Electrical short power question

[birdhunter]

I know we have a lot of electrical wizards here. I have a question relating to the shore power/park side: We have an older RV park with pedestals that have a 30 amp and a 20 amp outlet on each side of each pedestal to serve two spaces. We are thinking of upgrading the park. Would it be possible to use the existing underground wiring to the pedestals to put one 50 amp outlet on each pedestal instead of the two 30 amp outlets? I would think perhaps it would since RV 50 amp is two 120 volt legs. This would avoid us having to rip up and replace all the underground wiring.

Rich 2002 Magna

[mmartin]

to run 50 amp service you will need a minimum of 3 legs ( 2 hot legs and 1 neutral) of #6 wire and a minimum of 1 leg of #8 wire for the ground. Right now you probably have for the 30 amp service 2 legs of # 10 wire and #12 for the ground. You will need to pull all new wire

[twinboat]

Only if the underground wiring can handle 240 volt 50 amp service.

30 amp service is 120 volts only. Typically run with 10 gauge wire. It supplies 3600 watts of power.

50 amp is 120/240 volt service, typically run with 6 or 8 gauge wire. It supplies 12,000 watts of power.

50 amp is 2 lines at 50 amp each, not 2 lines at 25 amps.

Your best bet, having asked this question, is to consult a licensed electrician.

[cavie]

Originally Posted by birdhunter

I know we have a lot of electrical wizards here. I have a question relating to the shore power/park side: We have an older RV park with pedestals that have a 30 amp and a 20 amp outlet on each side of each pedestal to serve two spaces. We are thinking of upgrading the park. Would it be possible to use the existing underground wiring to the pedestals to put one 50 amp outlet on each pedestal instead of the two 30 amp outlets? I would think perhaps it would since RV 50 amp is two 120 volt legs. This would avoid us having to rip up and replace all the underground wiring.

Rich 2002 Magna

The existing 30 amp wire is not large enough for 50 amp service. You need all new wiring. ALSO you must take into account voltage drop. The longer the distance from the main service increases the need for larger wire than the standard #8 wire for 50 amps. There is a voltage drop chart for that.


You have 3 wire now. You need 4 wire for 50 amp.
You need an electrician engineer to design the system. Typically X size wire to each ped to feed 2 sites.

[Old-Biscuit]

Nope.......


Gonna need to pull ALL NEW WIRE
Probably need to dig up and run new conduit also


Then that brings into question....would you have to bring other utilities up to current code(s)
Opens up a can of worms........so best to check ahead with local building department and permitting section as to requirements to meet local laws/statues

[KanzKran]

It would depend on how your pedestals are fed. Many use 250-350 MCM feeders to multiple pedestals on one feeder, with pedestal bus terminals rated to 250A or more, and with circuit breakers at the pedestal for each receptacle. 250 MCM is about 1/2” in diameter (copper), not counting insulation, by the way. So if that’s the case, you may be able to reconfigure some of the pedestals, but you probably won’t be able to reconfigure all of them on a single feeder as the max total current won’t be there, and you’ll probably have to remove some from that chain.

Two 30A 120V receptacles combined are only good to 7.2kW while a single 50A 120/240V receptacle is good to 12kW, for discussion purposes. Or looked at another way, 30A on each of two hot legs for the existing 30A setup on two sides, vs 50A on each of those same two hot legs for a single 50A receptacle. That doesn’t include load diversity, of course, but you’d almost certainly need to run more wiring by breaking up those feeders to supply fewer pedestals and adding more feeders to supply fewer pedestal on each feeder. Again, that’s assuming pedestals are powered by large feeders.

If pedestals are fed by individual feeders to a distribution panel somewhere instead, then you’d need to pull the wiring and run new, and possibly upsize the conduit, as already mentioned.

You need to call an electrical contracting firm or design engineer experienced with RV park systems for a definitive answer I’m afraid. But it’s all but a certainty that you’ll have to do some big work to get what you want.

[KanzKran]

Doing some back of napkin calcs based on the NEC requirements for RV park site power yielded some interesting stuff.

If the present setup consisted of a 120/240V feeder powering 40 sites (two sides of 20 pedestals), each site consisting of 120V 20A plus 120V 30A, you'd need:

3600VA per site* x 0.41** = 1476VA per site (=12.3A at 120V)

1476VA x 40 = 59kVA total

59kVA / 240V = 246A on a 120/240V feeder

If you cut the system in half and run another feeder of the same size to the cut off half, and only use one 120/240V 50A receptacle on each pedestal:

9600VA per site* x .50** = 4800VA per site (=20A at 240V)

4800VA x 10 =48kVA total

4800kVA / 240V = 200A on a 120/240V feeder

I won't bother repeating the math, but 12 sites comes out to 240A, just under the 246A of the 40 sites in the first example.

All that is only valid for multiple power pedestals chained together on a heavy feeder, and even then, I was just playing with the number of sites. I have no idea how many the OP actually has. Nor the ampacity of the feeder. Nor even if that's how the sites are wired; they could be small individual feeders to one pedestal each (demand factor is 1.0 for one site per feeder, and 0.9 for two sites per feeder), in which case this is an exercise in futility. But it's just fun with numbers, and the interesting bit about demand factors [NEC Table 551-73] sheds some light on how load diversity can reduce the design demand on the system.

And for this and many other reasons, the OP has to call in a pro. But it may be possible to do minimal trenching and rewiring to add more capacity, but then, that means the service to the park may have to be increased, since the total load is being increased, and that will cost, too. And these are only minimums based on NEC values and requirements, and don't count voltage drop (though that's not code enforceable), and the OP may want to provide better than code minimums.

*3600 volt-amps per site for one 20A plus one 30A receptacle each at 120V, or 9600VA for one 50A (120/240V)

**0.41 demand factor for 36 or more sites on a feeder and 0.50 demand factor for 10-12 sites on a feeder

[cavie]

Originally Posted by Old-Biscuit

Nope.......


Gonna need to pull ALL NEW WIRE
Probably need to dig up and run new conduit also


Then that brings into question....would you have to bring other utilities up to current code(s)
Opens up a can of worms........so best to check ahead with local building department and permitting section as to requirements to meet local laws/statues

No need for conduit. There are Direct Bury cables available in all size wire and types. Ease of installation more than offsets the cost of the wire.

[dons2346]

Originally Posted by cavie

No need for conduit. There are Direct Bury cables available in all size wire and types. Ease of installation more than offsets the cost of the wire.

Direst bury could be a problem depending on what type of soil it is being buried in. I know of a park that direst buried and the insulation was penetrated by some of the small rocks causing problems. Park wound up rewiring within conduit

[birdhunter]

Thank you all for your excellent input! Doesn't sound good. I should have mentioned we currently have 12 pedestals each with two 30 amp and one 20 A. So there are 24 30 amp outlets. As far as I can see they are fed by two double pole 100 amp breakers which come off of a 200-amp breaker. Our goal would be to reduce things to twelve 50 A outlets although we could probably get away with 10. it would be done by a professional but I'm trying to determine if we could avoid trenching and running new wire and conduit which would cost a lot.

Rich 2002 Magna

Originally Posted by KanzKran

Doing some back of napkin calcs based on the NEC requirements for RV park site power yielded some interesting stuff.

If the present setup consisted of a 120/240V feeder powering 40 sites (two sides of 20 pedestals), each site consisting of 120V 20A plus 120V 30A, you'd need:

3600VA per site* x 0.41** = 1476VA per site (=12.3A at 120V)

1476VA x 40 = 59kVA total

59kVA / 240V = 246A on a 120/240V feeder

If you cut the system in half and run another feeder of the same size to the cut off half, and only use one 120/240V 50A receptacle on each pedestal:

9600VA per site* x .50** = 4800VA per site (=20A at 240V)

4800VA x 10 =48kVA total

4800kVA / 240V = 200A on a 120/240V feeder

I won't bother repeating the math, but 12 sites comes out to 240A, just under the 246A of the 40 sites in the first example.

All that is only valid for multiple power pedestals chained together on a heavy feeder, and even then, I was just playing with the number of sites. I have no idea how many the OP actually has. Nor the ampacity of the feeder. Nor even if that's how the sites are wired; they could be small individual feeders to one pedestal each (demand factor is 1.0 for one site per feeder, and 0.9 for two sites per feeder), in which case this is an exercise in futility. But it's just fun with numbers, and the interesting bit about demand factors [NEC Table 551-73] sheds some light on how load diversity can reduce the design demand on the system.

And for this and many other reasons, the OP has to call in a pro. But it may be possible to do minimal trenching and rewiring to add more capacity, but then, that means the service to the park may have to be increased, since the total load is being increased, and that will cost, too. And these are only minimums based on NEC values and requirements, and don't count voltage drop (though that's not code enforceable), and the OP may want to provide better than code minimums.

*3600 volt-amps per site for one 20A plus one 30A receptacle each at 120V, or 9600VA for one 50A (120/240V)

**0.41 demand factor for 36 or more sites on a feeder and 0.50 demand factor for 10-12 sites on a feeder

[birdhunter]

I meant to say each pedestal currently has two 30 amp and TWO 20 amp outlets.

[cavie]

Originally Posted by dons2346

Direst bury could be a problem depending on what type of soil it is being buried in. I know of a park that direst buried and the insulation was penetrated by some of the small rocks causing problems. Park wound up rewiring within conduit

Direct bury is never a problem if done correctly. You bury the 1st 12 " with clean sand then put back the existing garbage soil. I watch the idiot at my camp move a ped and bury wire with the existing rocks and roots. It will come back and bite him. I tried to tell him to no avail.

He did a complete camp rewire before I came there. Now there is a problem to his personal site right in front of my trailer. I now know exactly what the problem is. He didn't allow for voltage drop either. I have an autotransformer on my site.

[KanzKran]

Originally Posted by birdhunter

I should have mentioned we currently have 12 pedestals each with two 30 amp and one 20 A. So there are 24 30 amp outlets. As far as I can see they are fed by two double pole 100 amp breakers which come off of a 200-amp breaker. Our goal would be to reduce things to twelve 50 A outlets although we could probably get away with 10. it would be done by a professional but I'm trying to determine if we could avoid trenching and running new wire and conduit which would cost a lot.

Your present setup is:

12 x 3600VA x .50 = 21.6kVA. At 240V, that's 90A. There are two such feeders. Sounds right.

What you want is:

6 x 9600VA x .60 = 32.6kVA. At 240V, that's 144A, so that won't fly. And the wiring between pedestals won't handle it, either (most likely, since I don't know what it actually is).

5 x 9600VA x .65 = 31.2kVA. At 240V, that's 130A. Still no good.

If you cut each circuit in half, and power only three pedestals with each feeder, you get:

3 x 9600VA x .80 = 23.0kVA. At 240V, that's 96A. That sounds like it may work. You'd need to run two new 100A feeders, and cut the existing chains in half, but that's only two new feeders to run and no changes to the feeders between pedestals (other than disconnecting the feeders between the two in the middle).

As to the 200A main breaker, you'll have to upsize the feeder to that breaker, but now you have 12 sites powered, so the demand factor drops down to 0.50 for the whole bunch (I think).

12 x 9600kVA x .5 = 57.6kVA. At 240V, that's only 240A. The existing feeder may be able to handle an increase. That's up to whoever you bring in to evaluate and estimate of course.

Originally Posted by birdhunter

I meant to say each pedestal currently has two 30 amp and TWO 20 amp outlets.

NEC 551 requires site service estimates based on 9600VA per site for 120/240V 50A, 3600VA per site for 120V 30A with 120V 20A on the same site (no spec for lack of 20A), 2400VA for 120V 20A, and 600VA for 20A serving tent-only sites.

Demand factors are based on Table 551-73, and go up as the number of sites on a feeder go down, all the way up to 1.0 when there's only one site, and all the down to 0.41 for 36 or more sites on a feeder. Demand factors work the same way the service for your house works - if you add up all the circuit ampacity, it would come out to twice or more what your service is, but you never use all that at once, so there's an involved formula for determining what you actually need. And the results are conservative even at that.

Do call a pro who knows how RV parks are wired, though. Don't trust internet engineering. It's worth what you paid for it.

HTH.

[Old-Biscuit]

Originally Posted by cavie

No need for conduit. There are Direct Bury cables available in all size wire and types. Ease of installation more than offsets the cost of the wire.

Yeah........


But conduit allows you to replace bad wire W/O digging plus conduit protects the wire