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Thread: Standby generator and article 220.87 sizing

  1. #1

    Standby generator and article 220.87 sizing

    I am installing a standby 20Kw LP generator using a 200amp (non load shedding) service entrance automatic transfer switch on a home with an existing 200amp panel. Our local inspector informed me that the generator needs to have the capacity to support everything in the 200amp service panel - referencing the 2008 NEC code changes. I would like to use a load calculation (article 220.87) to comply. Provided figures from the local electric company prove the highest month usage was 59KWH per day in a 30 day period last month. Can I use that figure to equate a 15 minute useage to be .6KWH and do you think this satisfies the requirement as stated in this code change?

    Here is the change :
    702.5 — CAPACITY AND RATING
    The sizing for Optional Standby Power Systems is now based on the type of transfer switch used; manual versus automatic.

    (A) Available Short-Circuit Current. Optional standby system equipment shall be rated for the maximum available short-circuit current at its terminals.

    (B) System Capacity. The calculated load on the standby source shall be in accordance with Art. 220 or by a method approved by the authority having jurisdiction.

    (1) Manual Transfer Equipment. The optional standby power source shall have adequate capacity for all equipment intended to operate at one time. The user of the optional standby system selects the loads to be connected to the system.

    Author's comment: When a manual transfer switch is used, the user of the optional standby system selects the loads to be connected to the system, which determines the system size.

    (2) Automatic Transfer Equipment.

    (a) Full Load. The optional standby power source shall have adequate capacity to supply the full load transferred.

    Author's comment: If an automatic transfer switch is used in an optional system, the power source — typically a generator — must be capable of supplying the full load transferred. The load is determined by using Art. 220 as a basis on system sizing or an alternate method approved by the AHJ.

    For existing facilities, the optional standby source can be sized to the maximum demand data available for one year or the average power demand of a 15-minute period over a minimum of 30 days [220.87].

    This section was extensively revised and new subsections added in response to the growth of generator installations and the concern about the sizing of an optional standby source that uses automatic transfer switching.

    If an automatic transfer switch is used in an optional system, the power source — typically a generator — must be capable of supplying the full load transferred. The load is determined by using Art. 220 as a basis on system sizing or an alternate method approved by the AHJ. However, the conditions are not the same for optional standby power supply when a manual transfer switch is used. In this case, the user of the optional standby system selects the loads to be connected to the system, which determines the system size.

  2. #2
    Join Date
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    Massachusetts
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    Why is the inspector telling you that the generator needs to be sized to handle the whole house? He may be telling you to size the ATS the same as the 200a main breaker.

    - JWC

  3. #3
    Join Date
    May 2005
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    Richmond, Virginia
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    Trac, welcome to the zoo! :smile:

    Are there loads you can shed to bring down the demand? For example, HVAC is easy; all you have to do is interrupt the 24v red wire with an auxillary contact.

    Quote Originally Posted by JWCELECTRIC View Post
    Why is the inspector telling you that the generator needs to be sized to handle the whole house? He may be telling you to size the ATS the same as the 200a main breaker.
    No, it says standby source.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #4
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    Quote Originally Posted by tractormechanic View Post
    I am installing a standby 20Kw LP generator using a 200amp (non load shedding) service entrance automatic transfer switch on a home with an existing 200amp panel. Our local inspector informed me that the generator needs to have the capacity to support everything in the 200amp service panel - referencing the 2008 NEC code changes. I would like to use a load calculation (article 220.87) to comply. Provided figures from the local electric company prove the highest month usage was 59KWH per day in a 30 day period last month. Can I use that figure to equate a 15 minute usage to be .6KWH and do you think this satisfies the requirement as stated in this code change?.
    NO.
    220.87 is referring to the maximum KW demand over a 15 minute period for 30 days. KWH's has nothing to do with the maximum demand.

    (B) System Capacity. The calculated load on the standby source shall be in accordance with Art. 220 or by a method approved by the authority having jurisdiction..
    This is pretty straight forward. To determine the source requirements, you must calculate the load using NEC Art 220

    (1) Manual Transfer Equipment. The optional standby power source shall have adequate capacity for all equipment intended to operate at one time. The user of the optional standby system selects the loads to be connected to the system.

    (2) Automatic Transfer Equipment.

    (a) Full Load. The optional standby power source shall have adequate capacity to supply the full load transferred.
    Most of the time a new panel is installed with the selected load to be transferred. Your installation is transferring the entire household load.

    For existing facilities, the optional standby source can be sized to the maximum demand data available for one year or the average power demand of a 15-minute period over a minimum of 30 days [220.87].
    As previously stated, the demand data is the maximum demand over 15 minutes for a 30 day period. KWH's are not used.

  5. #5
    Unfortunetely, I do not have anything like an hvac to work with on this install. When this gen set and switch were ordered, the customer did not know or couldn't afford a load shedding switch. This install would have met NEC code 2005, as it did not address the issue of automatic service entrance switches. As I read the code revision and to meet code in this case, I will need to do a load calculation and am wondering what the correct method is to use the aformentioned load calculation in article 220.87 and if utilizing past utility consumption rates is acceptable, and whether anyone has used it. This load calculation is needed to to prove the standby power source can support a normal load. It may not work anyway, but any help would be much appreciated.

  6. #6
    Join Date
    Oct 2007
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    New Jersey
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    Quote Originally Posted by tractormechanic View Post
    Unfortunetely, I do not have anything like an hvac to work with on this install. When this gen set and switch were ordered, the customer did not know or couldn't afford a load shedding switch. This install would have met NEC code 2005, as it did not address the issue of automatic service entrance switches. As I read the code revision and to meet code in this case, I will need to do a load calculation and am wondering what the correct method is to use the aformentioned load calculation in article 220.87 and if utilizing past utility consumption rates is acceptable, and whether anyone has used it. This load calculation is needed to to prove the standby power source can support a normal load. It may not work anyway, but any help would be much appreciated.
    I am by no means an expert on the NEC, but reading between the lines, it seems that if the rating can be based on the 15 minute max demand, you need a demand meter at the customer. If this is a residential customer, it is highly unlikely there is a demand meter present. No meter, no demand recorded. Residential meters don't record "consumption rate" so the utility will not be able to provide you with the data. You'll need to calculate it the via Article 220, unless you want to lease a demand meter for a year and stick it in line with the service.

  7. #7
    Join Date
    Aug 2006
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    Miami Fla.
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    Quote Originally Posted by tractormechanic View Post
    Our local inspector informed me that the generator needs to have the capacity to support everything in the 200amp service panel - referencing the 2008 NEC code changes.
    Is this job in Florida?

  8. #8
    Quote Originally Posted by chris kennedy View Post
    Is this job in Florida?

    No, this is in Washington state. Has Florida not adopted the current NEC changes?

  9. #9
    Join Date
    Mar 2004
    Location
    Arizona
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    Trac,

    Just do a load calc., you have a 20,000w gen / 240v would be 83amps. You say there is

    no A/C system, I can't imagine the house using anywhere near 83amps, unless it is a big

    monster with all electric equipment.
    Frank Arizona,USA

  10. #10
    Join Date
    Sep 2004
    Posts
    1,432
    If the service load calc comes in over 83A, then either you need to change the transfer switch to a manual one, or rent a power logger for 30 days and hope the highest 15min average comes under 83A.

    I don't know why the CMPs adopted this new rule. Doesn't seem like a safety thing to me -- just people getting ripped off by contractors who put in too small of a system. The NEC isn't supposed to be a design or convenience manual.
    Mark
    Kent, WA

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