Mobile Home Calculations – 550.18

MOBILE HOMES CALCULATIONS – STANDARD METHOD

I would like to start off this article by saying that it would be exceedingly rare to use 550.18 to perform a load calculation for a mobile home in the real world. In fact, most mobile homes have a nameplate with the ampacity values given and you would simply size the Service and Feeders accordingly. However, it is common on electrical exams to have a calculation question using 550.18 so this article is dedicated to that effort and is for exam preparation purposes only.

The reason we need to understand 550.18 is when calculating a mobile home part, the calculation method in 550.18 is referenced in 550.31. As a result, we need to make sure you understand how to calculate the loads in 550.18 to give yourself a better chance of answering the question on the exam.

Demand load calculations for mobile homes are like those for one-family dwellings. The size of the service is based on many of the same conditions as a standard home. Standard calculation forms for mobile and manufactured homes can be used to keep track of each load. See Informative Annex D11 in the 2023 NEC for an example. The load of many of the sections is calculated in amps. Convert the volt-amps: into amps by dividing the volt-amps by the voltage of the appliance.

Once the load for each section is calculated, the total amperage is obtained by adding the current of each section. The loads must be balanced between the two phases as closely as possible to provide a reasonable service size. For example, once the lighting load is determined, the load will be divided by 240 V and distributed equally between the two phases. The 120 V loads will be placed on the phase that better balances the system.

Lighting, Small-Appliance, and Laundry Loads for Mobile and Manufactured Homes

Per 550.18(A)(1), the lighting load for a mobile or manufactured home is found by multiplying the square feet of the home (outside dimensions) by the volt-amps per square foot that is given in 220.41. As with other dwellings, the lighting and receptacle loads require 3 VA for each square foot.

Per 550.18(A)(2), each small-appliance circuit is calculated at 1500 VA. Unlike a typical one-family dwelling, there is not a two-circuit requirement. However, keep in mind that at least one is required and if there happens to be more than one then you would need to add 1500 VA for each additional small-appliance circuit.

Per 550.18(A)(3), each mobile home is required to have one laundry circuit. This circuit is calculated at 1500 VA. Once each area is calculated above, the total connected load is found by adding them together. Now, on many electrical exams they may not specify a laundry circuit but unless told to not include it always include the 1500 VA for at least one. If they happen to tell you there is more than one then simply add an additional 1500 VA for each additional they include in the electrical exam question.

EXAM PREP TIP – In the real world, if you happen to forget about the laundry circuit at 1500 VA or the Largest Motor at 25%, it should not “Steam Roll” your overall calculation, but in the exam prep world it may affect your conductor sizing to the mobile home so read the questions carefully. This author has a routine of tricking students by making up various loads and then omitting some required loads to see if they challenge me. The best way to learn is to simply ask the right questions.

Calculating Total Demand Load for Mobile and Manufactured Homes

The same derating factors that apply to standard one family dwellings apply here as well. Per 550.18(A)(5), the first 3000 VA is rated at 100%, the remaining volt-amp is rated at 35%. The total volt-amps are divided by the supply voltage, which is typically 240 V, to obtain the current of each supply phase. The key here is that once the current (amps) for the two legs in the 240 V service is established by doing the calculation, the values moving forward are the current (amps) only and not the volt-amperes.

Sizing Service Load for Mobile and Manufactured Homes

Other items such as cooking equipment, clothes dryers, water heaters, disposals, and microwaves as well as any other item that might be installed are covered in 550.18(B). Once each area is calculated, the total current is obtained by adding the current of each area.

Per 550.18(B)(2), use the nameplate rating for electric heaters, A/C units, and blower motors. Use the largest between the heating and A/C. If the blower motor is used for both heating and A/C, it must be counted separately. If there is no A/C, use 15 A per phase for the A/C load for a 40 A service.

Per 550.18(B)(3), the largest motor shall have a demand factor of 25%. Use the largest motor from 550.18(B)(2). Per 550.18(B)(4), use the nameplate rating for fixed appliances. This includes disposals, dishwashers, water heaters, clothes dryers, wall-mounted ovens, and counter-mounted cooking units. Where there are four or more of these loads, apply a 75% demand factor to the total connected load. Notice: There is an error in the 2023 NEC for 550.18(B)(3), it reads Table 550.18(B) but it should read 550.18(B)(2). This error has been corrected in the ERRATA published March 24, 2023.

If there is a free-standing range, a demand factor is applied from 550.18(B)(5). The demand factor is determined by the rating of the range.

Per 550.18(B)(6), if other circuits are provided that are not factory-installed, estimate the load size of each additional circuit, and add them to your calculation.

Before we do an actual example, I want to clarify some items that students tend to ask when looking at Mobile Home calculations. We will examine them in the nature they are asked for in this article.

1. Doesn’t 550.32(C) require a minimum mobile home service of 100 Amps?

Answer: Yes and No, as stated in 550.32(C), this is the minimum rating of the service equipment. The circuit’s actual ampacity, the actual load calculation for the mobile home could be less than 100 amps. However, the service equipment must be rated 100 amps, but the feeder overcurrent protection may be less than 100 amps but more on that later.

2. If my load calculation performed by 550.18 is less than 100A and the mobile home is permanently wired, do the rules of 230.79(C) for dwelling units apply?

Answer: No, the code arrangement rules in 90.3 dictate that Chapters 1-4 are applied generally to all electrical installations. However, Chapters 5, 6 and 7 can modify and/or supplement any of the rules in Chapters 1-7. So, in our case Chapter 5 is modifying the general rules in Chapter 2.

3. So, I am confused. Is 550.18 about the service or a feeder?

Answer: The rules for what circuits and so forth for a mobile home, if you are designing and wiring one, are found in Part II of Article 550. When calculating the service loads or feeder loads you are firmly in Part III of Article 550. It states in 550.18 that the calculation is for supply-cords and distribution panels, such as often called sub-panels.

While 550.32(C) demands the “service equipment” to be rated at least 100 amps, and also clear that the feeder conductor capacity, other than for power cord allowances in 550.10(A), must have a “capacity” of at least 100 Amps per 550.33(C) in the 2023 National Electrical Code, even if the actual calculated load itself is less than 100 amps. However, 550.18 is specifically about calculating the power cord or feeder conductors to the distribution panel in the mobile home.

4. Could I use a Power Cord instead of permanent wiring?

Answer: Sure, as long as your calculated load based on 550.18 is 50 amps or less as noted in 550.10(A). However, if the calculated load performed in 550.18 is over 50 amps, then a permanent method of installation would be required, and the power-supply cord is not an option.

At this stage in the article, it is important to also address some additional thoughts on the above information.

For example, when you read 550.32(C) it states the minimum rating of the service equipment is 100 amps, then per 550.33(C) [550.33(B) in the 2020 National Electrical Code] it states the feeder conductors to the mobile home must have a capacity and ampacity of at least 100 amps as well. At this point the question often gets asked, can I have a calculated load, based on 550.18, of 60 amps to a mobile home?

So, if an electrician subsequently installs service equipment on a pole adjacent, within sight of the mobile home (the within 30′ was removed for the 2023 NEC), and be rated minimum 100 amps as per 550.32(C), the feeder conductors, where calculated per 550.18 for a mobile home, which must have a minimum capacity for the load served and minimum ampacity rating of 100 amps per 550.33(C) as stated in the 2023 NEC, the electrician ultimately decides to terminate the mobile home feeders to a 60 Amp rated overcurrent protection device located in the service equipment. Is this an acceptable practice?

Yes, if the 60-amp circuit breaker or fuse lugs can accommodate the conductor’s size without violating the devices and the lugs listings then of course. Remember the adage, a 200A rated panelboard being utilized as a remote distribution panel (sometimes called sub-panel) that is being fed on the supply side by a 100 amp rated feeder overcurrent protection device and provided with 100 amp rated feeder conductors is quite common and acceptable. Just because the remote distribution panel has an equipment rating of 200 amps, the load is still limited to 100 amperes where the overcurrent protection on the feeder is set at 100 amps.

Now, upon inspection of the service equipment rated at least 100 amps, the feeder conductors having a capacity to handle the load and an ampacity rating of at least 100 amps, while in our example the overcurrent protection is rated at 60 amps based on the calculated load, the installation is still compliant even with the 60 amp feeder overcurrent device installed.

OK, as the author of this article I must be honest, if our electrician is going through the steps of compliance to meet the minimum 100 amp service equipment ratings per 550.32(C), and the feeder conductor capacity and ampacity requirements of 100 amps per 550.33(C), then why not just install a 100 amp overcurrent protection device and be done with it? Well, we will take a stab at that later.

CODE TIP – It is clear in the 2020 NEC per 550.33(B) and per 550.33(C) per the 2023 NEC that section 310.12, the 83% rule, can be applied to these calculations in terms of the feeder conductor sizing where all the applicable rules in 310.12 are applied, such as being at least 100 amperes feeder rating, which is established by the feeder’s overcurrent protection device.

Calculation Example: [Based on the 2023 National Electrical Code]

• Size the service for the following 1120 sq. ft Mobile Home:
  1. (2) Small Appliance Branch Circuits
  2. (1) Range @ 240V = 6000 VA

• (1) Heater @ 240V = 1500 VA

1. (1) Blower Motor @ 120V = 500 VA
2. (1) Dishwasher @ 120V = 500 VA

Now, before we get started let’s not forget that even if we did not express a laundry load at 1500 VA it must be included regardless in our calculation.

Step 1) [550.18(A)(1)] – 1,120 sq ft x 3 VA = 3,360 VA

Step 2) [550.18(A)(2)] – 1,500 VA x 2 = 3,000 VA (small appliance branch circuits)

Step 3) [550.18(A)(3)] – 1,500 VA x 1 = 1,500 VA (laundry circuit)

Step 4) [550.18(A)(4)] – 3,360 VA + 3,000 VA + 1,500 VA = 7,860 VA

Step 5) [550.18(A)(5)] – 7,860 VA – 3000 VA (100%) = 4,860 VA x .35(35%) = 1,701 VA + 3,000 (the original 100%) = 4,701 VA / 240 V = 19.58 VA or 20 VA per leg of the 240 V system, herein referred to as Leg A and Leg B.

Now, the additional loads.

Range [550.18(B)(5)] – Per table 550.18(B), Range load of 6000 VA x .80 = 4,800 VA / 240 = 20 Amps on Leg A and Leg B

Heater [550.18(B)(2)] – Since no AC loads was given, we assume the heat is the greater of the compared loads. Heater load of 1,500 VA / 240 = 6.25 amps on Leg A and Leg B

Blower Motor [550.18(B)(2)] – Blower motor load of 500 VA / 120V = 4.16 amps on Leg A

Dishwasher [550.18(B)(4)] – Dishwasher load of 500 VA / 120V = 4.16 amps on Leg B

Largest Motor [550.18(B)(3)] – Since the blower motor is the largest motor, you need to only capture 25% of the motor’s load. 600 VA x .25 = 150 VA / 120V = 1.25 amps on Leg A or B, our choice but in our example, we will put it on Leg A.

Sum – Leg A equals = 20 amps (lighting, small appliance, and laundry), 20 amps (range), 6.25 amps (heater), 4.16 amps (blower motor), and 1.25 amps (largest motor) which totals 51.66 amps or 52 amps.

            Leg B equals = 20 amps (lighting, small appliance, and laundry), 20 amps (range), 6.25 amps (heater), and 4.16 amps (dishwasher) which totals 50.41 or 50 amps.

Based on our load calculations using 550.18 the minimum size service is 60 Amps. Now, as we read previously, the service equipment must be rated 100 amps, the feeder conductors must have a capacity of at least 100 amps as well. However, you could terminate the feeder conductors on a 60 ampere rated overcurrent device, provided the lugs permitted this. Keep in mind, if you did this you would not get the benefits of applying 310.12 since the overcurrent device is what sets the ratings, and you choose to use the 60 ampere device which doesn’t meet the allowances in 310.12(B).

Why is the last sentence above important? Well, the minimum size for 100 amp feeder ratings per 310.12 is 4 AWG CU or 2 AWG AL. if I used the 60 amp rating instead then we are forced to size the conductors based on at least 100 amp capacity as stated in 550.33(C), per Table 310.16 which would be 3 AWG CU or 1 AWG AL.

Ultimately you get to pick your own destiny. But I would be remise if I did not reiterate what I stated at the beginning of this article. All of this is really an exam preparation exercise. In the real world you would look at the nameplate of the mobile home and size the service accordingly and not to the actual calculation per 550.18. In fact, the only time you would use 550.18, in my opinion, is when you are sizing the service or feeders for a Mobile Home Park where 550.31 comes into play and the choice of 16,000 VA per mobile home lot versus the calculated load of 550.18. Whichever is larger would be used in the park’s service and feeder load calculations.

Hope you enjoyed this little exercise as there is not a lot on the internet that breaks down mobile home calculations, so this was the inspiration for this article.