Residential Service Calculations in the National Electrical Code

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Residential Service Calculations in the National Electrical Code
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Residential Service Calculations in the
National Electrical Code
Christel Hunter
16-20 minutes
Load calculations in the National Electrical Code have evolved over many
decades. It was in the 1933 NEC that load calculation requirements began to
resemble a format that the modern code user would find familiar. Since then,
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many things have changed, but the primary requirement remains the same —
service equipment and conductors must be sized to handle the expected load.
Article 220 of theNational Electrical Code lays out the primary requirements for
performing load calculations that are necessary for determining the size of a
residential service. The calculations are based on the expected loads present
in a dwelling unit, along with appropriate demand factors that are used to
account for the diversity of electrical use by occupants. There are two
methods available, standard and optional calculations. Optional calculations
require fewer steps and generally result in smaller conductors, but the dwelling
unit must meet more restrictive requirements. We will only be considering onefamily dwelling units in this article, including single family residences,
apartments, etc.
Be aware that some authorities having jurisdiction adopt the International
Residential Code for One- and Two-Family Dwellings (IRC) and use the method
for calculating the service size using the requirements found in Chapter 36. The
IRC calculations are based on the National Electrical Code, but are not
identical. Always check with your local jurisdiction to find out what method(s)
are acceptable.
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Photo 1. Electric range
Standard Method
The standard method for calculating service sizing is found in Part III of Article
220. Of course, we can’t find all the requirements in this Part, so we will also
be looking at additional requirements in Articles 210, 220, 230, 250 and 310.
An example load calculation using the standard method is shown in Table 1.
Table 1
Lighting Load
The first thing we need to determine is the lighting load. Table 220.12 requires
that for dwelling units, we multiply the floor area (based on the outside
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dimensions of the dwelling unit) times 3 volt-amperes/square foot. Section
220.14(J) states that the following loads are also included in the general
lighting load calculations:
all general-use receptacle of 20-ampere rating or less, including the
receptacles connected to the bathroom branch circuit required in 210.11(C)(3),
the outdoor outlets in 210.52(E),
the receptacle outlets in basements, garages, and accessory buildings in
210.52(G), and
the lighting outlets required in 210.70(A), which includes habitable rooms, a
variety of additional locations, and storage or equipment spaces.
Table 220.42 gives us demand factors for lighting loads. Most homes will take
the first 3000 VA at 100% and the remainder at 35%. (If you are calculating a
multifamily dwelling service, you might use the third demand factor category,
where anything over 120,000 VA is taken at 25%.)
Small Appliance and Laundry Loads
Section 210.11(C)(1) requires a minimum of two small appliance branch
circuits. Section 220.52(A) tells us that we must use a minimum load of 1500
VA for each of these circuits, but also allows the small appliance branch
circuits to be included with the general lighting load when applying the demand
factors in Table 220.12. Section 220.52(B) requires that 1500 VA be added for
the required laundry circuit in 210.11(C)(2). This circuit can also be added to
the general lighting load and demand factors may be applied.
Photo 2. Washer/dryer
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Electric Dryers and Cooking Appliances
Section 220.14(B) refers us to requirements for electric dryers in 220.54 and
electric cooking appliances in 220.55. Electric clothes dryers are calculated at
either the minimum of 5000 watts or the nameplate rating, whichever is larger.
The demand factors in Table 220.54 may be helpful if there are more than four
dryers, but this is unlikely in a one-family dwelling unit, so we will not use this
table for the examples in this article. Electric ranges and other electric cooking
appliances (rated in excess of 1.75 kW) shall be permitted to be calculated in
accordance with Table 220.55, which takes up an entire page and has five
notes. There are also informational notes directing the code user to Annex D
for examples. It is worthwhile to review this table and read all the notes and
examples to become familiar with the various options.
Fixed-appliance load
If there are four or more fixed appliances in the residence, 220.53 permits all
of these loads to be totaled and then a demand factor of 75% applied. Fixedappliance loads include items such as a water heater, garbage disposal,
dishwasher, microwave, etc.
Photo 3. Garbage disposal
Largest motor load
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Section 220.14(C) tells us that motor loads shall be calculated in accordance
with the requirements in 430.22, 430.24 and 440.6. For the service
calculation, this means that we must determine the largest motor load and add
25% of its value to the total calculation. Common motor loads in residential
applications include air conditioning, water pumps, disposals, blowers, etc.
Often, the largest motor load in a home is the air conditioner. Even if the air
conditioning is dropped from the total load calculation in favor of electric
heating (see below), you may still be required to use the AC motor load for this
calculation. Check with your jurisdiction to see what the policy is locally. Many
jurisdictions publish residential load calculation worksheets to help with
determining the size of the service.
Noncoincident loads
When two loads are not likely to be energized at the same time, 220.60 allows
us to use only the largest load for the calculation of the service. This is
typically applied to dwelling units with both electric heating and air conditioning,
since they are not expected to run at the same time.
Specific appliances or loads
There are certain loads that may be found in residences that are not included
in the previous list. Section 220.14(A) requires that an outlet for a specific load
or appliance not covered elsewhere must be calculated based on the ampere
rating of the load served. Some examples might include a spa, RV hookup, etc.
These must be included in the load calculation at their full value.
Optional Method
The optional method is much simpler than the standard calculation, but is
restricted in 220.82 to “… a dwelling unit having the total connected load
served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or
feeder conductors with an ampacity of 100 or greater.” Most one-family
dwelling units meet this requirement, so the optional method is used frequently.
An example calculation using the optional method is shown in Table 2.
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Table 2
General Loads
For the purposes of the optional method, everything except heating and air
conditioning is considered to be a general load. For this method, the general
calculated load shall be not less than 100 percent of the first 10 kVA plus 40
percent of the remainder of all loads other than heating and air conditioning.
Lighting and general-use receptacles are again based on the outside
dimensions of the dwelling unit multiplied by 3 volt-amperes/square foot. The
small-appliance branch circuits and laundry branch circuit are each included at
1500 VA.
The next step is to determine the nameplate rating of each of the following
items:
all appliances fastened in place, permanently connected, or located to be on a
specific circuit
ranges, wall-mounted ovens, counter-mounted cooking units
clothes dryers that are not connected to the laundry branch circuit
water heaters
For all permanently connected motors not included in the previous list, the
nameplate or kVA rating must be included in the calculation.
Heating and Air Conditioning
The largest heating and air-conditioning load must be chosen from six options:
100 percent of the nameplate rating of the air conditioning and cooling
100 percent of the nameplate rating of the heat pump when it is used with no
supplemental electric heating
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100 percent of the nameplate rating of the heat pump compressor and 65
percent of the supplemental electric heating for central electric space-heating
systems (If the heat pump compressor is prevented from operating at the same
time as the supplementary heat, it does not need to be added to the
supplementary heat for the total central space heating load.)
65 percent of the nameplate rating(s) of electric space heating if less than four
separately controlled units
40 percent of the nameplate ratings of electric space heating if four or more
separately controlled units
100 percent of the nameplate ratings of electric thermal storage and other
heating systems where the usual load is expected to be continuous at the full
nameplate value.
Comparing Standard and Optional Calculations
To see how the two methods compare, let’s take a look at a 2900 square foot
residence with the following loads:
lighting load
4 small appliance branch circuits
laundry circuit 1500 W
natural gas heating
air conditioner 6000 VA
electric range 11,000 W
hot tub 8000 W (2 hp motor)
Level II electric vehicle charger 7200 W
electric dryer 5000 W
garbage disposal 800 W
microwave 1500 W
dishwasher 1200 W
electric water heater 4500 W
The standard calculation method is shown in Table 1 and the optional
calculation method is shown in Table 2. Using the standard calculation, our total
load is 47,520 VA. Dividing that by 240 volts gives us 198 amps. Using the
next standard service rating requires that we use a 200-amp service. Since we
have a 120/240-volt single-phase dwelling service, we are allowed to use NEC
Table 310.15(B)(7) and use either 2/0 AWG copper or 4/0 AWG aluminum
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service conductors.
Using the optional calculation, our total calculated load is 34,160 VA. Dividing
that by 240 volts gives us 142 amps. Using the next standard service rating
requires that we use a 150-amp service. Once again, we are allowed to use
NEC Table 310.15(B)(7), which requires either 1 AWG copper or 2/0 AWG
aluminum conductors.
For this example, it is clear that the optional calculation permits a smaller
service. From a practical perspective, due to equipment availability, it is likely
that a 200-amp service will be installed rather than a 150-amp service.
Neutral Load
Neutrals are permitted to be smaller than the phase conductors in most
residential service installations. Section 220.61 requires that the neutral load
be determined by calculating the maximum unbalanced load between the
neutral and any one ungrounded conductor. The values used for calculating the
neutral size when using the standard or optional methods will often be different,
as shown in Tables 3 and 4.
Section 230.42 states that the grounded conductor for a service shall not be
smaller than the minimum size as determined in accordance with 250.24(C). If
we have a single raceway (as is most common for service conductors),
250.24(C)(1) tells us that the conductor cannot be smaller than specified in
NEC Table 250.66, but is not required to be larger than the ungrounded
conductors.
For our standard service calculation, our minimum ungrounded conductor size
was a 2/0 AWG copper or a 4/0 AWG aluminum. Using NEC Table 250.66
would require a neutral no smaller than a 4 AWG copper or a 2 AWG aluminum.
In Table 3, we found that our calculated neutral load is 28,035 VA. Dividing that
by 240 volts gives us 117 amps, which will require either a 2 AWG copper or
1/0 AWG aluminum from Table 310.15(B)(7). These sizes are larger than the
required minimum, so we choose one of these conductors.
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Table 3
For our optional service calculation, our minimum ungrounded conductor size
was a 1 AWG kcmil copper or 2/0 AWG aluminum. Using NECTable 250.66
would require a neutral no smaller than 6 AWG copper or a 4 AWG aluminum.
In Table 4, we found that our calculated neutral load is 30,320 VA. Dividing that
by 240 volts gives us 126 amps, which will require either a 1 AWG copper or
2/0 AWG aluminum from NEC Table 310.15(B)(7). Since these sizes are larger
than the required minimum, we would choose one of these conductor sizes.
Table 4
Note that for this example in our optional method calculation, the neutral
conductor is the same size as our phase conductors. However, if a 200-amp
service is installed based on the standard calculation, the neutral is
significantly smaller due to the calculation method. Table 5 shows a summary
of the ungrounded and neutral conductor sizes for our example using both the
standard and optional calculation methods.
Table 5
Conclusion
To accurately calculate the service size for residential installations, the
designer and installer must be familiar with many requirements in the National
Electrical Code. The requirements are not necessarily straightforward, and it is
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recommended that additional resources be reviewed. Available resources
include the examples in Informative Annex D of the NEC, the IAEI publication
One- & Two-Family Dwelling Electrical Systems, and other published examples.
Sidebar
310.15(B)(7) – Changes for the 2014 NEC
For most residential services, the service conductors and main power feeders
are allowed to be sized based on Table 310.15(B)(7) instead of Table
310.15(B)(16), which permits a smaller size conductor to be used in many
cases. This allowance has been in the NEC since the 1950s in recognition of
the fact that only a small portion of the electrical loads in homes are typically
used at the same time, so the load on the service conductors at any one time
is generally much
smaller than the total calculated load.
The language in Section 310.15(B)(7) and the associated table have been a
subject of great debate in code-making panel 6 (CMP-6) over the last few
cycles. CMP-6 has
considered each of the proposals and comments received over the last few
years
and come up with new wording to address the concerns and suggestions
submitted.
CMP-6 has agreed to delete the existing wording and table and replace them
with the following language:
For one-family dwellings and the individual dwelling units of two-family and
multifamily dwellings, service and feeder conductors supplied by a single
phase, 120/240-volt system shall be permitted be sized in accordance with
310.15(B)(7)(a) through (d).
(a) For a service rated 100 through 400 amperes, the service conductors
supplying the entire load associated with a one-family dwelling or the service
conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling shall be permitted to have an
ampacity not less than 83% of the service rating.
(b) For a feeder rated 100 through 400 amperes, the feeder conductors
supplying the entire load associated with a one-family dwelling or the feeder
conductors supplying the entire load associated with an individual dwelling unit
in a two-family or multifamily dwelling shall be permitted to have an ampacity
not less than 83% of the feeder rating.
(c) In no case shall a feeder for an individual dwelling unit be required to have
an ampacity greater than that of its 310.15(B)(7)(a) or (b) conductors.
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(d) Grounded conductors shall be permitted to be sized smaller than the
ungrounded conductors provided the requirements of 220.61 and 230.42 for
service conductors or the requirements of 215.2 and 220.61 for feeder
conductors are met.
Informational Note No. 1: It is possible that the conductor ampacity will require
other correction or adjustment factors applicable to the conductor installation.
Informational Note No. 2: See example DXXX in Annex D.
In effect, the same size conductors that are allowed in the 2011 NEC will still
be allowed in the 2014 NEC, assuming that temperature correction factors or
adjustment factors are not required for the installation. The changes to the
code language were necessary to take into account certain limitations inherent
in the language in previous code cycles. Because Table 310.15(B)(7) is based
on service or feeder ratings and not the temperature rating of conductors,
there is no clear way to apply adjustment or correction factors for installations
at higher temperatures or if there are more than three current-carrying
conductors in a conduit.
It should be noted that the conductor sizing will still be based on the service or
feeder rating, not the calculated load. For example, if you have a calculated
load of 184 amps and are required to install a 200-amp service, the conductors
would be required to have an ampacity of 166 amps or more:
200 amps times 83 percent equals 166 amps. So, for a 200-amp service, you
would still be allowed to choose a 4/0 AWG aluminum or 2/0 AWG copper, but
you would choose it from the 75 degree C column in Table 310.15(B)(16).
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