Size Disconnect & Fuse required for 460V 3ɸ 50HP Wound Rotor, continuous duty

[Daniel Palmquist]

Size Disconnect & Fuse required for 460V 3ɸ 50HP Wound Rotor, continuous duty

Size Disconnect & Fuse required for 460V 3ɸ 50HP Wound Rotor, continuous duty

A) 100A disconnect, 90A fuse

B) 80A disconnect, 85A fuse

C) 100A disconnect, 65A fuse

D) 60A disconnect, 65A fuse

From 2008 NEC please and please provide articles.

 

[Smart $]

Why?

Looking strictly from a Code-compliance perspective, two choices are definitely non-compliant prior to energizing the circuit. Upon energizing, the other two choices could also be non-compliant.

 

[Daniel Palmquist]

This is how I am considering to work it; please let me know if this is the right way...

Table 430.250: 460V - Column, 50HP - Row shows (65) Full Load Current. 430.110 states disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115% of full-load current rating of the motor therefore 65 x 1.15 = 74.75 then per 240.6? standard rating would be 80 but for the fuse it would need to be sized according to the conductor or load which would be 81.25 A and again according to 240.6 Standard sizing would need to be 90 so the answer I think would be correct would be A) 100A Disconnect, 90A Fuse...?

 

[T.M.Haja Sahib]

This is how I am considering to work it; please let me know if this is the right way...

Table 430.250: 460V - Column, 50HP - Row shows (65) Full Load Current. 430.110 states disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115% of full-load current rating of the motor therefore 65 x 1.15 = 74.75 then per 240.6? standard rating would be 80 but for the fuse it would need to be sized according to the conductor or load which would be 81.25 A and again according to 240.6 Standard sizing would need to be 90 so the answer I think would be correct would be A) 100A Disconnect, 90A Fuse...?

Just out of curiosity,have you considered that omnipotent 125% factor for continuous duty?

 

[Daniel Palmquist]

According to 430.32 (A) (1) it would still be 115% since it is not 'service factor 1.15' or 'marked temperature rise 40 degrees Celsius' therefore by default it would be 'All other motors' thus 115%.

Couldn't this disconnect be anything over the minimum rating (FLC x 115%) since no where in part IX. does it mention a maximum Ampere Rating or Interrupting Capacity. What really confuses me is the Fuse. Is it really 115% also or something else?

 

[david luchini]

What really confuses me is the Fuse. Is it really 115% also or something else?

See Table 430.52. It seems to me a 100A fuse would be acceptacle, but I don't see that as an option in your list.

 

[kwired]

This is how I am considering to work it; please let me know if this is the right way...

Table 430.250: 460V - Column, 50HP - Row shows (65) Full Load Current. 430.110 states disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115% of full-load current rating of the motor therefore 65 x 1.15 = 74.75 then per 240.6? standard rating would be 80 but for the fuse it would need to be sized according to the conductor or load which would be 81.25 A and again according to 240.6 Standard sizing would need to be 90 so the answer I think would be correct would be A) 100A Disconnect, 90A Fuse...?

Minimum disconnectin means must be 74.75 amps as you indicated. This is the rating of the disconnect that may not even have fuses in it. So likely the only thing you will find that exceeds this as far as NEMA safety switch type disconnects go is a 100 amp switch. The only correct answer for this part is it must be at least 74.75 amps.

The fuse is branch circuit short circuit and ground fault protection. It is sized per 430.52. For a wound rotor motor T430.53 says 150% of full load current for both time delay and non time delay fuses. That comes to 97.5 amps - you are allowed to go to next higher standard size which is 100. Exception says if that does not allow the motor to start it can be increased.

I don't see any of the possible answers to be correct.

 

[Jraef]

You also must understand that the code discusses MINIMUMS, not exactly proper design. Article 110 discusses the concept of "suitable for intended use" (guessing on the exact words here) and you will find that disconnect switches have HP ratings that may or may not correspond to amp ratings. In other words your 60A disconnect is not likely to carry a 50HP rating anyway. And the 80A disconnect will likely be one of those rotary IEC style that may not be rated for 50HP either, plus it would not be UL98 rated, meaning it has to have another OCPD ahead of it anyway. I hate to say it, but you can usually get the correct info on the disconnect switch size by reading the catalog for the switch. Fuse sizing is then per 430.52. It isn't any more complicated than that really.

 

[kwired]

You also must understand that the code discusses MINIMUMS, not exactly proper design. Article 110 discusses the concept of "suitable for intended use" (guessing on the exact words here) and you will find that disconnect switches have HP ratings that may or may not correspond to amp ratings. In other words your 60A disconnect is not likely to carry a 50HP rating anyway. And the 80A disconnect will likely be one of those rotary IEC style that may not be rated for 50HP either, plus it would not be UL98 rated, meaning it has to have another OCPD ahead of it anyway. I hate to say it, but you can usually get the correct info on the disconnect switch size by reading the catalog for the switch. Fuse sizing is then per 430.52. It isn't any more complicated than that really.

True about not needing to calculate disconnect size - normally max HP is marked on switch and is easier to just go by that instead of doing any calculations. You are not likely to have a calculation come up with a larger switch than you will get just selecting from switch data only. And if it is larger there is exception that says unless otherwise marked - or something very close to that.

 

[Daniel Palmquist]

Thanks I think I have my answer.

 

[hankhill]

Isn't the correct answer 125 amp fuses in a 200 amp disconnect?

From Table 430.250 a 3PH 50HP 460V motor full load current is 65 amps.

From Table 430.52, squirrel cage motor, multiply the full load current by 1.75: 1.75 x 65 = 113.75

From 240.4(B) Language that allows the selection of the next higher standard overcurrent device: 113.75 amps, use 125 amp fuses. Use a 200 amp disconnect.

I think this is the correct way to size these items, please let me know if I'm wrong.

 

[david luchini]

Isn't the correct answer 125 amp fuses in a 200 amp disconnect?

From Table 430.250 a 3PH 50HP 460V motor full load current is 65 amps.

From Table 430.52, squirrel cage motor, multiply the full load current by 1.75: 1.75 x 65 = 113.75

From 240.4(B) Language that allows the selection of the next higher standard overcurrent device: 113.75 amps, use 125 amp fuses. Use a 200 amp disconnect.

I think this is the correct way to size these items, please let me know if I'm wrong.

Hank, you missed a small detail in the OP

Size Disconnect & Fuse required for 460V 3ɸ 50HP Wound Rotor, continuous duty

Per Table 430.52, the maximum fuse size for a wound rotor motor is 150%. 1.5*65=97.5. A 100A fuse would be the maximum permitted fuse size.

 

[hankhill]

David,

You're exactly right. That "small" detail I missed is glaringly obvious in the original post. Thanks for letting me know.