Branch Circuit Sizing - Part 1
A core fundamental for every electrical apprentice, journeyman electrician, and master electrician is knowing how to perform electrical calculations. It is very common to find electric cooking equipment, such as electric ranges, electrical in-wall mounted ovens, counter-mounted cooking equipment (Cooktops), and more on an electrical exam.
The question for today is do you know how to size the branch circuits for these electric cooking appliances we just mentioned? How about knowing how to size the service or feeder load contribution for the dwelling service or feeder, as it pertains to these cooking equipment loads?
In this article, we are going to dig into the basics of first sizing the cooking equipment branch circuits. However, we will continue this series in part 2 on calculating the demand load in a single-family dwelling scenario.
At this point, it would be appropriate to introduce an extremely important section in the National Electrical Code, as it pertains to Electrical Cooking Appliances, which is Sec. 220.55. This section is necessary to understand not only how to calculate the branch circuit loads for cooking equipment but also the required volt-amperes or watts necessary to complete that task. This section also is what is utilized for applying demand factors, where applicable, to the load calculations for standard method calculations supply dwelling units.
"220.55 Electric Cooking Appliances in Dwelling Units and Household Cooking Appliances Used in Instructional Programs.
The load for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances individually rated in excess of 13∕ 4 kW shall be permitted to be calculated in accordance with Table 220.55. Kilovolt-amperes (kVA) shall be considered equivalent to kilowatts (kW) for loads calculated under this section.
Where two or more single-phase ranges are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the maximum number connected between any two phases." - 2020 National Electrical Code (Fair Use Extract)
The above code language directly permits the electrician to use Table 220.55 when calculating the loads for electric ranges, wall-mounted ovens, counter-mounted cooking units (cooktops), and other household cooking appliances that are individually rated more than 1,750 watts (1 3/4kW). This will come in handy as we begin our process with the branch circuit calculation for a 12 kW Electric Range.
What size branch circuit is needed for a 12 kW Electric Range?
Visit 220.55 and notice it gives "permission" to use Table 220.55 to perform the calculation to determine the loads.
Looking at Table 220.55 (Figure 1), the electrician will notice the heading of the table, more specifically the portion that is in parenthetical text. It says "(Column C to be used in all cases except as otherwise permitted in Note 3.)". Since Note 3 doesn't apply to this question we are working in Column C.
Column C says "Not over 12 kW Rating", which we are not over 12 kW.
The number of appliances, in the left column of Table 220.55, is (1) for our 12kW Range, we follow the horizontal plane of the table to the right and we select the Maximum Demand to be utilized.
The resulting calculated load or demand load is 8 kW, which we promptly convert to 8000 watts by taking 8kw x 1000 = 8000 watts. Also as stated in 220.55, kilovolt-amperes are equivalent to kilowatts, also making watts equivalent to volt-amperes. Now, I understand that the original electric range was 12 kW but based on demand allowances that have been reduced to 8 kW at this point.
Figure 1 - Extracted for use under the Fair Use Law*
Now is an appropriate time to introduce the "notes" at the bottom of Table 220.55. These notes, as the brief mentioned previously regarding Note 3, are 100% enforceable, unlike informational notes scattered through the NEC. The notes under this table will be vital to working with table 220.55.
Since we are working on the branch circuit calculation, look at Note 4 of Table 220.55 in the 2020 NEC. The note states the following:
"4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range." - 2020 National Electrical Code (Fair Use Extract)
Admittedly there is a lot going on with Note 4 and we will cover all of the aspects of this in future articles. However, the underlined extraction is what we are using for this single Electric Range.
What does "shall be permissible" mean. Well, it means you are given the option to use the values from Table 220.55, which is the 8 kW (8000 VA) we previously calculated or the actual nameplate rating of 12 kW. As we always strive for the best possible answer, keep in mind that the NEC is a minimal safety standard. However, we always gravitate to the permissive allowance and use the 8 kW in these cases.
The resulting math would be 8000 VA ÷ 240 Volts (Line to Line Voltage) = 33.3 Amps
At this point, most electricians would simply jump to Table 240.6(A) to locate the overcurrent protective device (OCPD) and follow the rules in 240.4(B), where applicable, to then notice that a 33.3 Amp-rated OCPD does not exist. Here are the rules in 240.4(B) for review.
"240.4(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes." - 2020 National Electrical Code (Fair Use Extract)
So the "next size up" rule appears to allow you to move to the next available standard-sized protection, which would be 35 Amps. However, that would be an incorrect move at this point.
Remember, we are working on a branch circuit calculation and Article 210 of the National Electrical Code deals with branch circuits. Let's now look at 210.19(A)(3). The charging statement o 210.19 is "Minimum Ampacity and Size". The rules that are specific to the branch circuits of household ranges and cooking appliances are in (A)(3).
"210.19(A)(3) Household Ranges and Cooking Appliances.
Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 83∕ 4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes." - 2020 National Electrical Code (Fair Use Extract)
Examining the code language, our branch circuit conductor ratings and the overcurrent device ratings, which dictate the circuit rating, have to be rated no less than the maximum load to the served. In our case, it was 33.3 Amps. However, since our original range, which is actually 12 kW, is clearly above 8 3/4 kW, bringing the last part of the paragraph into play. It says "For ranges of 8 3/4 kW or more rating the minimum branch-circuit rating shall be 40 amperes.".
The conclusion is that the 35 Amp OCPD is not acceptable. It would need to be at least 40 amperes since our original range, not the load calculation, was indeed over 8 3/4 kW.
The actual conductor has to handle the maximum load to be served, which is 33.3 Amps. Since nothing was stated about the terminal limitations and nothing about the conductor insulation type being used, we will default at 110.14(C)(1)(a) and choose a conductor from the 60°C column of Table 310.16.
The resulting conductor size would be an 8 AWG CU or 6 AWG AL.
What size branch circuit is needed for a 4 kW Electric Counter-Mounted Cooking Appliance (CookTop)?
Since this article is about branch circuits we are still in that branch circuit mode. The only difference in the above question is that now we are talking about the counter-mounted cooking unit, such as a cooktop. The nameplate rating of the appliance is given as 4 kW. Let's look at Note 4 again, with a focus on the underlined text.
"4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range." - 2020 National Electrical Code (Fair Use Extract)
When you have (1) wall-mounted oven or (1) counter-mounted cooking unit, which is what we have, the code says the "nameplate" rating of the appliance shall be used.
So, the nameplate of the "cooktop" is 4 kW or 4000 Watts, which is equivalent to 4000 VA. Taking 4000 ÷ 240 V = 16.66 Amps. Using the exact same rules we previously used for the range, go to 240.6(A) and select the OCPD. Since a 16.66 Amp OCPD doesn't exist and since we meet the rules in 240.4(B) the next size-up OCPD would be 20 Amps.
For the conductor sizing, staying in the 60°C column of Table 310.16, we see that the conductor needed to handle the 16.66 amps is a 12 AWG copper. At this point, you may also notice the asterisks next to the 12 AWG in the left column. That reference sends you to the bottom of the table where the notation is given to review 240.4(D), which we call the small conductor rules. Here is the relevant language from 240.4(D).
"240.4(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.
240.4(D)(5) 12 AWG Copper.
20 amperes" - 2020 National Electrical Code (Fair Use Extract)
As a result of our efforts, the branch circuit is a 12 AWG and the OCPD is 20 Amperes for this branch circuit supplying the electric counter-mounted cooking unit.
In the next part (Part 2) of this series, the discussion will shift to load calculations that pertain to the service and feeder loads for a One-Family Dwelling.
Paul Abernathy, CMECP® | CEO & President
Electrical Code Academy, Inc. | www.FastTraxSystem.com Office: 214-945-0653
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