Hello all . First post will be asking about 430.24 (feeder sizing)

Leatherbark

Member
Location
Beckley, WV
I have been inspecting installations that use dual 200 HP, 460 volt (3 phase) motors. The motors are protected by a single OCPD adjustable (instantaneous) CB. The motor branch circuit conductors are 4/0 (ICEA cables) The feeder is a 350 KCMIL. According to 430.24 the feeder should be a 500 kcmil. Calculations are as follows


200 HP 460 volt FLC= 240x1.25=300amps+240amps for the other motor =540amp feeder ampacity. The owners are using a 350 MCM (ICEA cable) with an ICEA ampacity of 465 amperes. In an underground mine we use the ICEA ampacity charts for a 20 degree ambient. The cables that are used underground are 90 Degree insulated 0-2000 volt. The ICEA chart for this type of cable needed for the above motors is 540 amperes requiring a 500 KCMIL (MCM for us old guys). If I were to use 310.16 the size needed would be 750 KCMIL. Our regulations allow the ICEA chart to be used but I digress.

My question is. In 240.24 there is an exception No. 3 that I have trouble defining. The dual 200 hp motors above are not started simultaneously. There is some delay for the inrush of the first motor. What does Exception No. 3 mean?

Exception 3.
"Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted based on the summation of currents of the motors and other loads to be operated at the same time that results in the highest total current."

Does this exception allow a smaller feeder for this application because the motors are not started together? It seems like it is saying that the feeder size will be determined by just adding the FLC of each motor without the 125% factored in if the motors start separately. These motors are not VFD controlled

Thank you all.

Bob
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
That exception permits a smaller feeder when both motors cannot be running at the same time. it would require an electrical control circuit that does not permit both motors to run at the same time.

As far as the ICEA conductor ampacities...what temperature are they based on and if they are based on something higher than 75°C do your rules eliminate the requirements of 110.14(C)?
 

Smart $

Esteemed Member
Location
Ohio
Staggered starting has no bearing on minimum conductor ampacity if both motors can or will be operated concurrently.

What regulations permit you to use ICEA ampacity while imposing the NEC otherwise?
 

kwired

Electron manager
Location
NE Nebraska
I have been inspecting installations that use dual 200 HP, 460 volt (3 phase) motors. The motors are protected by a single OCPD adjustable (instantaneous) CB. The motor branch circuit conductors are 4/0 (ICEA cables) The feeder is a 350 KCMIL. According to 430.24 the feeder should be a 500 kcmil. Calculations are as follows


200 HP 460 volt FLC= 240x1.25=300amps+240amps for the other motor =540amp feeder ampacity. The owners are using a 350 MCM (ICEA cable) with an ICEA ampacity of 465 amperes. In an underground mine we use the ICEA ampacity charts for a 20 degree ambient. The cables that are used underground are 90 Degree insulated 0-2000 volt. The ICEA chart for this type of cable needed for the above motors is 540 amperes requiring a 500 KCMIL (MCM for us old guys). If I were to use 310.16 the size needed would be 750 KCMIL. Our regulations allow the ICEA chart to be used but I digress.

My question is. In 240.24 there is an exception No. 3 that I have trouble defining. The dual 200 hp motors above are not started simultaneously. There is some delay for the inrush of the first motor. What does Exception No. 3 mean?

Exception 3.
"Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted based on the summation of currents of the motors and other loads to be operated at the same time that results in the highest total current."

Does this exception allow a smaller feeder for this application because the motors are not started together? It seems like it is saying that the feeder size will be determined by just adding the FLC of each motor without the 125% factored in if the motors start separately. These motors are not VFD controlled

Thank you all.

Bob
First question is if this is for mining does NEC even apply?


It appears that you only have one branch circuit overcurrent device - but essentially have what should be two branch circuits. If there is interlocked circuitry to prevent both from running at same time as the mentioned exception says, then you can treat it as one branch circuit.

How high over the setting otherwise needed for a single motor do you need to set that branch device to make sure it will start the second motor?
 

Leatherbark

Member
Location
Beckley, WV
To answer a few of the questions.

  • MSHA regulation 75.513-1 states essentially that electric conductor size is not adequate if it is smaller than what is required by the NEC 1968 version. MSHA Policy for the above standard states that if the power cable is manufactured in accordance with the ICEA (Insulated Cable Engineers Association then the ICEA tables will be used for determining compliance with the regulation. Most all power cables in an underground coal mine are ICEA cables.
  • The 1968 NEC is the only version MSHA can enforce. Enforcing any other version is tantamount to making federal regulation without proposed rules and with public comment. Later versions of the NEC are used to assist in determining compliance because the usually go into greater detail explaining things. Plus if the later codes says its OK then MSHA is usually satisfied.
  • We all know the NEC has a statement that says "does not apply to underground mines" Well MSHA applies 2 parts of the NEC by Code of Federal Regulations 30 CFR 75.513 (ampacity) and 75. 518(short circuit protection). Enforcement of the rest of the NEC is off limits underground. MSHA can enforce all applicable parts of the NEC on the surface area of an underground mine or a surface mine. So basically Article 240, 430 and any other part of the NEC where ampacity/overcurrent applies.
  • I forgot to mention that the 350 mcm feeder OCPD (adjustable set 700-1300%) provides the 460 volt power to a starter a few hundred feet away that contains another OCPD which then feeds both contactors which in turn provides the 460 volt power to each of the 200hp motors via 4/0 ICEA cables. This OCPD in the starter box is basically a switch. FWIW by regulation all three phase power in an underground coal mine is resistance grounded. Ground fault protection required is achieved with ground fault relays and balanced flux relaying. The exception No. 3 in 430.24 got me sort of bumfuzzled. I don't believe whether the motors start together or separately makes a difference. Both motors can become overloaded while they are both running to overtax a smaller than required feeder regardless as to how they start. Starting them in sequence is to stop nuisance tripping from inrush and has nothing to do with using a smaller feeder. I am searching for the true meaning of that exception. The way it is written it can be construed different ways.

Appreciate everyone's input for sure.

Bob
 

Leatherbark

Member
Location
Beckley, WV
That exception permits a smaller feeder when both motors cannot be running at the same time. it would require an electrical control circuit that does not permit both motors to run at the same time.

As far as the ICEA conductor ampacities...what temperature are they based on and if they are based on something higher than 75°C do your rules eliminate the requirements of 110.14(C)?

Thank you Don

Both motor do run at the same time they just start several seconds apart. The ICEA cables have 90 degree insulation and are used in a 20 degree ambient. When these cables are used outside on the surface we use an ICEA derating table. (.93). If I understand 110.14 C it matters not that the cable is 90 Degree if the terminals are of a lower value?

Bob
 

kwired

Electron manager
Location
NE Nebraska
110.14(C) basically means if you have 75C terminations on equipment, then the attached conductors need to be minimum sized based on 75C ampacity chart, even if their insulation is 90C. We can use 90C ampacity values for other adjustments to other conditions that conductor may encounter like if it is in a cable or raceway that has more then three current carrying conductors - we can make adjustments from 90C values - but adjustments taken can not reduce size to less then what is still required by the 75C termination.
 

Leatherbark

Member
Location
Beckley, WV
Thank you KWired

110.14 sure has changed since 1968. 110-14 of the 1968 version has one paragraph which essentially states that "connection of conductors to terminal parts shall insure a thoroughly good connection without damaging the conductors". That is essentially it in a nutshell. It does have precautions about mixing copper and aluminum conductors.

Bob
 

Leatherbark

Member
Location
Beckley, WV
Isn't it wonderful how the more you read something the plainer the meaning. I can see now that the exception is for only if interlocking circuitry allows only one motor can be ran at a given time the exception applies. Therefore the largest FLC x 1.25 plus the other motor's FLC applies in my case for sure

Bob
 

kwired

Electron manager
Location
NE Nebraska
I was born in 1968, but started using 1968 NEC immediately:cool:. Just kidding I never heard of NEC until started using it in 1987.

I don't know when 90C conductors started to become normal, but don't think they were available as common "building conductors" so to speak in 1968, or even for another 10 years or so, which means some changes to wording of NEC had to occur later if they didn't exist yet.
 
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