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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3401
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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Powering rv sites in a rv park expansion, load calculations

Customer Question

powering rv sites in a rv park expansion
JA: Sometimes electrical issues that that you think will be really complicated end up being easy to fix. The Electrician I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so we can help you best.
Customer: load calculations for medium size RV sites and extra-large rv sites
JA: Is there anything else the Electrician should be aware of?
Customer: yes
JA: What else should I tell the Electrician?
Customer: the owner of the park is asking to power the medium-size sites with a nominal service of 100 Amp and an actual of 50 Amp (204 V/1 Ph), but for the large or extra-large sites he wants 100 Amp nominal and 100 Amp actual . When I read table 551.73 on NEC, I do not get these same recommendations. If I listen to the owner, the size of the service is considerably higher than if I calculate it with the NEC.
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Electrician about your situation and then connect you two.
Submitted: 1 year ago.
Category: Electrical
Expert:  Kevin replied 1 year ago.

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) I agree with you. Nowhere within Article 551 does it state that RV sites must have 100 amp feeders. The NEC only states 20 amp, 30 amp and 50 amp sites. 100 amp feeder sites would be considered as "optional" in my code interpretation.

2) Article 551.71 (Type Receptacles Provided) states that a percentage of the total sites must be a mixture of the various feeder amperages.

3) Table 551.73(A) Demand Factors would also be applicable when calculating the site feeder and service entrance conductors.

Expert:  Kevin replied 1 year ago.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.


Customer: replied 1 year ago.
Hello Kevin:
So what is your recommendation for the extra-large sites. For the medium sites, I have: 5 straight runs of 17 sites, 1 straight run of 16 sites, and 1 straight run of 15 sites. For the large and extra-large sites, I have 1 straight run of 9 sites, 1 straight run of 16 sites, and 1 straight run of 21 sites.All sites have a power tower comprised of (1) 20-Amp 1-pole breaker, (1) 30-Amp 2-pole breaker, and (1) 50-Amp 2-pole breaker.For the medium sites, I can run a feeder from a 200 Amp breaker to feed a group of 4 sites, or a 250 Amp breaker for 5 sites.
What would you do with the large and extra large? 200 Amp for 2 sites?..F.
Expert:  Kevin replied 1 year ago.

1) Yes, installing either a 200 amp or a 250 amp feeder will satisfy the 50 amp sites.

2) For the medium sites you will have no other choice but to 1st satisfy article 551.71 (percentage breakdown on receptacle types) and then apply the demand factors.

3) I would then apply article 551.73 (D) Feeder circuit capacity. and calculate the larger sites (100 amp) separately and then apply the demand factor table. Since the total number of large sites = 46, use a demand factor of 41% ------> (36 plus sites) as shown in table 551.73(A).

4) Then total the 2 load calculations from above and size the service based on these 2 load calc's. By separating the 2 load calc's and then totaling them, this will satisfy 551.73(D).

Your thoughts?

Expert:  Kevin replied 1 year ago.

1) Since the RV park owner is requesting for 100% demand on each of the 100 amp sites, that's gonna be a large increase on the total park service, not to mention your individual feeder runs. I recommend to show the customer article 551.73 and explain that the probability of all sites being filled to capacity and all using 100 amps each and simultaneously is highly unlikely. Thus the reason why the NEC developed the demand table.

Customer: replied 1 year ago.
The problem is that all the extra large sites run the heating during the night. If you read the fine print notes FPN on the bottom of 551.73(C). It says (I use NEC 2002) that he demand factor may be inadequate in extreme weather areas. When the park has no extra large vacancies, the breakers trip. I need give them more than the 41% that you mentioned above. But the question is how much. If you go back to my listing, each straight run has its own panel, or I can add one if they don't already have it. This means that the demand factor is for fewer than 36-plus units. Also, if you read 551.73(A), what does "supply facility" mean? a power supply tower unit next to each RV? If that is the case, what load should I assume for each one of these stations? 9600 VA? or is this load for the 50 Amp breaker only? the power supply tower station does not have only a 50 Amp breaker but also a 30 Amp and a 20 Amp. So what load is the actual basis of calculation.
Expert:  Kevin replied 1 year ago.

Thanks for the replies.

1) Where the tower has more than 1 receptacle, the calculated load shall be calculated only for the highest receptacle. If a 50 amp breaker, then the 9600 VA value will be used. 9600 VA will the basis of calculation per 551.73(A) sentence in the 1st paragraph (highest rated receptacle).

2) Yes, the supply facility will be the tower (the plug-in pedestal) located next to each RV site.

3) I'm using the 2011 NEC and it also shows the FPN for extreme hot or cold temperatures. In consideration of the FPN and this being a case-by-case basis. I would ask the RV owner to quantify the average site capacity or future site capacity during the summer and winter months. Logically, I would start out by splitting the 36 sites by 50% down to 18 sites, thus increasing the demand factor from 41% to 47% as a minimum. This will provide an increase in amps. Once again, in cases such as this, I think obtaining an average capacity during the summer and winter months would be in order and appropriate to confirm from the park owner. So the $64K question for the owner is "will all 46 of the large sites be utilized all of the time" during summer and winter months? I doubt it:)

Keep in mind, you can only design the electrical distribution system based upon the NEC requirements as well as the estimated loads. How many RV rigs that stay in the park are going to have 240 volt electric hot water heaters, 240 volt electric dryers, 240 volt ranges, 240 volt heaters, etc. all running simultaneously? Far and few in between will have these amenities in my opinion. For example, even a 100 amp service on a 1700 sq feet house with electric appliances will only pull an average of high 80 or low 90 amps in most cases under average Full Load.

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