If fire flows are an issue I could calculate it for my own purposes but would want verification by a PE or FPE before proceeding. After reading everything about it I could find I concluded ISO fire flows are an area sprinkler layout technicians aren't qualified to navigate. What the heck, a set of plans with $300 or so on a $10 million project could save me a lot of headache down the road so why take the risk?
Why the 1500 gpm pump on a project that requires but a 30 minute supply? Sounds light hazard to me unless there are ISO fire flows involved.
From NFPA #20 appendix A-2-3
A stationary pump for fire protection should be selected in the range of operation from 90 percent to 150 percent of its rated capacity. The performance of the pump when applied at capacities over 140 percent of rated capacity can be adversely affected by the suction conditions. Application of the pump at capacities less than 90 percent of the rated capacity is not recommended.
Seems a 500 gpm pump would handle anything up to OH2.
My understanding the correct way is to calculate total sprinkler demand plus hose stream allowance for whatever time requirement there was. 400 gpm sprinkler + 250 hose = 650 gpm which would call for a 39,000 gallon water storage regardless of the pump size.
Personally I like to size the tank for the pump at 150% for whatever time is required. In the case of a 500 gpm pump I prefer to size the tank at 750 gpm for 60 minutes and provide a 45,000 gallon tank. Come on, the cost difference between a 39,000 gallon and 45,000 is very minimal. What are we talking about, $6,000 more on a project that might be sellign for $10 million?
I've always been able to offer it to the owner as an alternate which they always accept when you explain the larger tank would take care of any additions or a density increase should something change down the road.
But the answer to the question (not addressing ISO fire flows) is sprinkler demand plus inside and outside hose stream allowance if any.