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:
 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:
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:

Important Definitions:
Important Conversions:

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
storagetype water heater, please follow the following steps:
Example#2:
A Swimming pool with dimensions 40’ x 18’ x 5’, Average
water temperature at initial heatup is 60 deg F & a Desired 80 deg F
water temperature in pool. Calculate the required input KW.
Solution:

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:
Table1: By knowing GPH and temperature rise at deg F
Table2: By knowing amount of water inside the tank (cubic
feet or gallon) and temperature rise at deg F
Table3: 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
Table1 as follows:
In table1, 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.
ProSize
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 ProSize will guide you through the process.
Bradford White's RightSpec^{®} is the
most comprehensive and intuitive sizing program available and the allnew
design makes it easier to use from virtually anywhere. There is no signup
required or program to download.

Method#4: Using Excel Sheets
The standard data used in this excel sheet are from:
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:
2 ASPE Table7 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 Table7
How to use the Calculations Spreadsheet for Minimum Number of Plumbing
Facilities Required and KW Calculation:
This excel sheet includes (6) sheets as follows:
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:
Note: for Dishwasher, the Input GPM of Dishwasher and verify it from Manufacturers
catalog data.
The result data from “KW Calculation” worksheet are:

To download your copy of Calculations Spreadsheet for Minimum Number of Plumbing Facilities Required and KW Calculations, please click on the link.
Note:
Nonregistered members in ElectricalKnowhow website
will not be able to open and use this excel spreadsheet.
To register as a member of ElectricalKnowhow
website, do the following:

Method#5: Using Rules
of thumb
1 Rules Of Thumb for Single Home/Apartment
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.
3 Rules Of Thumb for Hotels & Motels
4 Rules of Thumb for Apartments
5 Rules of Thumb for Dormitories
6 Rules of Thumb for Restaurants
8 Rules of Thumb for Schools & Gymnasiums

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. Branchcircuitrated circuit breakers listed and marked for use with 18 AWG copper wire b. Branchcircuitrated 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. Branchcircuitrated circuit breakers listed and marked for use with 16 AWG copper wire b. Branchcircuitrated 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 CopperClad Aluminum. 15 amperes (5) 12 AWG Copper. 20 amperes (6) 10 AWG Aluminum and CopperClad Aluminum. 25 amperes (7) 10 AWG Copper. 30 amperes
422.10
BranchCircuit Rating. (A) Individual
Circuits:
The
branchcircuit rating for an appliance that is a continuous load, other than
a motoroperated appliance, shall not be less than 125 percent of the marked
rating, or not less than 100 percent of the marked rating if the
branchcircuit device and its assembly are listed for continuous loading at
100 percent of its rating.
422.11
Overcurrent Protection:
Appliances
that are Single Non–MotorOperated Appliance shall be protected against overcurrent
as in (E)
(E)
Single Non–MotorOperated Appliance.
If the
branch circuit supplies a single non–motoroperated 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
StorageType Water Heaters:
A
fixed storagetype 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 VoltAmperes:
For
permanently connected appliances rated over 300 voltamperes, the
branchcircuit 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: BranchCircuit Rating
As per
NEC 422.13, An electric water heater is a continuous load and as per NEC 422.10
the branchcircuit 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 branchcircuit 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: BranchCircuit Overcurrent Protection
To
size the BranchCircuit 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 BranchCircuit
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
branchcircuit 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.

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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