Blog of Aaron J.: water supply

Ian Keating, "Fire Hose (Yellow & Orange)"

The basics of fire extinguishment come down to this, "big fire, big water; little fire, little water". But, how can you know what amount of water you will need? The fire service stresses the importance of pre-planning. How can you pre-plan your water need?

A good tool for determining minimum water supply can be found in NFPA 1142, Standard on Water Supplies for Suburban and Rural Firefighting. This standard provides a simple formula than can be used to determine the minimum water supply that will be needed for a given structure on the fireground.

MWS = [(total volume of structure) / occupancy hazard)] x construction classification x 1.5 if exposure hazard

In short, the formula can look like this:

MWS = (TCF / OH) x (CC) x (EH)

Step 1. Determine total cubic feet of the structure (TCF).

This can be done by multiplying the length by the width, then adding the height of each floor plus 1/2 height to the ridgepole.

TCF = (L x W) x (height of each floor + 1/2 distance to ridgepole)

Step 2. Determine the occupancy hazard classification number (OH).

The occupancy hazard classification should be based on the number and description as assigned in, NFPA 1142, Standard on Water Supplies for Suburban and Rural Firefighting.

3 - severe hazard occupancy

4 - high hazard occupancy

5 - moderate: combustibility of contents expected to develop moderate rate of heat and flame spread

6 - low: combustibility of contents expected to develop low rate of heat and flame spread

7 - light: combustibility of contents expected to develop light rate of heat and flame spread

Step 3. Determine the construction type classification number (CC).

The construction type classification should be based on the number and description as assigned in, NFPA 1142, Standard on Water Supplies for Suburban and Rural Firefighting.

0.5 - Type I construction
0.75 - Type II construction
1.0 - Type III construction
0.75 - Type IV construction
1.5 - Type V construction

Step 4. Determine if there are any exposures.

If exposures exist, the final step of the formula is to multiply by 1.5. However, the total MWS for buildings with exposures can never be less than 3,000 gallons.

Step 5. Complete the formula.

Plug the appropriate numbers into the correct spot on the formula. Work the simple math. The number produced will indicate the minimum amount of water needed for extinguishment.

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Sample problem:

The dwelling has the following characteristics:

20' x 20'
single story - 10'
pitched roof - 4' to ridgepole
wood frame construction
one exposure

(TCF / OH) x CC = MWS

20 x 20 x 12 = 4,800 cu.ft. (TCF)

4800 / 7 (OH) = 685

685 x 1.5 (CC) = 1,028

1,028 x 1.5 (EH) = 1,542 gallons

The minimum water supply needed for this structure is 1,542 gallons. However, since there is an exposure a minimum of 3,000 gallons of water must be available.

Download the minimum water supply formula cheat sheet.

How much fire extinguishing agent should an ARFF department have available? Is there a set amount of agent that is needed? What are some guidelines for determining agent quantity?

NFPA 402, Guide for Aircraft Rescue and Firefighting Operations, acknowledges that it would be impractical to keep a stock of extinguishing agent on hand for the worst case scenario. To mitigate any issue of an extinguishing agent shortage, pre-arrangements should be made. Pre-fire plans and mutual aid agreements should detail the expectations for additional agencies to provide aid in the form of bringing additional agent to the scene. Support should be requested early in an incident.

The initial water supply on a piece of apparatus should be assumed to be used up within 5 minutes of the incident. Based on the fact that ARFF apparatus carry enough agent (foam) for at least 1 water refill, NFPA 403, Standard for Aircraft Rescue and Firefighting at Airports requires 100% refill capability within the critical rescue and firefighting access area.

The critical rescue and firefighting access area is a rectangular area extending 500 feet out from the centerline of the runway, and 3,300 feet beyond each end of the runway. It is within this space that most aircraft accidents are expected to occur.

To determine the 100% capability a needs analysis must be conducted. All water sources and refill capabilities should be assessed. Recommendations for meeting the 100% refill requirements should be made. These recommendations may include water sources from: