2 a/c units on 1 gen?

[TravelinDog]

This is a hypothetical question, for now.
Has anyone tried to run 2 a/c units with 1 large gen? 15k and 13.5k btu.
I have a 3500 watt gen with 4600 watt surge. I'm wondering if it would be possible to run both a/c units using the 30 amp twist-lock outlet if both a/c units had a soft-start installed?

[Chum_lee]

Hummmmmm. Why do people assume that the wattage (capacity) of a given generator means that it will provide that wattage at ALL temperatures, altitudes, and humidities. This is simply not true. Generally, (unless you are turbocharged) as the temperature, elevation, and humidity go up, rated output goes down. That's why some people can get it to work, for some people it works, . . . . sometimes, while, others can't get it to work at all. Everybody isn't in the same location, on the same day, at the same time.

It's true that sometimes the breakers/wires/plugs can't handle the load, but sometimes the genny can't produce enough horsepower to generate the wattage either. (which can cause conductors to overheat)

Chum lee

[TravelinDog]

Just an observation here.
2 weeks ago in 95° temps I was running the rear a/c in our 5er which is 15k btu off my gen using the 20 amp outlet. I had to use a 50ft extension cord and an adapter to the 50 amp power cord because the gen is too heavy to lift out of the truck bed by myself.
The gen ran the a/c with no problem. It did pull down quite a bit at start-up but then ran for 7+ hours until I shut it off. It still had about one gallon of gas left.
Obviously it would best without an extension cord but it worked just fine.

[MrWizard]

Ive always needed 5000watts or more to power (2) roof top A/Cs in hot weather, in fact the best situation was using a 6500 watt Onan

[Gdetrailer]

BB_TX wrote:

Sorry, but I don't use a 50 amp cord on 30 amp service. I use a 25' Marinco 30 amp power cord with a 50 amp connector on the trailer end and a 30 amp plug at the shore power end instead of my 50 amp cord with a 50/30 dogbone.

In your case, the 30A breaker on the shore power side OR generator breaker will pop before your 50A main breaker, therefore protecting your 30A shore cord from overload..

But once again, the 30A breaker or even the breaker in your gen are thermal/magnetic trip. The thermal trip once you get to 80% continuous for more than a couple of hrs will trip much easier as the breaker warms up.

You shore power cord even though it may be 10ga and rated for 30A, does indeed dissipate heat as you draw power, the more power drawn, the more heat it must dissipate.

The same insulation that protects you from touching live electrical wires becomes a insulator of heat. Making it difficult for the copper wire to dissipate any heat build up over time near full current rating. So, it is normal for a continuous heavy load to heat up your shore cord more than enough it will be warm to touch.

Uninsulated wire can handle more current for a longer time than insulated wire in the same wire gauge.

Heat dissipation and heat build up are the reasons why non insulated open air 10 ga wire can handle more current over a longer time than 10ga wire with insulation..

[BB_TX]

afidel wrote:

BB_TX wrote:

valhalla360 wrote:

BB_TX wrote:

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.

I believe we had this discussion on a different thread. If you want to melt you cords, have at it.

There were statements made that a 30 amp breaker was only good for 24 amps. And that is not true either. NEC specifies a breaker be sized for 125% of the designed max load, or the max load be sized for 80% of the rating of the breaker. Same thing. But a breaker can carry full load for an extended period of time before it trips. If you look at breaker trip curves you will see the initial trip point will be in excess of the breaker amp rating. Not less than rating. Some time delay at small amounts over rating and faster trip as current increases over rating. And yes, breakers can trip at loads slightly below rating due to heat buildup over time, especially where multiple breakers are installed side by side sharing heat.

Properly designed wire sizing must be large enough to carry the full rating of the protection device safely whether it be a breaker or fuse. So with all these statements made I am simply asking for some specifications that state a power cord is only good for 80% of the specified rating. Simple request. I have searched and can not find anything that states a power cord or extension cord can only carry 80% of stated load without burning up.

And I have run my 50 amp trailer on 30 amp service at near max for days (even tripped the park 30 amp breaker a few times) and have never burned my power cord or adapter.

Marinco 30 amp cord

NEC says outlets and wiring shall be derated to 80% for continuous load and that breakers shall be designed to enforce that. If you were using a 50A cord on a 30A plug then duh you'd trip the breaker before your cord melts as your cord isn't the weak point, that's the outlet and the wiring behind it, that's what the breaker's thermal trip is protecting. The time period for continuous is 3 hours at the 80% threshold or above.

Sorry, but I don't use a 50 amp cord on 30 amp service. I use a 25' Marinco 30 amp power cord with a 50 amp connector on the trailer end and a 30 amp plug at the shore power end instead of my 50 amp cord with a 50/30 dogbone.

[Gdetrailer]

afidel wrote:

NEC says outlets and wiring shall be derated to 80% for continuous load and that breakers shall be designed to enforce that. If you were using a 50A cord on a 30A plug then duh you'd trip the breaker before your cord melts as your cord isn't the weak point, that's the outlet and the wiring behind it, that's what the breaker's thermal trip is protecting. The time period for continuous is 3 hours at the 80% threshold or above.

Which is why some devices like portable electric heaters that may be operated continuously which may be plugged into 15A circuits are not allowed to be made to draw above 1500W or 12.5A.. 80% of 15A just happens to be 12A.. If a portable heater were to have higher wattage than 1500W, it will have to have a 20A plug on it which will not fit in a 15A outlet. Tis the reason for the oddball non compatible plugs.

80% of 30A just happens to be of all things.. 24A! Which follows NEC requirements.

Run more than 24A continuous on 30A breaker for more than a few hrs and it will get trip happy..

[afidel]

BB_TX wrote:

valhalla360 wrote:

BB_TX wrote:

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.

I believe we had this discussion on a different thread. If you want to melt you cords, have at it.

There were statements made that a 30 amp breaker was only good for 24 amps. And that is not true either. NEC specifies a breaker be sized for 125% of the designed max load, or the max load be sized for 80% of the rating of the breaker. Same thing. But a breaker can carry full load for an extended period of time before it trips. If you look at breaker trip curves you will see the initial trip point will be in excess of the breaker amp rating. Not less than rating. Some time delay at small amounts over rating and faster trip as current increases over rating. And yes, breakers can trip at loads slightly below rating due to heat buildup over time, especially where multiple breakers are installed side by side sharing heat.

Properly designed wire sizing must be large enough to carry the full rating of the protection device safely whether it be a breaker or fuse. So with all these statements made I am simply asking for some specifications that state a power cord is only good for 80% of the specified rating. Simple request. I have searched and can not find anything that states a power cord or extension cord can only carry 80% of stated load without burning up.

And I have run my 50 amp trailer on 30 amp service at near max for days (even tripped the park 30 amp breaker a few times) and have never burned my power cord or adapter.

Marinco 30 amp cord

NEC says outlets and wiring shall be derated to 80% for continuous load and that breakers shall be designed to enforce that. If you were using a 50A cord on a 30A plug then duh you'd trip the breaker before your cord melts as your cord isn't the weak point, that's the outlet and the wiring behind it, that's what the breaker's thermal trip is protecting. The time period for continuous is 3 hours at the 80% threshold or above.

[pianotuna]

TravelinDog,

Please let us know the results--and the altitude.

[TravelinDog]

Well I guess I'll just have to give it a try and see what happens. My gen does have a 30 amp twist-lock outlet and I have an adapter for use with my rv 50 amp cord.

[BB_TX]

valhalla360 wrote:

BB_TX wrote:

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.

I believe we had this discussion on a different thread. If you want to melt you cords, have at it.

There were statements made that a 30 amp breaker was only good for 24 amps. And that is not true either. NEC specifies a breaker be sized for 125% of the designed max load, or the max load be sized for 80% of the rating of the breaker. Same thing. But a breaker can carry full load for an extended period of time before it trips. If you look at breaker trip curves you will see the initial trip point will be in excess of the breaker amp rating. Not less than rating. Some time delay at small amounts over rating and faster trip as current increases over rating. And yes, breakers can trip at loads slightly below rating due to heat buildup over time, especially where multiple breakers are installed side by side sharing heat.

Properly designed wire sizing must be large enough to carry the full rating of the protection device safely whether it be a breaker or fuse. So with all these statements made I am simply asking for some specifications that state a power cord is only good for 80% of the specified rating. Simple request. I have searched and can not find anything that states a power cord or extension cord can only carry 80% of stated load without burning up.

And I have run my 50 amp trailer on 30 amp service at near max for days (even tripped the park 30 amp breaker a few times) and have never burned my power cord or adapter.

Marinco 30 amp cord

[Stetson12]

On my rv ,I run a 13.5k and 15k ac and a 18 cubic feet residential refrigerator on a Cummins onan 4300 watt inverter generator. I have soft starts on both ac units and have not had any problems. I run my generator on eco mode and set my air conditioners to kick on at slightly different temps. Generator specs say 3600 continuous running watts.

[Gdetrailer]

BB_TX wrote:

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.

I suspect that was in literal sense..

Cord and plug will get pretty hot, the plug most likely will heat up enough to soften the plastic holding the pins when full load is applied for long periods of time..

But, from the breaker point of view, yes, it is very possible that running more than 24A on a 30A breaker continuous can cause the breaker to trip before 30A.

The breakers used are thermo-magnetic trip, the thermo part creates a small amount of heat under loads near the rating.. Do that long enough and the breaker will trip due to thermal overload. The small amount of heat generated inside the breaker never has time to dissipate under continual loads and that increases the chance the breaker will trip under lighter loads than at rating.

Thermo trip is slow and takes time (think of it as a time delayed blow) but magnetic trip is for those very fast current spikes like what happens when you have a short.

I have seen 16A-18A loads on a 20A breaker run for a long period of time trip the breaker..

[valhalla360]

BB_TX wrote:

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.

I believe we had this discussion on a different thread. If you want to melt you cords, have at it.

[BB_TX]

valhalla360 wrote:
Probably yes...until the cord melts. As skibine showed, you are probably looking at 27-30amps in ideal conditions. A 30amp is rated for 24amps continuous load.

......

Please show me the specifications that show a 30 amp cord is only rated for 24 amps continuous. I really want to know.