Generator serving fire pump - Minimum Tank Size

[_Ix]

Hey all,

The AHJ recently tagged us for an undersized fuel tank for a mid rise core/shell office building. The generator will be serving a fire pump and Article 701 loads.

I've reached out to peers in the industry, as well as my Cummins rep and none of them have had a problem with this before. I would like to see if anyone else has had experience in this matter.

The AHJ is citing NFPA 110 - 5.5.3: "The main fuel tanks shall have a minimum capacity of at least 133 percent of either the low-fuel sensor quantity specified in 5.5.2 or that specified in Table 4.1(a)."

Referring to Table 4.1 appears to indicate that the generator needs to run for 8 hours as indicated by NFPA 20 - 9.6.2.3: "The generator fuel supply capacity shall be sufficient to provide 8 hours of fire pump operation at 100 percent of the rated pump capacity in addition to the supply required for other demands."

Basically, the minimum tank size for any generator serving a fire pump is going to be 8 hours X 133% = 10.64 hours. This far exceeds the NEC's minimum run time of 2 hours. My gut feeling is that this seems excessive and perhaps there is some code misinterpretation, but I wanted to see if anyone else has experience dealing with this.

Thanks.

 

[kingpb]

Have you submitted a calculation that shows it meets these requirements? if not, that could be the issue; he wants to see it in writing.

 

[hbendillo]

What size tank did you actually specify?

 

[_Ix]

Sorry for the delay. Please see below for responses:

kingpb

Have you submitted a calculation that shows it meets these requirements? if not, that could be the issue; he wants to see it in writing.​

We did not show calculations because we did not originally meet these requirements. I thought that a generator tank sized for 8hrs of use at full load would be adequate. Apparently I'm mistaken.

hbendillo

What size tank did you actually specify?​

The owner requested code minimum tank size, so we went down to 280 gallons for a 500KVA genset. The generator fuel efficiency at full load is 31.2 gph. 31.2gph X 8 hours = 249.6 gallons. However, due to this 133% rule which I was not familiar with, we now need to size the tank for 31.2 X 8 hours X 133% = 331.968 gallons. This is problematic because we are using (2) 500KVA gensets operating in parallel, thus we would need a total of 331.968g X 2 = 664g of fuel. This now pushes us into a class H-2 occupancy due to having over 660 gallons of fuel stored inside (we're using belly tanks) and is increasing the cost of the project by a fair margin.

Has anyone had experience with this 133% multiplier? I feel like we must obey the AHJ, but my gut is telling me that this is a misinterpretation; 133% is quite the safety factor.

Thanks.

 

[GoldDigger]

133% is quite the safety factor.

Well, that depends on the context. In many cases 200% is considered an appropriate safety factor.
I do have a hard time visualizing a situation in which a fire pump will be called upon to run continuously at full volume for 8 hours! Either the fire will be out or the building will not be there anymore.