Wrong rated hp disconnect

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Davebones

Senior Member
Have wrote on here before . Have 150HP motor fed from 200Amp (125HP) rated bus fusible switch. (Correct Bus switch would be a 400 amp )Motor starts and runs no problem as it was installed like this 10 years ago . Will give this to management to decide what to do .Just would like some thought's on why we need to change this to the correct rated switch , legal , risks involved leaving it in , etc ..... Want to show them some thoughts from this forum on why we need to install correct HP rated switch ...
 

skeshesh

Senior Member
Location
Los Angeles, Ca
I think its safe to assume its a 3phase motor at 480V based on your values. Take a look at 430.32(A)(1) and table 430.250. I guess there's a small chance that this motor may fall under "all other motors" (requires only 115% of nameplate FLA) + run at the higher end of the operating voltage (i.e. 490-500 or so) which could possibly justify 200A fusible, but I think chances for this condition are slim. Also take a look at the existing starter, if its rated at 5 it should be protected by a 400A.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I appears your prime concern would be legal (liability).
Obviously it works :) but it doesn't meet 430.110. Were there a failure it would be a violation that could have legal bearing in litigation.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
From my experience, you are now at the mercy of management attitude.
"It works--why change it ! " is most often the attitude. Your having to bring to to managememnts attention to get permission to change it, makes me think the maintenace budget is not that large and you may have a tough sell.
Liability is always a trump card but playing it can put you in disfavor also.
Often they want to kill the messanger :)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
From my experience, you are now at the mercy of management attitude.
"It works--why change it ! " is most often the attitude. Your having to bring to to managememnts attention to get permission to change it, makes me think the maintenace budget is not that large and you may have a tough sell.
Liability is always a trump card but playing it can put you in disfavor also.
Often they want to kill the messanger :)
Its been working fine for all this time, and it is not a serious safety hazard.

I'd be inclined to put it on the list of "things that need to get done but are not a priority".
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Its been working fine for all this time, and it is not a serious safety hazard.

I'd be inclined to put it on the list of "things that need to get done but are not a priority".

Is that the same list as the one replacing that "temporary" feeder we put in 5 years ago :grin:
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
If the switch is not being used to actually start or stop the motor (in other words it is used for no-load break isolation) then a HP rating is not necessary.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If the switch is not being used to actually start or stop the motor (in other words it is used for no-load break isolation) then a HP rating is not necessary.

can you give me a Code reference for that ?
All I can find is in 430 part IX and all I find is be "capable of disconnecting" the the ampere and HP rating requirement.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
can you give me a Code reference for that ?
All I can find is in 430 part IX and all I find is be "capable of disconnecting" the the ampere and HP rating requirement.
You are right that Part IX deals with switches that are being used to disconnect the motor,, and that 430.108 requires all disconnect s to meet the requirements of 430.109. But, an isolation switch does not need to be HP rated per 430.109(E).
 

Davebones

Senior Member
Does 430.6 (A) (1) not say that table 430.250 shall be used to determine the ampere ratings of switches ! If thats the case 150 hp (480) motor disconnect would need to be at least rated for 207 amps . 180 amps ( table 430.250 ) x 1.15 (430.110) = 207 amps . ???????????????? The way I 'm reading this a 200 amp switch would not be correct .????????
 

yired29

Senior Member
IBEW Local Union 701 Electrican

IBEW Local Union 701 Electrican

It is my understanding we use the design letter from the motor to size HP rating of disconnect for design letters F-V. We need to refer to Table 430.7 (B). We will use the locked rotor formula:

LRC = (kVA per HP x 1000 x HP) / (E x 1.73)
LRC = Locked Rotor Current
kVA per HP = Value from Table 430.7 (B) based on design letter of motor
1000 = multiplier to convert from kVA to VA
HP = Horsepower of motor in question
E = Voltage of motor
1.73 = multiplier for 3 phase

For example a 60 HP 460 volt 3 phase design letter N would be as follows. From Table 430.7 (B) kVA per HP for design letter N would be 11.2-12.49 I would use the maximum rating of 12.49. The formula will be:

LRC= (12.49 x 1000 x 60) / (460 x 1.73) = 941.69

Now take 941.69 to Table 430.251 (B) cross reference 460 volts down until you find a amperage rating that meets or exceeds 941.69. The answer would be you need a 150 Amp HP disconnect.

If it were a design letter B it would have been:

LRC = (3.54 x 1000 x 60) / ( 460 x 1.73) = 266.90
Table 430.251 (B) would result in a 40 HP rated disconnect

If we use the LRC formula for design letter A-E we could end up with an HP rating lower than the motor HP.

This would be a violation of 430.111 (A), 430.83 (A) (1)
430.81 requires suitable controllers for all motors
430.82 (A) Shall be capable of interrupting the LRC of the motor
NEC 2005

If I've missed something please set me straight.

Yired29@hotmail.com
LU 701
 

skeshesh

Senior Member
Location
Los Angeles, Ca
You are right that Part IX deals with switches that are being used to disconnect the motor,, and that 430.108 requires all disconnect s to meet the requirements of 430.109. But, an isolation switch does not need to be HP rated per 430.109(E).

I don't see why you assume it's an isolation switch.

If thats the case 150 hp (480) motor disconnect would need to be at least rated for 207 amps . 180 amps ( table 430.250 ) x 1.15 (430.110) = 207 amps . ???????????????? The way I 'm reading this a 200 amp switch would not be correct .????????

You're right, but the item not being per code doesn't mean it hasn't been working and, as noted by Gus, a lot of times people that are handling the budget are willing to overlook issues like this.
 

yired29

Senior Member
IBEW Local Union 701

IBEW Local Union 701

I misread the question the switch would have to have a minimum ampacity of FLC x 115% times the FLC determined by tables 430.247-430.250 430.6 (A) (1). 430.110 (A) If 150 amps were the FLC off of the tables than 150 x 1.15 = 172.5 amps minimum ampere rating and HP rating based on motor code letter and LRC formula and table 430.251 (A) (B) from my first post.

Yired29@hotmail.com
LU701
 

Jomaul

Senior Member
Location
Ocala, Fl
wouldn't you have to go back to the code that it was installed under to figure out if it is a violation? correct me if I am wrong but the code articles that are being cited now are current codes and not codes of 10 years prior. It dosen't appear that anyone has gone back to see what would have been legal at that time.
 

yired29

Senior Member
Once again I made a mistake. We would not us the HP of 150 x 115%. If it is a 150 HP 3 Phase 460 volt motor 430.250 per 430.6 (A) (1) FLC would be 180 amps x 115% = 207 amps mimnimum disconnect ampacity per 430.110 (A). HP rating would have to be at least 150 amps per 430.83 (A) (1) if it is a design letter B,C,D. If it was a design letter N the HP rating would have to be as follows:
(12.49 x 1000 x 150) / (460 x 1.73) = 2354 amps than to 430.251 (B) 460 volts 350 HP disconnect.

Sorry for any confusion.
Yired29@hotmail.com
LU701
 

Davebones

Senior Member
I'm a little confused .Why would we look at (430.83) for the motor disconnect ? Does this section not reference the size of the controller ? This is a 200 amp fusible bus switch ( the rating inside says 125 hp ) that feeds a starter then the motor .Why does this have to be so hard to figure out the correct size of a disconnect .Why doen't they just have the code state the disconnect must be hp rated to match the size of the motor ! We have 5 of these test stands and I can't imagine trying to go to management and tell them we need $ 27,000 for new disconnects ,
 

yired29

Senior Member
I'm a little confused .Why would we look at (430.83) for the motor disconnect ? Does this section not reference the size of the controller ? This is a 200 amp fusible bus switch ( the rating inside says 125 hp ) that feeds a starter then the motor .Why does this have to be so hard to figure out the correct size of a disconnect .Why doen't they just have the code state the disconnect must be hp rated to match the size of the motor ! We have 5 of these test stands and I can't imagine trying to go to management and tell them we need $ 27,000 for new disconnects ,
VII. Motor Controllers
430.81 General.
Part VII is intended to require suitable controllers for all motors

430.111 Switch or Circuit Breaker as Both Controller and Disconnecting Means.
A switch or circuit breaker shall be permitted to be used as both the controller and disconnecting means if it complies with 430.111(A) and is one of the types specified in 430.111(B).

A) General. The switch or circuit breaker complies with the requirements for controllers specified in 430.83, opens all ungrounded conductors to the motor, and is protected by an overcurrent device in each ungrounded conductor (which shall be permitted to be the branch-circuit fuses). The overcurrent device protecting the controller shall be permitted to be part of the controller assembly or shall be permitted to be separate. An autotransformer-type controller shall be provided with a separate disconnecting means.
 
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