AUTOMATIC GENERATORS

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afried9499

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I installed a 40kw automatic generator with 2 200 amp automatic transfer switches in a town house that has all gas appliences and heat. The calculated load is only 120 amps. the accual load with everything on is under a 100 amps. The BALTIMORE COUNTY INSPECTION department is telling me I can not do this. They are telling me the generator must be large enough to handle the entire 400 amp service. I disagree with them. Article 702.5 says the unit must be able to handle the load that is INTENDED to run at the same time. Please help me prove them wrong.This is an optional standby generator and the customer should be able to choose the load on the generator. This unit will never be over loaded but the county says it has to handle the entire service that is connected to it if we are picking up the entire house.
ARNOLD FRIEDLANDER

{Moderator's Note: Edited to remove personal contact information. If you wish to contact this person directly, then start by sending a Private Message.}

[ October 20, 2004, 11:20 AM: Message edited by: charlie b ]
 
Re: AUTOMATIC GENERATORS

When you say that ?the calculated load is only 120 amps,? do you mean the load on the entire house, or the load that you intend to place on the generator? Put another way, if there is only 120 amps of load in the house, why are you using a pair of 200 amp transfer switches?

If the total load is above 120 amps, and you plan on only loading the generator to 120 amps, how do you plan to prevent an overload? How is the system to be controlled, so as to prevent the owner from turning on more equipment than the generator can handle? If the power is lost when nobody is at home, how will the system automatically limit the load to within the rating of the generator?
 
Re: AUTOMATIC GENERATORS

I have the same situation with a generator that I am pricing for a customer with two dwellings each with a 200 amp service which needs to be feed by one generator. As I understand it the generator does not have to be rated for the full size of the combined services (400 amps). The generator has a breaker sized for the generators capacity. In this case 125 amps. So if the duplex were to draw more than the generator can supply the generators breaker would trip. Sure this is inconvenient but it is safe. Its the same situation as a 200 amp panel. If you add up all the breakers in a full panel to their rated values, they easily total more than 200 amps. 42 circuits times 15 amps equals 630 amps. even if you balance the load on the two phases that still 315 amps. So if all the breakers draw their full rated value the main trips. Inconvenient but safe.
The reason for two 200 amp transfer switches is because each dwelling has a 200 amp service. You need a separate transfer switch for each and it must be rated for the highest connected supply amperage.
 
Re: AUTOMATIC GENERATORS

Arnold, I apologize for not getting back to you via PM request. Try you tomorrow time permitting.

I do not agree with the AHJ interpretation, but if they refuse to budge, you may only have a couple of options:

1. Manual system.
2. Auto system installed after the service disconnect with a separate emergency panel with emergency loads only to impose the limits.

Personally I do not see a problem with the project as long as the generator has it's own OCPD to limit fault current preferable a shunt operated type. What you are proposing is quite common practice for optional systems. Although most systems with undersized generators (like a Honda 5KW) I have seen use a manual transfer switch, maybe as Charlie eluded too, possible it is the automatic part the city has a problem with. Just a guess.
 
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