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Old 07-03-2018, 01:21 AM   #1
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30 amp vs 50 amp in motorhomes

I notice Motorhomes what ever year . 50 amp Motorhome has 5500 watt generator .
That's only 30 amp per leg .. then there's ones with 8000 watt generators . Now that's 50 amp per leg . I understand the issue on cheaper generator smaller etc do some have boosters for startup on 5500 watt generators ? Next question what number wire does the 50 amp cord have on most Motorhomes . Or is it length dependant ? I know 30 amp max is 50 ft with number 10/3 wire that might be pushing it .. Is there a chart for RV 50 amp and 30 amp cord length and wire size ?

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Old 07-03-2018, 04:49 AM   #2
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My previous coach and this coach (both 50 amp) had a 6 gauge for the cord. I happened upon some 6 gauge shielded and made a 40 ft extension cord for a couple places I camp at in the summer.
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Old 07-03-2018, 04:58 AM   #3
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Old 07-03-2018, 05:04 AM   #4
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6 Gauge would not carry 50 amps with a reasonable voltage drop on a 20 foot shore cable if there is an actual 50 amp load on one side or the other of the 50 Amp shore cable. But as prior poster noted, a more "normal" max load for all but the largest Class As is more like 30 amps per each side of the shore cable. The 6 gauge cord may get warm to the touch if the RV has everything going at once but should not get so hot that it melts anything or catches fire.

That said if I was buying or building a extension cord with male and female 50 amp connectors I would want 4 gauge to keep the additional voltage drop caused by the longer cable run to a minimum.
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Old 07-03-2018, 05:45 AM   #5
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My 2017 Dutch Star comes with 50 foot shore reel and I believe the wire size is #4.
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Old 07-03-2018, 06:06 AM   #6
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Originally Posted by powercat_ras View Post
6 Gauge would not carry 50 amps with a reasonable voltage drop on a 20 foot shore cable if there is an actual 50 amp load on one side or the other of the 50 Amp shore cable. But as prior poster noted, a more "normal" max load for all but the largest Class As is more like 30 amps per each side of the shore cable. The 6 gauge cord may get warm to the touch if the RV has everything going at once but should not get so hot that it melts anything or catches fire.

That said if I was buying or building a extension cord with male and female 50 amp connectors I would want 4 gauge to keep the additional voltage drop caused by the longer cable run to a minimum.
Are you sure about this. I ran the calculations for 100 amp load which seems unlikely in most cases, 50’ in length, conductor temp at 175 degrees F, copper conductor, no conduit and came up with only 2.1% drop. Well with the max drop target of 5%. Worst case I could produce would be a 4.2% drop over 50’ with a 100 amp load. Still under 5%.

Almost all 50 amp extensions cords offered for sale are 6/3.

I’m no expert but I have 15 and 30’ extension cords made of 6/3 at home that friends plug their 50 amp rigs into when staying at the house, 30’ of extension plus their own hard wired cord is going to be 50’ give or take a foot. No overheating no blown breakers.

I could see using 4 on the supply side from main panel to the post if it were in conduit where it could not disapte the heat but from the post to the coach there would be no disapation issue.
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Old 07-03-2018, 10:46 AM   #7
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Originally Posted by Itasca66 View Post
I notice Motorhomes what ever year. 50 amp Motorhome has 5500 watt generator. That's only 30 amp per leg. Then there's ones with 8000 watt generators. Now that's 50 amp per leg. I understand the issue on cheaper generator smaller etc do some have boosters for startup on 5500 watt generators? ,,,
Don't compare installed generator capacity to installed shore power capacity and consider converting the capacities to watts (volts X amps = watts) which will give you an easier overview when dealing with generators. At least it does for me. When designers need to determine what size generator to install shore service configuration has a very minor roll while the demand of installed appliances will be driver.

For class A rigs the prevailing shore power choices are 3600 watts (30 amp) or 12,000 watts (50 amp is two hot lines for a combined capacity of 100 amp). For generators there's a bit more flexibility with the current sizes being in the range of 3600, 5500 and 7000 watts for gas and 6000, 8000, 10,000 for diesel. Sizes will vary with generator manufactures.

For the final product and power needs use an inexpensive 30' gas coach as the first example. Single roof A/C, AC/DC power converter, gas water heater and such which 3600 watts shore service and matching generator will easily support. Works great and keeps cost down. Same coach but add a second A/C and electric hot water. Power demand now might be 4200 watts. With shore power there's one option, 12,000 watts, or nearly 3 times the demand while a 5500 watts generator will easily support the coach. Move into a higher end 36'+ gas rig with a washer/dryer along with bigger A/Cs and strong potential that you're probably beyond 5500 watt capacity and will need 7000 watts. 42' DP with three A/C's on the roof along with all the bells and whistles and a 10K generator might be best to take full advantage of all the options and convinces installed. 45' Prevost, who knows what might be in the aircraft hanger size below floor area besides a small car.

For power cords I try and keep it simple. The coach came with a 25' 6/3 cord for 50 amp service. I have a 25' extension that is also 6/3. If I need more cable than 50' I need to move the coach.
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Old 07-03-2018, 11:30 AM   #8
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6 Gauge would not carry 50 amps with a reasonable voltage drop on a 20 foot shore cable if there is an actual 50 amp load on one side or the other of the 50 Amp shore cable.
The ampacity tables in the NEC and elsewhere disagree with you. 6/3 has 6 gauge hots and neutrals and each is rated for 65A. That will deliver power for 25-30 feet with less than 5% voltage drop (see sdennislee reply).


If another 25 ft 6/3 extension was added, it could come up a bit short, but if the actual loads are even a little under 50A, no worries. I've used a 35 ft 6/3 shore cord plus a 15 ft 6/3 extension on numerous occasions and the voltage drop isn't worth mentioning.
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Old 07-03-2018, 11:38 AM   #9
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Quote:
Originally Posted by Itasca66 View Post
I notice Motorhomes what ever year . 50 amp Motorhome has 5500 watt generator .
That's only 30 amp per leg .. then there's ones with 8000 watt generators . Now that's 50 amp per leg . I understand the issue on cheaper generator smaller etc do some have boosters for startup on 5500 watt generators ? Next question what number wire does the 50 amp cord have on most Motorhomes . Or is it length dependant ? I know 30 amp max is 50 ft with number 10/3 wire that might be pushing it .. Is there a chart for RV 50 amp and 30 amp cord length and wire size ?

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I'm not sure where you came up with your numbers, but they are wrong. 30A is equal to 3600Watts. Two 50 Amp legs is equal to 12,000 Watts.
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Old 07-03-2018, 06:29 PM   #10
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You can’t use the “conductor” rating in the NEC. You need to use the table for “cords”.
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Old 07-03-2018, 06:41 PM   #11
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Quote:
Originally Posted by Itasca66 View Post
I notice Motorhomes what ever year . 50 amp Motorhome has 5500 watt generator .
That's only 30 amp per leg .. then there's ones with 8000 watt generators . Now that's 50 amp per leg . I understand the issue on cheaper generator smaller etc do some have boosters for startup on 5500 watt generators ? Next question what number wire does the 50 amp cord have on most Motorhomes . Or is it length dependant ? I know 30 amp max is 50 ft with number 10/3 wire that might be pushing it .. Is there a chart for RV 50 amp and 30 amp cord length and wire size ?

Itasca
Well first off a 30 amp RV has a maximum usage of 3,600 watts. ( 120V x 30) since there is only one leg.
A 50 amp RV has a maximum usage of 12,000 watts. (120 v x 50 x 2) since there are two legs with 50 amps each.
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Old 07-03-2018, 06:44 PM   #12
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Originally Posted by Gary RVRoamer View Post
The ampacity tables in the NEC and elsewhere disagree with you. 6/3 has 6 gauge hots and neutrals and each is rated for 65A. That will deliver power for 25-30 feet with less than 5% voltage drop (see sdennislee reply).


If another 25 ft 6/3 extension was added, it could come up a bit short, but if the actual loads are even a little under 50A, no worries. I've used a 35 ft 6/3 shore cord plus a 15 ft 6/3 extension on numerous occasions and the voltage drop isn't worth mentioning.
Gary
I just like reading these answers who are think they are Electrical Engineers. lol

I'm an Electrical Engineer and a Master Journey Level Electrician for over 40 plus years working from outlets to 60,000 volt power lines.
You are correct about the #6 AWG can handle 65 amps.
Our 50 amp cable can actually carry much more being it is free air.
Meaning it is not in a wall or on conduit.
Now if you need to have 50 amps for a long distance over the normal length of your cables (25-30ft) you should consider a large conductor size like #4 awg but I'm talking if you have to be 100 foot from power and want 120/208 volts at 50 amp at your rig.

Thanks Gary
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Old 07-03-2018, 06:44 PM   #13
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Quote:
Originally Posted by Sweetbriar View Post
Don't compare installed generator capacity to installed shore power capacity and consider converting the capacities to watts (volts X amps = watts) which will give you an easier overview when dealing with generators. At least it does for me. When designers need to determine what size generator to install shore service configuration has a very minor roll while the demand of installed appliances will be driver.

For class A rigs the prevailing shore power choices are 3600 watts (30 amp) or 12,000 watts (50 amp is two hot lines for a combined capacity of 100 amp). For generators there's a bit more flexibility with the current sizes being in the range of 3600, 5500 and 7000 watts for gas and 6000, 8000, 10,000 for diesel. Sizes will vary with generator manufactures.

For the final product and power needs use an inexpensive 30' gas coach as the first example. Single roof A/C, AC/DC power converter, gas water heater and such which 3600 watts shore service and matching generator will easily support. Works great and keeps cost down. Same coach but add a second A/C and electric hot water. Power demand now might be 4200 watts. With shore power there's one option, 12,000 watts, or nearly 3 times the demand while a 5500 watts generator will easily support the coach. Move into a higher end 36'+ gas rig with a washer/dryer along with bigger A/Cs and strong potential that you're probably beyond 5500 watt capacity and will need 7000 watts. 42' DP with three A/C's on the roof along with all the bells and whistles and a 10K generator might be best to take full advantage of all the options and convinces installed. 45' Prevost, who knows what might be in the aircraft hanger size below floor area besides a small car.

For power cords I try and keep it simple. The coach came with a 25' 6/3 cord for 50 amp service. I have a 25' extension that is also 6/3. If I need more cable than 50' I need to move the coach.
There are lots of MH's out there with the Onan 12.5 kw liquid cooled diesel gens and your Prevosts may have a 20 kw one.
My rig has the 12.5 and my dryer is 240 volts. The Onan 10 and 12.5 units can both support 240 volt appliances.
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Old 07-03-2018, 07:52 PM   #14
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Are you sure about this. I ran the calculations for 100 amp load which seems unlikely in most cases, 50’ in length, conductor temp at 175 degrees F, copper conductor, no conduit and came up with only 2.1% drop. Well with the max drop target of 5%. Worst case I could produce would be a 4.2% drop over 50’ with a 100 amp load. Still under 5%.

Almost all 50 amp extensions cords offered for sale are 6/3.

I’m no expert but I have 15 and 30’ extension cords made of 6/3 at home that friends plug their 50 amp rigs into when staying at the house, 30’ of extension plus their own hard wired cord is going to be 50’ give or take a foot. No overheating no blown breakers.

I could see using 4 on the supply side from main panel to the post if it were in conduit where it could not disapte the heat but from the post to the coach there would be no disapation issue.
No conductor should see 100 Amps. There is a maximum of 50 amps per hot leg with the neutral carrying any difference.
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