circuit, you are REQUIRED to provide running thermal overload protection for that motor, usually either in the form of an "overload relay" or on small motors, embedded thermal protection switches (called "Klixons" because that's a common brand name) , but only if the motor nameplate SPECIFICALLY says it is "Thermally Protected". Either way, the thermal protection device for the MOTOR is understood to also provide thermal over CURRENT protection for the conductors going to that motor. The OL relay will not however provide Branch Short Circuit and Ground Fault Protection, so that function must be provided by fuses or a circuit breaker. THEN, because the starting current for a motor can be very very high (albeit for a very short amount of time), you are allowed to OVER SIZE that SC/GFP device within certain limits. You size your conductors for a minimum of 125% of the motor HP FLC from table 430.150 of the NEC, which says 77A for a 60HP motor, so 96.25A minimum, which, not knowing anything else, would fall into the area of #3 cable rated at 75C. But technically, although you have sized your conductors for 100A, it is the running overload protection for that motor which is also protecting those conductors, the 250A breaker would ONLY be protecting everything from Short Circuit.

What others have questioned is that the basic rule says the CB can be sized at 250% of the motor FLC, so 192.5A, rounded up to the next size makes it 200A. The only time you can go higher is if you PROVE to an AHJ that a 200A breaker will not hold in under starting current conditions, which is unlikely. So yes, the breaker SHOULD have been 200A, not 250A.

The next question is, why have you put in 100A fuses AND a 250A circuit breaker? Do you not have a motor starter with an overload relay? If not, how is the motor turning on and off and what is protecting it? Are you using the disconnect switch for that, and assuming the 100A fuses will act as the motor thermal overload protection? That's remotely possible to do but much much trickier to determine if it will do so properly to protect the motor and not clear on starting current. ASSuming is not allowed.