RV Park Feeder Question

[charlie b]

NEC 2008 applies. Consider a single site at an RV park. The power pedestal has breakers and outlets for 20, 30, and 50 amps. 551.73(A) tells me to calculate the feeder load at not less than 9600 VA. At 240 volts, that equates to 40 amps. I would like to supply this power pedestal with a 50 amp, 120/240 volt, single phase breaker, using a wire with an ampacity of 47.5 amps.

Whatcha think? Legal? Illegal? Cheating? I?ll tell you why I am asking, after I get a few opinions.

 

[jap]

Why would what you want to do be Illegal or cheating?

 

[charlie b]

A power pedestal with a 50 amp receptacle being fed with a conductor that only has an ampacity of 47.5 amps. That is the issue.

 

[jap]

What kind of wire are you planning on using that is only rated for 47.5 Amps?

 

[jap]

I generally use #8 thhn for a 50 amp feeder or #6 XHHW Aluminum for a 50 amp Feeder.

Unless we're derating for some reason.

I feel like I'm being set up here since i know how smart you are.

 

[Dennis Alwon]

210.23(B)(3) allows a 40 amp circuit to supply a 50 amp receptacle so why would the 47 amp wire matter. In fact you could use 240.4(B)

 

[charlie b]

What kind of wire are you planning on using that is only rated for 47.5 Amps?

#4 XHHW, with 12 conductors in the same conduit. 50% derating from 95 amps gives you 47.5.

 

[don_resqcapt19]

Since this is a feeder and not a branch circuit, 240.4(B) will let you round up to 50 amps for the OCPD for the 47.5 amp feeder conductors. The calculated load is less than the ampacity of the conductor. I think you are good to go.

 

[jumper]

Since this is a feeder and not a branch circuit, 240.4(B) will let you round up to 50 amps for the OCPD for the 47.5 amp feeder conductors. The calculated load is less than the ampacity of the conductor. I think you are good to go.

Calculated load is not really stated in this thread, it only says "not less than 9600 w". Could be more, no?

 

[charlie b]

This is an existing RV park. A 225 amp panel with 40 amp breakers supplies power pedestals with 120 volt, 20 and 30 amp breakers and outlets. A gutter underneath the panel distributes the feeders into conduits that head out in many directions. The owner wants to upgrade to pedestals with those same outlets plus a 50 amp, 120/240 volt outlet. The question relates to whether I can use the existing wire or whether I have to replace it with larger wire.

Some (but not all) handholes were opened during a site visit earlier this week. I took photos of the wiring. In one handhole, it looks like twelve conductors enter via one large conduit. Nine of them leave via another large conduit, and three leave via a smaller conduit. The three are presumed to be heading towards the power pedestal of one of the RV sites. I infer that in the next handhole downstream, nine conductors enter, six continue on, and three peel off towards the next RV site.

My concern is that I did not see the first handhole in any sequence. What if as many as five RV sites get their power starting from the same conduit? That’s 15 conductors, 10 of which are CCCs. Since the conductors would be derated to 47.5 amps, could I still use them, when I replace the panel (giving me 50 amp breakers) and the pedestals (giving me 50 amp receptacles)?

 

[Dennis Alwon]

I still say you are good to go.

 

[jumper]

I still say you are good to go.

Even if the load is 49 amps?

 

[defears]

My concern is that I did not see the first handhole in any sequence.

Assuming leads to disaster. Nobody can answer without knowing the facts.

 

[jap]

I never understood where they came up with the calculations for an RV site anyway.

We have a festival every year and when those 40 campers show up and all plug in and turn on all the air conditioners in the 107 degree weather, and start cooking, you would have wished you had calculated the load at 250%

 

[charlie b]

Even if the load is 49 amps?

The book only requires me to calculate it at 9600 VA. I don't have a specific reason to use a higher number. This particular RV park is not far from my own home in Seattle. We don't often get temperatures in the high 80's, let alone the 90's and beyond. But the park is going to be full a high percentage of summer days. Tell you what, how's about I calculate the load at 11,400 VA per site? :happyyes:

 

[charlie b]

Assuming leads to disaster. Nobody can answer without knowing the facts.

I know. That is why, in my design narrative, I stated that a field verification of existing conditions would have to precede any design efforts.

 

[jumper]

The book only requires me to calculate it at 9600 VA. I don't have a specific reason to use a higher number. This particular RV park is not far from my own home in Seattle. We don't often get temperatures in the high 80's, let alone the 90's and beyond. But the park is going to be full a high percentage of summer days. Tell you what, how's about I calculate the load at 11,400 VA per site? :happyyes:

Nope IMO. you do not have a calculated load. So...

(D) Feeder-Circuit Capacity. Recreational vehicle site
feeder-circuit conductors shall have an ampacity not less
than the loads supplied....

What are the loads supplied? Don't know? Then full bore: 12kVA.

 

[jap]

Nope IMO. you do not have a calculated load. So...

(D) Feeder-Circuit Capacity. Recreational vehicle site
feeder-circuit conductors shall have an ampacity not less
than the loads supplied....

What are the loads supplied? Don't know? Then full bore: 12kVA.

I agree with this.
I even called the power company once to ask how they would calculated the load on RV Outlets and they indicated without knowing what was going to pull up, they calculate at 100%.

They would calculate a 100 amp feeder to a pedestal with a 50 amp 30 amp and a 120v GFI in it.

I know the 50 and 30 are in the pedestal for convienience, but, if there there, by rights there should be enough power run to the pedestal to service all receptacle outlets in the pedestal.

That would include enough power to run the 50,30 and 120v gfi all at the same time.

 

[charlie b]

. . . by rights there should be enough power run to the pedestal to service all receptacle outlets in the pedestal. That would include enough power to run the 50,30 and 120v gfi all at the same time.

I would call that a good design practice. I do not believe, however, that it is a code requirement.

 

[marti smith]

" not less than 9600 volt amps per site..." It doesn't say that you can't go higher. I think often we get hung up on one number and forget that the code is the minimum. Of course, if the calculation were based on greater than 9600 va, that wire wouldn't do. But you know that.