Electrical Water Heaters Power Rating Calculations – Part Four




This is the fourth Article for helping designers to choose the appropriate type and calculate the required power rating for the chosen type of Electrical Water Heater.

In the first Article " Electrical Water Heaters Power Rating Calculations – PartOne", we gave you a brief about the following points:
  • Hot Water System Components,
  • Different types of Water Heaters used in domestic and commercial buildings,
  • How to choose the best type of water heater for any application?


With this brief, you will be familiar with the different types and construction of common Water Heaters.

Also, in the second article "Electrical Water Heaters Power Rating Calculations –Part Two", we explained the Design Methodology of Electrical Water Heaters for any Building which is represented in Fig.1, where we are going to calculate the following:
  • The Minimum number of plumbing facilities for a given type of building/occupancy,
  • The total demand of Water in GPH or GPM,
  • The size (volume) in gallons or liters for the required electrical water heaters,
  • The power rating in KW for the required electrical water heaters.




Fig (1): Design Methodology of Electrical Water Heaters for any Building

And in this second article, we explained the first two steps which were:
  • Step#1: Determination of the Building/Occupancy Type,
  • Step#2:  Calculation of The Minimum Number of Plumbing Facilities for a Given Type of Building/Occupancy by using the Calculations Spreadsheet for Minimum Number of Plumbing Facilities Required

Also, in the third article "Electrical Water Heaters Power Rating Calculations –Part Three", we explain the following points:
  • Step#3: Calculation of the total demand of water in GPH or GPM.
  • Step#4:  Calculation of The size (volume) in gallons or liters for the required electrical water heaters

In Step#3, there are many methods used like:

  • First: Methods used for new/existing buildings:
        - International Codes Methods,
  • Second: Methods used only for existing buildings.

And in this third article, we explain some of the International Codes Methods which are:
  • Method# 1: Estimating Hot Water Demand on Fixture Types
  • Method# 2: Estimating Hot Water Demand on Fixture Units
  • Method #3: Estimating Hot Water Demand on Occupants/Units
  • Method #4: Estimating Hot Water Demand on Daily Use
  • Method #5: Generic Curves Method
  • Method# 6: Estimating Hot Water Demand on Building's operating characteristics


Today, we will explain in detail the following:
  • Step#3- Second: Methods used only for existing buildings,
  • Step#5: Calculate the power rating in KW for the required electrical water heaters,
  • Water Heater Branch Circuit and Sizing Calculations.

  


Step#3: Calculation of The Total Demand Of Water in GPH or GPM
Second: Methods Used Only For Existing Buildings




Second: Methods used only for existing buildings, which include:
  1. Actual Water Meter History Method,
  2. Clock Recording Method,
  3. Comparison with Similar Buildings.


1- Actual Water Meter History Method:

In determining water consumption of any application, it is more desirable to obtain the actual water meter history. Generally, a six month history will be representative of the applications requirements. This can easily be accomplished by contacting the water service supplying the application. Such requests are considered public information. Many of these services record usage in cubic feet.
To convert volume given in cubic feet to gallons, multiple by 7.5

Example: 50 cubic feet x 7.5 = 375 gallons.

2- Clock Recording Method:

Another procedure in determining consumption, and in particular when a meter reading is not available such as on a well system, is the use of a clock recording method.
Upon determining the GPM rating of a well pump, connect an inexpensive clock to the pump circuit. Set at 12:00 o’clock and record daily the number of minutes the pump ran. Multiply these minutes recorded by the GPM rate and the average total daily consumption can then be estimated more realistically.

3- Comparison with Similar Buildings:

A third method that can lend credibility to an estimated daily usage is through comparison. By obtaining an actual meter recording usage of a similar operation, the customer will have more confidence in your projections.






Step#5: Calculate the power rating in KW for the required electrical water heaters





We have many methods to calculate the power rating in KW for the required electrical water heaters which is the aim of this course, these methods are:
  • Method#1: Using equations,
  • Method#2: Using Sizing Tables,
  • Method#3: Using Online calculators,
  • Method#4:  Using Excel Sheets,
  • Method#5: Using Rules of thumb.






Important Definitions:

  • Draw Rate: The rate at which hot water is drawn from a water heating system, usually expressed in gallons per minute (GPM).
  • Temperature Rise: The difference in temperature between the desired hot water and the incoming cold water, expressed in degrees Fahrenheit. For example, the desired temperature at the faucet is 120°F and the incoming cold water is 70°F. The required temperature rise is 50°F (120 - 70 = 50).
  • BTU: The amount of heat required to raise the temperature of one pound of water to 1 degree F. Since one gallon of water weighs 8.33 pounds, it would require 8.33 BTU to raise one gallon of water one degree F.
  • GPH (Gallons Per Hour): The amount of water, in gallons, that is used each hour by the plumbing fixtures and equipment, such as dish machines.
  • GPM (Gallons Per Minute): The amount of water, in gallons, flowing through a plumbing fixture or through an instantaneous water heater per minute.
  • Storage Water Heater: A water heater that incorporates a thermostat, a storage tank, and a burner or heating elements, to heat and maintain the water within the tank at a specific temperature.
  • Thermal Efficiency: The measure of the overall efficiency of the water heater, taking into consideration loss of energy due to combustion, radiation, convection and conduction of heat from the unit. The thermal efficiency for electric water heaters, unless otherwise listed other nationally recognized testing laboratories, will be assumed to be 98%.


Important Conversions:

  • 1kW = 3412 Btu/h
  • 1 gallon of water = 8.3 pounds (lbs)
  • 1 cu. ft. = 7.5 USgallon
  • 1USgallon = 3.79 liters




Method#1: Using equations

To compute KW rating for electric water heaters the following formulas should be used:

KW input = GPH X Temperature Rise (deg F)X 8.33 (lb./ gallon) / (Thermal Efficiency X 3412 BTU/KW )

Example#1:

For a food facility, a water heater would be required to recover 54 GPH. The desired temperature is 120°F and the incoming cold water is 70°F. Calculate the required KW rating for electric water heaters.

Solution:

KW input = GPH X Temperature Rise (deg F)X 8.33 (lb./ gallon) / (Thermal Efficiency X 3412 BTU/KW )
KW input = 54 X 50 X 8.33 /(.98 X 3412)
KW input = 6.7

Using Equations For Swimming Pools

KW input = (length X width X average depth x 7.5) x Factor/3412

Note: length, width & average depth are in feet.

To calculate the input KW power for electrical storage-type water heater, please follow the following steps:

  1. Calculate the Cubic feet of the swimming pool= length X width X average depth.
  2. Calculate the required Gallons of water = cubic feet X 7.5.
  3. Calculate the temperature rise in deg F
  4. For swimming pools, heating periods of 48 to 72 hours are usually acceptable.
  5. Find the Factor from Table-1, by noting the hours desired for heating the swimming pool or spa water, plus the temperature rise.
  6. Calculate input BTU/H = gallons X factor from Table in below.
  7. Calculate input KW = BTU/H ÷ 3412




Example#2:

A Swimming pool with dimensions 40’ x 18’ x 5’, Average water temperature at initial heat-up is 60 deg F & a Desired 80 deg F water temperature in pool. Calculate the required input KW.

Solution:

  1. Calculate the Cubic feet of the swimming pool= length X width X average depth = 40’ x 18’ x 5’ = 3600 cubic feet.
  2. Calculate the required Gallons of water = cubic feet X 7.5. = 3600 X 7.5 = 27,000 gallon capacity.
  3. Calculate the temperature rise in deg F = 80 – 60 = 20 deg F
  4. Need 48 hours to heat swimming pool.
  5. Find the Factor from Table, by noting the 48 hours desired for heating the swimming pool or spa water, plus the 20 deg F temperature rise. Factor = 5
  6. Calculate input BTU/H = gallons X factor from Table-1 = 27,000 gallons X 5 = 135,000 BTU/H
  7. Calculate input KW = BTU/H ÷ 3412 = 135,000 BTUH / 3412 = 39.57 KW





Method#2: Using Sizing Tables

There are many different sizing tables that can be used to determine the required KW rating for electrical water heaters as follows:
  • Table-1: By knowing GPH and temperature rise at deg F
  • Table-2: By knowing amount of water inside the tank (cubic feet or gallon) and temperature rise at deg F
  • Table-3: By knowing number of people in home and amount of water (in liters)



Table-1: By knowing GPH and temperature rise at deg F


Table-2: By knowing amount of water inside the tank (cubic feet or gallon) and temperature rise at deg F


Number of People in Home
Capacity
Litres
Element
Size kW
1 and 2
80
1.8
2 and 3
125
1.8
4 – 6
160
2.4
5 – 8
250
3.6
6 – 10
315
4.8
8 – 13
400
4.8
Table-3: By knowing number of people in home and amount of water (in liters)


Example#3:

Solve for example#1 but by using sizing tables.

Solution:
Since we know that the GPH = 54 and the temperature rise = 50 deg F , so we will use Table-1 as follows:
In table-1, Read across in table from nearest amount of GPH to desired temperature rise column and note kilowatts.
The Nearest amount of 54 GPH = 56 GPH & by using column of 50 deg F gives 7 KW
The KW input = (54/56) x 7 KW = 6.75 which is equal to the same result in example#1





Method#3: Using Online Calculators

There are many online calculators for sizing your needs for the electrical water heaters and helps you to find the suitable product for your case. Here we include two online calculators For example. You can find other online calculators on the web.






Pro-Size is the new online water heater sizing and product selection software from A. O. Smith. You can begin by selecting one of the Application types shown below, and Pro-Size will guide you through the process.




Bradford White's RightSpec® is the most comprehensive and intuitive sizing program available and the all-new design makes it easier to use from virtually anywhere. There is no sign-up required or program to download.




Method#4:  Using Excel Sheets

The standard data used in this excel sheet are from:
  • International Plumbing Code (IPC) that developed by the International Code Council (ICC) for calculating Minimum Number of Fixtures Required.
  • American Society of Plumbing Engineers ASPE for calculating the KW for electric water heaters. 




This Excel Spreadsheet uses  the following tables:

1- IPC - Table 403.1 for Minimum Number of Fixtures Required. The table specifying the minimum number of plumbing fixtures required based on the following factors:

The Building Occupancy Type,
The Occupancy Classification,
The Number of Building Occupants.

Minimum Number of Fixtures Required (Table 403.1)

Advantages of using table 403.1 for Minimum Number of Fixtures Required:
  • The IPC table is based on various studies and is intended to provide equal access to fixtures.
  • The IPC table is very accurate since it is directly related to the referenced building codes.
  • The IPC table takes into consideration the female population and the waiting period for the female population


2- ASPE Table-7 which gives Hot Water Demand per Fixture for Various Types of Buildings [Gallons (liters) of water per hour per fixture, calculated at final temperature of 140o F (60oC)]


ASPE Table-7

How to use the Calculations Spreadsheet for Minimum Number of Plumbing Facilities Required and KW Calculation:
This excel sheet includes (6) sheets as follows:
  1. Cover and General data sheet,
  2. Floor input data sheet,
  3. Floor Calculation results sheet,
  4. KW Calculation,
  5. Standard data sheet,
  6. ASPE Table-7.


Most of these sheets explained in article "Electrical Water Heaters Power Rating Calculations – Part Two".


KW Calculation” worksheet

the only worksheet that is new and not explained is “KW Calculation” worksheet, the input data are as follows:
  • Building Type: select the building type from the drop down list to be the same as in the floor input data sheet.
  • Delta T: The difference in temperature between the desired hot water and the incoming cold water, expressed in degrees Fahrenheit.
  • Thermal Efficiency: The thermal efficiency for electric water heaters, unless otherwise listed other nationally recognized testing laboratories, will be assumed to be 98%.
  • Number of Fixtures: take these numbers from the result section in the worksheet “Floor Calculation results sheet”.


Note: for Dishwasher, the Input GPM of Dishwasher and verify it from Manufacturers catalog data.

The result data from “KW Calculation” worksheet are:
  • BTU/H INPUT
  • KW INPUT







Note:
Non-registered members in Electrical-Knowhow website will not be able to open and use this excel spreadsheet.

To register as a member of Electrical-Knowhow website, do the following:
  • Click on the phrase “Follow" in bottom of the home page, below the images of our members.
  • After finishing your registration send email to ali1973hassan@yahoo.com, asking for your password and I will send it with email reply.






Method#5: Using Rules of thumb

1- Rules Of Thumb for Single Home/Apartment

the first two persons
Assume 20 gallons of water each
Each additional person
Assume 15 gallons of water per person

So a family of four would be using (2x20 +15 +15 =70) gallons of water per day. 

2- Rules Of Thumb for Hospitals & Nursing Homes

As a preliminary estimate, some designers use a rule of thumb of 125 gallons of 140ºF water per bed per day as basis for the hot water requirement.

Nursing Homes
Max. Hour
Max. Day
Avg. Day
4.5 gal/bed
30.0 gal/bed
18.4 gal/bed
(Source- ASHARE Chapter 45, Table 7)

3- Rules Of Thumb for Hotels & Motels

Motel/Hotels
Number of units
Max. hour
Max. Day
Avg. Day
20 unit or less
6.0 gal/unit
35.0 gal/unit
20.0 gal/unit

60 units
5.0 gal/unit
25.0 gal/unit
14.0 gal/unit

100 units or more
4.0 gal/unit
15 gal/unit
10.0 gal/unit
(Source- ASHARE Chapter 45, Table 7)

4- Rules of Thumb for Apartments

Apartments
Number of Units
Max. hour
Max. Day
Avg. Day
20 unit or less
12.0 gal/unit
80.0 gal/unit

42.0 gal/unit
50 units
10.0 gal/unit
73.0 gal/unit
40.0 gal/unit

75 units
8.5 gal/unit
66.0 gal/unit
38.0 gal/unit

100 units
7.0 gal/unit
60.0 gal/unit
37.0 gal/unit
200 units or more
5.0 gal/unit
50.0 gal/unit
35.0 gal/unit

(Source- ASHARE Chapter 45, Table 7)

5- Rules of Thumb for Dormitories

Dormitories
Max. hour
Max. Day
Avg. Day
Men's dormitory
3.8 gal/student
22.0 gal/student
13.1 gal/student

Women's dormitory
5.0 gal/student
26.5 gal/student
12.3 gal/student

(Source- ASHARE Chapter 45, Table 7)

6- Rules of Thumb for Restaurants

Food service establishments
Max. hour
Max. Day
Avg. Day
Type "A"
Full meal
restaurant/cafeterias
1.5 gal/max.
meal/hour
11.0 gal/max.
meal/hour
2.4 gal/max.
meal/hour
Type "B"
Drive-ins, grilles,
luncheonettes, deli shops
0.7 gal/max.
meal/hour
6.0 gal/max.
meal/hour
0.7 gal/max.
meal/hour
(Source- ASHARE Chapter 45, Table 7)

8- Rules of Thumb for Schools & Gymnasiums


Max. hour
Max. Day
Avg. Day
Elementary School
0.6 gal/student
1.5 gal/student
0.6 gal/student

Jr. and Sr. High
School
1.0 gal/student
3.6 gal/student
1.8 gal/student
(Source- ASHARE Chapter 45, Table 7)





Water Heater Branch Circuit and Sizing Calculations




All homes require a supply of hot water To meet this need, one or more automatic water heaters are generally installed as close as practical to the areas having the greatest need for hot water

First: NEC Rules used for Water heater branch circuit and sizing calculations

110.14 Electrical Connections. (C) Temperature Limitations:

The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
(1) Equipment Provisions.
The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).
(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60°C (140°F).
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

240.4 Protection of Conductors:
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(1) 18 AWG Copper. 7 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 5.6 amperes.
(2) Overcurrent protection is provided by one of the following:
a. Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire
c. Class CC, Class J, or Class T fuses
(2) 16 AWG Copper. 10 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 8 amperes.
(2) Overcurrent protection is provided by one of the following:
a. Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire
b. Branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire
c. Class CC, Class J, or Class T fuses
(3) 14 AWG Copper. 15 amperes
(4) 12 AWG Aluminum and Copper-Clad Aluminum. 15 amperes
(5) 12 AWG Copper. 20 amperes
(6) 10 AWG Aluminum and Copper-Clad Aluminum. 25 amperes
(7) 10 AWG Copper. 30 amperes

422.10 Branch-Circuit Rating.  (A) Individual Circuits:
The branch-circuit rating for an appliance that is a continuous load, other than a motor-operated appliance, shall not be less than 125 percent of the marked rating, or not less than 100 percent of the marked rating if the branch-circuit device and its assembly are listed for continuous loading at 100 percent of its rating.

422.11 Overcurrent Protection:
Appliances that are Single Non–Motor-Operated Appliance shall be protected against overcurrent as in (E)
(E) Single Non–Motor-Operated Appliance.
If the branch circuit supplies a single non–motor-operated appliance, the rating of overcurrent protection shall comply with the following:
(1) Not exceed that marked on the appliance.
(2) Not exceed 20 amperes if the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less; or
(3) Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.

422.13 Storage-Type Water Heaters:
A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load for the purposes of sizing branch circuits.

422.30 General:
A means shall be provided to simultaneously disconnect each appliance from all ungrounded conductors in accordance with the following sections of Part III

422.31 Disconnection of Permanently Connected Appliances.
(B) Appliances Rated over 300 Volt-Amperes:

For permanently connected appliances rated over 300 volt-amperes, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is lockable in accordance with 110.25.





Second: Branch-Circuit Rating

As per NEC 422.13, An electric water heater is a continuous load and as per NEC 422.10 the branch-circuit rating must not be less than 125 % of the nameplate rating.





Third: Disconnecting Means

As per NEC 422.30 the electric water heater must have a means of disconnecting it from the power source but if the branch-circuit switch or circuit breaker is within sight from the appliance or is lockable, it can be used as the disconnecting means as per NEC 422.31.




Fourth: Branch-Circuit Overcurrent Protection

To size the Branch-Circuit Overcurrent Protection for electrical water heater, we have two cases:

Case#1: the protective device rating is marked on the electric water heater‘s nameplate.

In this case and As per NEC 422.11(E) the overcurrent protective device shall not exceed the protective device rating marked on the appliance.

Example#4:
A water heater nameplate indicates that the rating of the maximum Branch-Circuit Overcurrent Protection fuse is 25 A. can we use 30 A fuse instead?

Solution:
No, As per NEC 422.11(E) the overcurrent protective device shall not exceed the protective device rating marked on the appliance.

Case#2: the protective device rating is not marked on the electric water heater‘s nameplate.

In this case:
As per NEC 422.10, the minimum protective device rating = 125 % of the ampere rating,
As per NEC 422.11.3(E), the maximum protective device rating = 150% of the ampere rating.

Example#5:
A 4500 watts, 240 volts water heater to be installed. What is the minimum and maximum size fuse permitted by the Code?

Solution:
I = 4500/240 = 18.75 A
The branch-circuit rating must not be less than 125 % of the water heater’s nameplate rating.
The minimum fuse rating= 18.75 X 1.25 = 23.4 A
The maximum fuse rating= 18.75 x 1.5 = 28 A

Select the fuse rating from the standard ratings to be 25 A.




Fifth: Conductor Size

In Table 310.15(b)(16), we find the allowable ampacity of conductors in the 60°C column as required by 110.14(C) and 240.4(D).

Table 310.15(b)(16)


Example#6:
For the water heater in example#5 in above, find the suitable conductor size?

Solution:
In Table 310.15(b)(16), in 60°C column, find value or next higher value for 25 A, it will be 30 A for the 10 AWG wire.




The following figure summarizes the Water heater branch circuit and sizing calculations.



Also, the following table is a guide for the Water heater branch circuit and sizing calculations.




This is the end of this course.


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