NEC Code few questions.

[reaplaypolo]

Hello,

I have few questions.

Q1) If The motor has 50HP and 65 FLC, Why Field disconnect rating -HP is 50HP. I think 65 HP instead of 50HP.

Motor Field Disconnect
Field disconnect rating - current rating	74.75	A	FD Rating >= 1.15 FLC for single motor [430.110(A)]??
Field disconnect rating - HP	50.00	hp	FD Rating >or = hp rating [430.110(A) Exception]??

Q2) As I know, Motor Starter consists of Motor Contactor and Motor Overload Protection//

Q2-1) Can I assume that Motor controller is the same as Motor Starter or Disconnection SW?

Q2-2) Can we use the same symbol of Motor Starter and Motor Contactor?


Q3) To calculate on the Feeder Conductor Ampacity by using Table 310.15(B)(16), A Question has only gave us for 75 degree. How can we choose Cooper 75 degree from two 75 degree column of the table 310.15(B)(16) eventhough we have only given condition as 75 degree?

Feeder conductor base ampacity	95.25	A	Ampacity >= 1.25*(Imtr,max) + 1.00 *(sum Imtr,fla) + 1.25 * (sum Ires) [409.20] (This is a superset of [430.24])
Feeder conductor temp derating	1.00		(Ambient Tem --> 30 deg C) 30 deg C => derating factor = 1.00 [Table 310.15(B)(2)(a) temp correction factors]
Feeder conductor ampacity Allowing for temperature derating	95.25	A	Ampacity from table = Base ampacity / derating factor [Table 310.15(B)/ Table 310.15 (16)]
Feeder conductor multconductor derating. Derating for Number of conductors	1.00		[Table 310.15(B)(3)(a)] 3 cond (3 phase) => 1.00, 4-6 => 0.8, 7-9 => 0.70, 10-20 => 0.50, 21-30 => 0.45, 31-40 => 0.40, >=41 => 0.35
Feeder conductor ampacity Allowing for multiconductor derating	95.25	A	Ampacity from table = Base ampacity / derating factor [Table 310.15(B)/ Table 310.15 (16)]
Feeder conductor Sizing	3	awg	[Table 310.15(B)(16) - 75 deg C] and [240.4(D) ] (Formerly [Table 310.16]) Note [NEC 240.4(D)(1-7)] the following limits: 18 awg Cu<=7A, 16 awg Cu<=10A, 14 awg Cu<=15A, 12 awg Cu<=20A, 10 awg Cu<=30A. Also see [430.53(D)] 75 dec C connectors assumed. 75 deg C used for all conductors [110.14(C)]

 

[don_resqcapt19]

1) a listed 50 hp disconnect switch is fine for a 50 hp motor per the exception.
2-1) yes
2-2) a motor controller is not the same...the controller is just the contactor and does not have overloads. A starter has a controller and overloads. Only a manually operated controller can be a disconnect.
2-3) Since they are not the same thing, you should not use the same symbol as that would add confusion to the drawings.
3) You simply find the required ampacity (motor full load current from code tables and multiply that by 1.25) and find a conductor that has at least that ampacity on the ampacity table.

 

[reaplaypolo]

Dear Don,

Thank you so much. I was made a clear some of them from my questions.

Based on your answer, I have few more question.

Q1) Regarding to answer for Q1, We need to choose the value (HP) greater than 74.75 A (=1.15 X FLC for single motor/460 V ) in the table 430.250. Right? If I'm okay, we can choose 65HP (480V/74.75A).Right?

Q2) Regarding to Answer for Q3, Finally we got 95.25 A. Next Step we need to find Feeder conductor Sizing (answer :#3) from the Table [Table 310.15(B)(16)]. My point of question is if we want to find #3, we need to know two assumption as 75 degree and Cooper. Right?

Cause In the table, they have two 75 degree column,

Regards
Kim.

 

[augie47]

If I may:
Your switch would need an ampere rating 1.15 X the motor FLA and have a HP rating equal to the motor HP.
In real life that pretty much takes care of itself by manufacturing specs. A switch with a 75 amp rating would be a 100 amp switch and those are listd for 60HP @ 480.
You don't need a 95 amp conductor, simply a conductor rated 81 amps (65 x 1.25 per 430.22). Assuming 75° terminations that would be a #4 Cu.
(I believe the (2) 75° columns you mention are (1) fo AL and (1) for Cu)

 

[don_resqcapt19]

Dear Don,

Thank you so much. I was made a clear some of them from my questions.

Based on your answer, I have few more question.

Q1) Regarding to answer for Q1, We need to choose the value (HP) greater than 74.75 A (=1.15 X FLC for single motor/460 V ) in the table 430.250. Right? If I'm okay, we can choose 65HP (480V/74.75A).Right?

Q2) Regarding to Answer for Q3, Finally we got 95.25 A. Next Step we need to find Feeder conductor Sizing (answer :#3) from the Table [Table 310.15(B)(16)]. My point of question is if we want to find #3, we need to know two assumption as 75 degree and Cooper. Right?

Cause In the table, they have two 75 degree column,

Regards
Kim.

1) If the switch is unfused, it can either have a HP rating equal to or greater than the HP of the motor, or it can have a current rating equal to or greater than the full load motor current. You do not have to look at the current rating where the unfused disconnect has an HP rating. if the switch is fused, it must have an ampere rating of at least 115% of the motor full load current. There is no requirement that the fused switch have a hp rating.
2) For most equipment the conductor is sized using the 75°C ampacities, and the code always assumes a copper conductor unless otherwise stated. The minimum conduct size is 125% of the full load ampacity shown in the motor full load tables in Article 430. A3 phase 480 volt, 50 hp motor has a full load current of 65 amps. The minimum conductor size is 81.25 amps. You pick a conductor from the table that has an ampacity equal to or greater than 81.25 amps.

 

[reaplaypolo]

1) If the switch is unfused, it can either have a HP rating equal to or greater than the HP of the motor, or it can have a current rating equal to or greater than the full load motor current. You do not have to look at the current rating where the unfused disconnect has an HP rating. if the switch is fused, it must have an ampere rating of at least 115% of the motor full load current. There is no requirement that the fused switch have a hp rating.
2) For most equipment the conductor is sized using the 75°C ampacities, and the code always assumes a copper conductor unless otherwise stated. The minimum conduct size is 125% of the full load ampacity shown in the motor full load tables in Article 430. A3 phase 480 volt, 50 hp motor has a full load current of 65 amps. The minimum conductor size is 81.25 amps. You pick a conductor from the table that has an ampacity equal to or greater than 81.25 amps.

Thank you so much!! Don!! I made a clear from your answer for Q2. But
I would like to confirm what you mention for answer from Q1. Is the rule for Branch Circuit instead of Feeder. right?

Again Thanks Don!!

 

[reaplaypolo]

If I may:
Your switch would need an ampere rating 1.15 X the motor FLA and have a HP rating equal to the motor HP.
In real life that pretty much takes care of itself by manufacturing specs. A switch with a 75 amp rating would be a 100 amp switch and those are listd for 60HP @ 480.
You don't need a 95 amp conductor, simply a conductor rated 81 amps (65 x 1.25 per 430.22). Assuming 75° terminations that would be a #4 Cu.
(I believe the (2) 75° columns you mention are (1) fo AOL and (1) for Cu)

Augie47 : Thanks you so much for your sharing about the real field experience!!

 

[reaplaypolo]

1) If the switch is unfused, it can either have a HP rating equal to or greater than the HP of the motor, or it can have a current rating equal to or greater than the full load motor current. You do not have to look at the current rating where the unfused disconnect has an HP rating. if the switch is fused, it must have an ampere rating of at least 115% of the motor full load current. There is no requirement that the fused switch have a hp rating.
2) For most equipment the conductor is sized using the 75°C ampacities, and the code always assumes a copper conductor unless otherwise stated. The minimum conduct size is 125% of the full load ampacity shown in the motor full load tables in Article 430. A3 phase 480 volt, 50 hp motor has a full load current of 65 amps. The minimum conductor size is 81.25 amps. You pick a conductor from the table that has an ampacity equal to or greater than 81.25 amps.

Hi Don,

Sorry for missing question!

So
Q1) If we want to use unfused Disconnect sw, we need to find HP from table 430.250 which should be less than 74.74 A (Current Rating). So this is why 50 HP. or Just Motor Size HP Rating. two approach right?
>= 50HP.

Q2)Motor Controller could be recalling as Motor Contactor or Branch Circuit Motor Field Disconnect SW instead of Motor Starter. Right?
I mean Motor Controller is not the same as Motor Starter which consists of Motor Contactor and Motor Overload Protector.

Thanks

 

[don_resqcapt19]

Thank you so much!! Don!! I made a clear from your answer for Q2. But
I would like to confirm what you mention for answer from Q1. Is the rule for Branch Circuit instead of Feeder. right?

Again Thanks Don!!

It is for the motor disconnect that is required by 430.102(B).

 

[don_resqcapt19]

Hi Don,

Sorry for missing question!

So
Q1) If we want to use unfused Disconnect sw, we need to find HP from table 430.250 which should be less than 74.74 A (Current Rating). So this is why 50 HP. or Just Motor Size HP Rating. two approach right?
>= 50HP.

Q2)Motor Controller could be recalling as Motor Contactor or Branch Circuit Motor Field Disconnect SW instead of Motor Starter. Right?
I mean Motor Controller is not the same as Motor Starter which consists of Motor Contactor and Motor Overload Protector.

Thanks

The disconnect horsepower is from the motor nameplate.