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Thread: Over current protection for fire pumps

  1. #1

    Over current protection for fire pumps

    I have a question regarding overcurrent protection for fire pumps. I was teaching a code class and pulled a question out of a book regarding fire pump overcurrent protection for a 25 HP 208 three phase code letter "C" fire pump.
    I know that 695.4(b)(1) says that the overcurrent device shall be selected to indefinetely carry the locked rotor current of the motor. However it does not tell me where to get the locked rotor current. I know that I can go to table 430.251(b) and get a locked rotor current of 404 amps. I would then be allowed to go to the next standard size of 450 amp overcurrent device. But above table 430.251(b) it tells me what code sections to use this table with and article 695 isn't one of them. The second choice is to go to table 430.7(b) the high end for code letter "C" is 3.99 kva per HP. 3.99 x 1,000 x 25 / 208 x 1.732 = 277 amps and I would go with a 300 amp overcurrent device. I am wondering why these two tables do not correspond with each other. I have seen similar questions and I see table 430.251(b) being used. Which one is to be used if any. I talked about this with my students and told them the best thing to do is get the information from the manufacturer. But if you had this question on a test how is the correct way to proceed? The answer in the book I was using was 450 amp overcurrent device based on table 430.251(b).

  2. #2
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    Quote Originally Posted by HEYDOG View Post
    I have a question regarding overcurrent protection for fire pumps. I was teaching a code class and pulled a question out of a book regarding fire pump overcurrent protection for a 25 HP 208 three phase code letter "C" fire pump.
    I know that 695.4(b)(1) says that the overcurrent device shall be selected to indefinetely carry the locked rotor current of the motor. However it does not tell me where to get the locked rotor current. I know that I can go to table 430.251(b) and get a locked rotor current of 404 amps. I would then be allowed to go to the next standard size of 450 amp overcurrent device. But above table 430.251(b) it tells me what code sections to use this table with and article 695 isn't one of them. The second choice is to go to table 430.7(b) the high end for code letter "C" is 3.99 kva per HP. 3.99 x 1,000 x 25 / 208 x 1.732 = 277 amps and I would go with a 300 amp overcurrent device. I am wondering why these two tables do not correspond with each other. I have seen similar questions and I see table 430.251(b) being used. Which one is to be used if any. I talked about this with my students and told them the best thing to do is get the information from the manufacturer. But if you had this question on a test how is the correct way to proceed? The answer in the book I was using was 450 amp overcurrent device based on table 430.251(b).
    Keeping in mind that the basic concept behind all things related to Fire Pumps is "run to destruction", the more conservative approach would be to use table 430.251(b). But I agree, they don't make it clear. Might just be an oversight in that when they added 695.4(b)(1), they forgot to go back and amend the application note on table 430.251(b).

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    Quote Originally Posted by HEYDOG View Post
    ... I am wondering why these two tables do not correspond with each other. ...
    One is Design Letter, the other is Code Letter. IIRC, the Design Letter has something to do with the motor's torque... not its LRC as with Code Letter.
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    Quote Originally Posted by Smart $ View Post
    One is Design Letter, the other is Code Letter. IIRC, the Design Letter has something to do with the motor's torque... not its LRC as with Code Letter.
    Please go to NFPA-20. In the 2007 edition it is table 9.5.1.1. For 25 HP motors at 460V 3-phase the locked rotor current is 183 amps. Note 9.5.1.4 says ratio the voltage to find the locked rotor current at other voltages so 460/208 = 2.21, so the locked rotor current for a 25 HP pump on 208 is 2.21x183 = 404.7. You can take it from there.

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    Quote Originally Posted by gadfly56 View Post
    Please go to NFPA-20. In the 2007 edition it is table 9.5.1.1. For 25 HP motors at 460V 3-phase the locked rotor current is 183 amps. Note 9.5.1.4 says ratio the voltage to find the locked rotor current at other voltages so 460/208 = 2.21, so the locked rotor current for a 25 HP pump on 208 is 2.21x183 = 404.7. You can take it from there.
    Well the first thing to bring up is if—and that's a fairly big if—the AHJ has adopted NFPA20 in to law, only then does it apply.

    Furthermore, NFPA20 also says "9.5.1.6 Code Letters of motors of all other voltages shall conform with those shown for 460V in Table 9.5.1.1." The table's code letter heading states "F" to and including "J" through "G", as applicable for the horsepower. So a fire pump motor with a Code Letter C would not fall within the applicable motors of this table. I do not know whether such a motor exists that meets the NEMA MG-1, Design B, and fire pump service listing requirement but I also do not see any requirement that a motor be Code Letter F through G conformance. Perhaps that requirement falls under the NEMA Design B standard, or the listing requirements...
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    Quote Originally Posted by Smart $ View Post
    Well the first thing to bring up is if—and that's a fairly big if—the AHJ has adopted NFPA20 in to law, only then does it apply.

    Furthermore, NFPA20 also says "9.5.1.6 Code Letters of motors of all other voltages shall conform with those shown for 460V in Table 9.5.1.1." The table's code letter heading states "F" to and including "J" through "G", as applicable for the horsepower. So a fire pump motor with a Code Letter C would not fall within the applicable motors of this table. I do not know whether such a motor exists that meets the NEMA MG-1, Design B, and fire pump service listing requirement but I also do not see any requirement that a motor be Code Letter F through G conformance. Perhaps that requirement falls under the NEMA Design B standard, or the listing requirements...
    The UL standard for fire pumps is 448, and it references NFPA 20. I'm going to suggest that on that basis, no listed fire pump is going to fall outside the requirements of NFPA 20 on the electric motor side of things. I won't say I'm positive, since I don't feel like dropping $392 for a PDF copy of the standard just to satisfy my curiosity.

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    Quote Originally Posted by gadfly56 View Post
    The UL standard for fire pumps is 448, and it references NFPA 20. I'm going to suggest that on that basis, no listed fire pump is going to fall outside the requirements of NFPA 20 on the electric motor side of things. I won't say I'm positive, since I don't feel like dropping $392 for a PDF copy of the standard just to satisfy my curiosity.
    Hmmm... I hear you on the $$$

    But aside from that, the UL Whitebook paints a slightly different picture than what you are saying. From the 2009 version...

    FIRE PUMP MOTORS (QXZF)


    USE

    This category covers motors intended for use in fire pump systems. These motors are used to drive centrifugal pumps used for fire service.

    PRODUCT MARKINGS

    This equipment is marked as follows:
    1. Manufacturer’s name or trademark
    2. Factory identifier (if produced at more than one factory)
    3. Model or catalog number
    4. Rated voltage
    5. Full-load input amperes or watts (or both)
    6. Rated full-load speed
    7. Rated temperature rise or the insulation system class
    8. Rated ambient temperature
    9. Time rating, or, if it is a continuous duty motor, then “Continuous” or “CONT”
    10. Rated horsepower when 1/8 hp (93 W) or more
    11. Code letter to indicate locked-rotor amperes in accordance with ANSI/NFPA 70, “National Electrical Code,” for an alternating-current motor rated 1/2 hp (373 W output) or more
    12. Secondary volts and full-load amperes, when product is a wound-rotor induction motor
    13. Rated frequency expressed in one of the following terms: hertz (Hz), cycles per second (cps or c/s), ac-dc, (number of cycles)/dc (e.g., 60/dc), or ac only – or direct current; and, for a motor intended for use on a polyphase circuit, number of phases
    14. Winding – straight shunt, stabilized shunt, compound, or series, for a direct-current motor;
    15. Service factor (1.15 or less)
    16. Amperes and horsepower at each speed, for a multi-speed motor other than a shaded-pole or a permanent-split-capacitor motor

    ADDITIONAL INFORMATION

    For additional information, see Pumping Equipment for Fire Service (QVUT) and Fire Protection Equipment (AAFP).

    REQUIREMENTS

    The basic standards used to investigate products in this category are UL 1004, “Electric Motors,” and UL 1004A, “Fire Pump Motors.”

    UL MARK

    The Listing Mark of Underwriters Laboratories Inc. on the product is the only method provided by UL to identify products manufactured under its Listing and Follow-Up Service. The Listing Mark for these products includes the UL symbol (as illustrated in the Introduction of this Directory) together with the word ″LISTED,″ a control number, and the product name “Fire Pump Motor.”
    * * * * * * * * * * * * * * * * * * * * * * * * *
    UL, in performing its functions in accordance with its objectives, does not assume or undertake to discharge any responsibility of the manufacturer or any other party. UL shall not incur any obligation or liability for any loss, expense or damages, including incidental or consequential damages, arising out of or in connection with the use, interpretation of, or reliance upon this Guide Information.
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  8. #8
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    Quote Originally Posted by Smart $ View Post
    Hmmm... I hear you on the $$$

    But aside from that, the UL Whitebook paints a slightly different picture than what you are saying. From the 2009 version...
    Interesting. However, unfortunately I'm the kind of guy who winds up reading the whole dictionary while looking up one word. In this case, I'm going to walk away from the rabbit hole.

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