Standby generator and article 220.87 sizing

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tractormechanic

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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.
 
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
 
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.

JWCELECTRIC said:
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.
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No, it says standby source.
 
tractormechanic said:
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?.
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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..
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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.
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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].
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As previously stated, the demand data is the maximum demand over 15 minutes for a 30 day period. KWH's are not used.
 
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.
 
tractormechanic said:
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.
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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.
 
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.
 
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.
 
I did a load calc for this house and it comes in at 102A. It has full electric wall heaters for heat, however, the homeowners do not use them. At this point the best bet may, infact, be to use a 'power deman logger' to get the highest 15 min reading over a 30 day period. Is this the correct way to read article 220.87 and do you guys suppose it would satisfy the inspector? It sure would save a lot of time and effort to get the issue resolved. Also, where can I get a 'power demand logger' at? Is it something I can rent or buy? Thanks for all of your efforts in helping me.
 
tractormechanic said:
I did a load calc for this house and it comes in at 102A. It has full electric wall heaters for heat, however, the homeowners do not use them. At this point the best bet may, infact, be to use a 'power deman logger' to get the highest 15 min reading over a 30 day period. Is this the correct way to read article 220.87 and do you guys suppose it would satisfy the inspector? It sure would save a lot of time and effort to get the issue resolved. Also, where can I get a 'power demand logger' at? Is it something I can rent or buy? Thanks for all of your efforts in helping me.
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Ranger has several data loggers that with the supplied Pronto software will do a demand report. We utilize the 6000 and 3000 series for data logging and occasionally do demand reports to check power company billing.
 
tractormechanic,
Your AHJ is not asking for anything extraordinary. If you have DEMAND data on the service for one year. And the peak demand x 1.25 is below the capacity of the 20kW genset you comply with all the requirements of NEC 220-87 and your AHJ. You're good.

If you do NOT have demand data, you will need a recorded, 30 day load reading that includes the use of any large equipment within that period. If this recording indicates that the peak demand in that period, (well actually a 15 min ave), again x 1.25, is less than the 20 kW supplied by your generator, again you are good and have complied with NEC 220-87 and your AHJ's request.

If your generator sizing fails these tests, then you cannot use it at the service entrance, and will either need to upsize it, or connect it to a selected load center downstream of the service panel. Say a branch panel that feeds 'critical' loads, lighting, refrigeration, etc....

I think the safety issue comes up when we have people trying to avoid the high utility costs these days and we see SEVERLY undersized equipment being installed without proper safety measures taken. The equipment fails, catastrophically in some cases and the resultant fires and other equipment and facilities being destroyed puts people at risk.
 
EEJaime,

I will get a demand logger and monitor the current demand and hope a 15 min average x 1.25 comes in less than 83.3A. In response to your last comment, I agree with you fully about sub standard installs and undersized units, however, in this case the generator has a 100A breaker installed. To overload it in an emergency situation (something only a knucklehead would do) would only cause this breaker to trip. Also, this particular install would have met the 2005 NEC code. I kind of makes you wonder....
 
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