New RV park

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Master B

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Hi guys.
A friend of mine is working with the county, the power company and several vendors to design and build a new RV park from the ground up and has asked me to advise on the electrical.
I have several questions.
First? Article 551 has some tables in it that detail what appears to be the absolute minimum for a park with 35 spaces. Table 551.73A indicates that the minimum demand factor for a 35 space park can be as low as %42. He intends to install UL listed manufactured electrical pedestals made by Midwest Industries. They are rated at 100 amps.
My question is, do we indeed have to install 100 amp breakers in the branch circuit distribution panel and connect it to 100 amp rated aluminum wire if we have all 35 pedestals include all three of the 20 amp 120 volt, 30 amp 120 volt and the 50 amp 120/240 volt receptacles? I ask because preliminary research has made it clear that the smaller tow behind RVs usually only have the 30 amp 120 volt power cords while the larger 5th wheel RVs usually have the 50 amp 120/240 volt power cords. As such it is more than unlikely that both the 30 amp and the 50 amp outlets will ever be used at the same time. Because he wants to have all three outlets available for his guests, if I calculate for both the 20 amp 120 volt outlet and the 50 amp 120 240 volt outlet, can I omit including the 30 amp 120 volt outlet in the demand factor calculation and supply for only the 50 and 20 amp outlets at 70 amps instead of 100 amps to save on the expense of the larger wire?
Thanks guys,
Norm
 
Doesn't directly answer your questions but many RV peds are dual-lugged for connection into a daisy-chain (some have lugs that go up to 2/0)- one branch circuit and 5-8(?) peds looped off of that. The demand factor acts on that circuit.

Look at the spec sheet for some of the pedistals and how their breakers are set up.
 
The pedestals are 100 amp rated from the feed+thru lugs at its base up to its OCPD section and receptacles. The feed thru lug section is rated 200 amp (maybe 225a, I cant remember). As far as your calculation, you only take the largest receptacle provided not 50a and 20a together ( in your case the 50amp to do your calculation as set forth in the article) 12,000va x number of sites in run x demand factor %.
Ex. A 200amp branch feeder from your distribution panel could supply a run of 7 sites.
7 sites X 12,000va ( for sites equipped with a max size recept of 50 amps) X demand factor ( 7 sites = 55%)
7 sites x 12,000 va = 84,000 total va
84,000 va x demand factor 55% =. 46,200 minimum allowable va
46,200 va / 240 volt = 192.5 Amp Use 200 amp feeder breaker.
 
Zbang, Steve, Thank you.
Yeah I re read most of the entire article and found that at the very last sentence in 551.73. So? The demand factor for this could be as low as 35X12,000 X .42 = 176,400 watts. 176,400 / 240 volts = 735 amps.
The spec sheets for the pedestals he wants indicate a 250 amp feed through using aluminum wire.
I guess the next question I have for the owner is does he want to provide for the additional 20 amp circuit as I believe he is negotiating with the contractors that are going to build the giant wind farm here in south east Wyoming for a lot of their tradesmen to stay at his place year round for as long as the next three or four years. I say this because Wyoming winters are harsh and most of his guests will probably want to plug in their vehicle block heaters.
There is also the informational note just before 551.72 warning that the demand for the 50 amp circuits could approach %100.
I'll keep you posted as this project evolves.
Thanks,
Norm
 
IIRC block heaters don't take much (a couple hundred watts), so they won't contribute too much overall. On the other hand, a lot of extra electric heaters in the RVs might (when propane isn't enough).

Is this extra 20a part of the manufactured pedestal or an add-on?
 
It's on the UL listed pedestals. If you look at figure 551.46(C) (1), you'll see the configurations required. The owner wants to have all three available for his guests. He also believes they will no doubt have all sorts of devices, like bug zappers and USB chargers plugged into them as well at differing times of the year.
Then there's always the need to use electric weed eaters or mowers on the property as well.
 
Master B said:
Hi guys.
A friend of mine is working with the county, the power company and several vendors to design and build a new RV park from the ground up and has asked me to advise on the electrical.
I have several questions.
First? Article 551 has some tables in it that detail what appears to be the absolute minimum for a park with 35 spaces. Table 551.73A indicates that the minimum demand factor for a 35 space park can be as low as %42. He intends to install UL listed manufactured electrical pedestals made by Midwest Industries. They are rated at 100 amps.
My question is, do we indeed have to install 100 amp breakers in the branch circuit distribution panel and connect it to 100 amp rated aluminum wire if we have all 35 pedestals include all three of the 20 amp 120 volt, 30 amp 120 volt and the 50 amp 120/240 volt receptacles? I ask because preliminary research has made it clear that the smaller tow behind RVs usually only have the 30 amp 120 volt power cords while the larger 5th wheel RVs usually have the 50 amp 120/240 volt power cords. As such it is more than unlikely that both the 30 amp and the 50 amp outlets will ever be used at the same time. Because he wants to have all three outlets available for his guests, if I calculate for both the 20 amp 120 volt outlet and the 50 amp 120 240 volt outlet, can I omit including the 30 amp 120 volt outlet in the demand factor calculation and supply for only the 50 and 20 amp outlets at 70 amps instead of 100 amps to save on the expense of the larger wire?
Thanks guys,
Norm
Click to expand...

While all sites might have both 50 amp and 30 amp receptacles installed, 551.72(F) allows for only one of these two receptacles to be in use at a time (however, the 20 amp receptacle CAN be used at the same time as either the 30 amp or 50 amp receptacle, and as you stated, probably will by some users). I don't see anything in the code that breaks the 20 amp receptacle out as a separate load from the 551.73 table values (non-pedestal loads ARE broken out), so it's assumed that the entire mix of loads is taken into consideration for the demand factor. In other words, it doesn't matter what ratio of 20 amp, 30 amp, or 50 amp loads exist among the various RV sites--the table values assume that it's a mix that's averaged to that demand factor.

HOWEVER, the table values are minimums, and there's nothing stopping you from exceeding these minimums if your knowledge of the likely use of the site dictates that larger load capacity is warranted. For example, if you own an RV park in south central Texas that's typically filled with large, 5th-wheel RVs with double rooftop AC units, then it's not unreasonable that the actual loading will be in excess of the demand factor in the table for much of the summer. In an extreme case, if you feel it's warranted (and if the owner wants to pay for it), there's nothing stopping you from using a 100% demand factor and sizing your wires and equipment accordingly.

In regard to your initial question about breaker sizing--every RV park pedestal that I've seen had separate breakers for the 50 amp, 30 amp, and 20 amp receptacles, but since the 50 amp and 30 amp are by code mutually exclusive (and rarely operating near peak load) and since the 20 amp convenience receptacle is considered part of overall loading, I don't see anything in the code that would prevent you from sizing the pedestal for just 50 amps. You may also want to dig into the pedestal specs more deeply and look at the actual ratings of the internal components, since I suspect that this could be a limiting factor in some cases. A 50 amp RV hookup cable is going to be designed and sold for that purpose and so a 50 amp breaker will protect that wiring (and the receptacle will be rated for it). Likewise for a 30 amp receptacle and the 20 amp convenience receptacle. But because the 20 amp convenience receptacle shares the same current path as the 50 amp and 30 amp receptacles, it's helpful to understand how your particular pedestal is set up internally. My gut instinct is that virtually all of these are overdesigned by quite a bit because the lugs are designed to be daisy chained together and its understood that someone might have both 50 amp and 20 amp loads simultaneously, but verify the values.

When it comes to the actual wiring, the voltage drop is likely going to be the limiting factor. That code chapter reminds you of this, but it's entirely possible for the wire to be well within the required current capacity from end to end across all of the pedestals, yet have 20% or more voltage drop. Consequently, in many cases, you're going to find that you need to use some ridiculous wire sizes relative to what you're used to if the sites are far apart. For this reason, it pays to take into consideration how the sites are laid out--far better to use a hub and spoke system when possible rather than a long line of RV sites.

Regarding "...Then there's always the need to use electric weed eaters or mowers on the property as well...", I would recommend installing separate 20 amp receptacles around the property in opportune locations around the sites for use by maintenance staff, rather than have the expectation of plugging these into the pedestals. If you do this, they'll need to be on their own circuit, per the same code section above.

/r
Brian
 
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Hi guys.
I've about got this figured out but I'm not finding either USE or URD aluminum wire dimensions in Chapter 9 or the tables in810.16. Everything else is in there. THHN, XHHW and all the rest. We want to use the quadreplex USE or URD in rigid nonmetalic schedule 40 PVC. The closest I've come is the fill tables in 810 with four # AWG 1/0 being at the %40 fill.
Any suggestions?
Thanks,
Norm
 
I think it’s comparable to RHW maybe that’s in the chart?
So what size of conduit are you running? Why are you only running 1/0?
The sites I’ve done we run conduit from distribution panel to a in-ground box near the first pedestal in a row. From there we just direct bury between sites. I recommend copper for longevity, aluminum underground will have problems within a year of any damage to it is caused during installation. ( had a customer that did most of the work to his campground, had damaged URD right off the spool he didn’t realize. I’ve had to fix a dozen or so locations where the Aluminum URD blew apart.
Definitely use long sweeps or fiberglass sweeps, esp if your feeder runs are longer than 100’.
 
We're only going to put two pedestals on each branch circuit and we figured the minimum demand factor above the allowed %42 at at least %75.
We're also hoping the local power provider will give us four pad mount 120/240 volt services, one for each branch circuit distribution panel, with five 1&1/4 inch conduits in each of them for a total of forty pedestals. If we have all of this in one service it will be well above the 1500 amp requirement for ground fault protection with all the additional associated expense. We are putting the USE/URD in conduit because the whole place out there is loaded with gophers, ground squirrels and prairie dogs. I've had a lot of bad experiences with them chewing through direct burial.

These are all going to be back in sites too. No pull throughs. Got my new code book too. "Bout time huh?
 
Been a while since I read up on this section, but I seem to recall a 15/20 amp 120 volt receptacle is required at every site, the 30 and 50 amp receptacles are what is optional.

There are "double sided" pedestals also, work great if you have two back to back sites only one pedestal to serve two sites.
 
Master B said:
We're only going to put two pedestals on each branch circuit and we figured the minimum demand factor above the allowed %42 at at least %75.
We're also hoping the local power provider will give us four pad mount 120/240 volt services, one for each branch circuit distribution panel, with five 1&1/4 inch conduits in each of them for a total of forty pedestals. If we have all of this in one service it will be well above the 1500 amp requirement for ground fault protection with all the additional associated expense. We are putting the USE/URD in conduit because the whole place out there is loaded with gophers, ground squirrels and prairie dogs. I've had a lot of bad experiences with them chewing through direct burial.

These are all going to be back in sites too. No pull throughs. Got my new code book too. "Bout time huh?
Click to expand...
Looks like you need to re-think the conduit size. 1 1/4 pvc sch40 by table C.11 only allows 2 RHW conductors (which I believe is the other listing typical of URD cable) . Don’t forget the EGC whether your running a 4 conductor URD or separate conductor for sizing the conduit.
 
Your branch circuit needs to be figured at 90% demand factor if you are only having 2 sites per branch circuit.
2 sites X (50a max provided circuit) 12,000va = 24,000va
24,000va X demand factor 90% = 21,600 va
21,600 va / 240 volts = 90 amps
Size wire accordingly for at least 90 amps and any voltage drop if so chosen.
 
mopowr steve said:
Your branch circuit needs to be figured at 90% demand factor if you are only having 2 sites per branch circuit.
2 sites X (50a max provided circuit) 12,000va = 24,000va
24,000va X demand factor 90% = 21,600 va
21,600 va / 240 volts = 90 amps
Size wire accordingly for at least 90 amps and any voltage drop if so chosen.
Click to expand...
That means you can have 90 amp conductor minimum, but keep in mind if you want a 100 amp breaker on it you still need 100 amp conductor.

You also can't have an 85 amp conductor with 90 amp breaker.
 
Hi have a question that MASTER B touched on.
He said "The spec sheets for the pedestals he wants indicate a 250 amp feed through using aluminum wire."
I can't find a rating on the feed through lugs.
I can find what what size they accept, but wire size goes up for voltage drop so it's not uncommon to use a large conductor to feed these.
Here is my question.
If I have 100A or 125A rated pedestal with the following breakers, 20A/1P, 30A/1P, 50A/2P. how can I put this on a 200 or 225A circuit breaker?
The wire from the feed through lugs to the breakers is only rated for 100A.
Is there something in the tap rules which allows this?
I know this is common from drawings I've seen but would like to know specific code references if possible.

thanks
 
msu-ee said:
Hi have a question that MASTER B touched on.
He said "The spec sheets for the pedestals he wants indicate a 250 amp feed through using aluminum wire."
I can't find a rating on the feed through lugs.
I can find what what size they accept, but wire size goes up for voltage drop so it's not uncommon to use a large conductor to feed these.
Here is my question.
If I have 100A or 125A rated pedestal with the following breakers, 20A/1P, 30A/1P, 50A/2P. how can I put this on a 200 or 225A circuit breaker?
The wire from the feed through lugs to the breakers is only rated for 100A.
Is there something in the tap rules which allows this?
I know this is common from drawings I've seen but would like to know specific code references if possible.

thanks
Click to expand...
If pedestal is rated 100 or 125 amps - I would think that generally would mean it has 100 or 125 amp panelboard bus in it and must be protected at no higher than that rating, unless it has a single main breaker to protect the bus then it's rating would be limited to that main breaker.

Even outside taps of unlimited length must still end in a single overcurrent device so a main lugs assembly with multiple breakers wouldn't qualify for that tap rule.
 
zbang said:
How many lugs carry a current rating at all? I was over looking at the Ilsco site, and none of the lugs I looked at had one, only conductor sizes. (Didn't check other mfgs.)
If you're concerned, contact the ped's manufacturer.
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A lug itself you would think can carry the same current as ampacity of any conductor it is rated to accept, at whatever temp rating it has.

But a "feed thru lug" in the situation of OP also has a bus in the equipment it is connected to to consider.
 
kwired said:
A lug itself you would think can carry the same current as ampacity of any conductor it is rated to accept, at whatever temp rating it has.

But a "feed thru lug" in the situation of OP also has a bus in the equipment it is connected to to consider.
Click to expand...
As I understand it, lugs are rated to carry the maximum current that could result from the largest wires being connected to each hole.

So if you have a lug with two 1/0 openings on it, the lug would be able to handle however much current two 1/0 wires could possibly handle. Interestingly, many (maybe most or all) lugs are actually listed by UL at 90 deg C. I don't know if it is true or not but somewhere along the way someone once told me that listed lugs are required to be listed for 90 deg C while recognized ones can be rated for a lower temperature.
 
petersonra said:
As I understand it, lugs are rated to carry the maximum current that could result from the largest wires being connected to each hole.

So if you have a lug with two 1/0 openings on it, the lug would be able to handle however much current two 1/0 wires could possibly handle. Interestingly, many (maybe most or all) lugs are actually listed by UL at 90 deg C. I don't know if it is true or not but somewhere along the way someone once told me that listed lugs are required to be listed for 90 deg C while recognized ones do not.
Click to expand...
A lug for a single conductor I would expect to be able to carry the current related to the conductor it can accept.

Many two port lugs with an attachment provision I would think probably are rated twice the largest conductor, but maybe need to take in some considerations.

A Polaris type multiport assembly - I don't know you can bring in say three 200 amp conductors in parallel on a unit that doesn't accept much more on conductor size and expect it to carry 600 amps, or if so you may at least need the supply conductors in the center ports and outgoing conductors in the outer ports. Three in on the left and three out on the right is asking the middle section to carry all 600 amps, which it may or may not be able to do so without excessive heating.
 
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