Portable Generator Question

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Esay

Esay

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Location
Rocky mount NC
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Industrial Electrician
I have a portable generator with an interlock device installed on my panel. I currently have a 30 amp breaker with four 10 gauge wires that go to a 20 amp reliance outlet. I would like to upgrade what I have to a 50 amp breaker so I can be able to power my heat pump with a larger generator. From what I gather, I will need at least a 10kw generator to be able to run a 2.5 ton HVAC. I read that 1 ton = 3,500 watts. 2.5 tons x 3,500 watts = 8,500 watts. I don't quite understand how to calculate the start up load for the heat pump. Attached is the tag for my unit.
 

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Sounds like you will need to upgrade the outlet, the wiring and the breaker together. As for the start-up load... Looks like they give that to you in the form of the LRA = 63A.
 
Esay said:
I have a portable generator with an interlock device installed on my panel. I currently have a 30 amp breaker with four 10 gauge wires that go to a 20 amp reliance outlet. I would like to upgrade what I have to a 50 amp breaker so I can be able to power my heat pump with a larger generator. From what I gather, I will need at least a 10kw generator to be able to run a 2.5 ton HVAC. I read that 1 ton = 3,500 watts. 2.5 tons x 3,500 watts = 8,500 watts. I don't quite understand how to calculate the start up load for the heat pump. Attached is the tag for my unit.
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Do you have heat strips in the air handler?
 
The nameplate you posted indicated that a 20A circuit can handle the HVAC equipment with MCA at 13A (9.5+0.9), means the combined running load will be less than 13A. The surge current (LRA) needed to be addressed to be sure the generator can handle a start-up.
 
ron said:
The nameplate you posted indicated that a 20A circuit can handle the HVAC equipment with MCA at 13A (9.5+0.9), means the combined running load will be less than 13A. The surge current (LRA) needed to be addressed to be sure the generator can handle a start-up.
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Yes, this is a concern for a 10kw generator, even if it can surge to 13kw. I'm not an HVAC guy, but couldn't that be addressed with a large starting capacitor?
 
Esay said:
I have a portable generator with an interlock device installed on my panel. I currently have a 30 amp breaker with four 10 gauge wires that go to a 20 amp reliance outlet. I would like to upgrade what I have to a 50 amp breaker so I can be able to power my heat pump with a larger generator. From what I gather, I will need at least a 10kw generator to be able to run a 2.5 ton HVAC. I read that 1 ton = 3,500 watts. 2.5 tons x 3,500 watts = 8,500 watts. I don't quite understand how to calculate the start up load for the heat pump. Attached is the tag for my unit.
Click to expand...
You need to factor in the SEER rating of the unit to come up with any type of useful watt/ton calculation. According to the nameplate in your photo your unit draws 10.4 amps at 200 volts. At 240 volts it will draw even less. I don't think that you even need a 10kw generator.
 
So would the start up wattage be 14,490 watts?
63A x 230V = 14,490 watt surge?

Most guides will tell you to pick a model with surge current matching inrush current of your motor. Well, with such an advice you may wind up with twice larger generator than you really need. The case is that nameplate LRA is given for full voltage starting. In reality, when you start a motor from a generator, the current surge causes voltage dip. When voltage drops, the consumed current is reduced proportionally. Most residential appliances can start with up to 30% voltage sag, that is at 30% lower currents. As the result, starting volt-amps could be 0.7*0.7=0.49 of nominal.

In my case it would be 63 LR amps at 230VAC. At 30% voltage dip it would require (63*0.7)*(230*0.7)=7,100 VA to start.
 
Esay said:
So would the start up wattage be 14,490 watts?
63A x 230V = 14,490 watt surge?

Most guides will tell you to pick a model with surge current matching inrush current of your motor. Well, with such an advice you may wind up with twice larger generator than you really need. The case is that nameplate LRA is given for full voltage starting. In reality, when you start a motor from a generator, the current surge causes voltage dip. When voltage drops, the consumed current is reduced proportionally. Most residential appliances can start with up to 30% voltage sag, that is at 30% lower currents. As the result, starting volt-amps could be 0.7*0.7=0.49 of nominal.

In my case it would be 63 LR amps at 230VAC. At 30% voltage dip it would require (63*0.7)*(230*0.7)=7,100 VA to start.
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You are on the right track here. Generator will almost always have more voltage sag when starting motor loads than when starting on utility power. This will have somewhat of a soft starter (reduced voltage starting) effect on the motor you are trying to start as well. Keep in mind it will not accelerate as fast as when starting on utility power either.

Other considerations is how much load is already running when attempting to start that AC compressor.

I have a 8000 watt portable generator, haven't had to try to start my 3 ton heat pump with it for several years now. But it will run my 3 ton heat pump. Has difficulty starting before it blows the 30 amp generator breaker if there is much other load running already when you try to start it. But once it is started that heat pump often only draws 6 to maybe 8 amps and there is capacity to run other loads once the heat pump is started.
 
Using a EasyStart 368 Soft Starter dropped my starting amps such that my Generator was easily able to handle the initial surge.
 
kwired said:
I have a 8000 watt portable generator, haven't had to try to start my 3 ton heat pump with it for several years now. But it will run my 3 ton heat pump. Has difficulty starting before it blows the 30 amp generator breaker if there is much other load running already when you try to start it. But once it is started that heat pump often only draws 6 to maybe 8 amps and there is capacity to run other loads once the heat pump is started.
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I've run a 3 ton AC with higher current values than the OP's heat pump without any issue.
 
infinity said:
I've run a 3 ton AC with higher current values than the OP's heat pump without any issue.
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Well I think he said his was a 2.5 ton, so it likely is going to draw less.

Mine is 3 ton but I can't ever recall it drawing more than maybe 8 amps when in heat mode. On low stage it usually draws only about 6 amps.
Though RLA of compressor is probably around 17 - 21ish? Haven't looked at it in for some time so just a guess. Pretty sure it normally draws more during cool mode, but still not all that close to RLA either.
 
kwired said:
Mine is 3 ton but I can't ever recall it drawing more than maybe 8 amps when in heat mode. On low stage it usually draws only about 6 amps.
Though RLA of compressor is probably around 17 - 21ish? Haven't looked at it in for some time so just a guess. Pretty sure it normally draws more during cool mode, but still not all that close to RLA either.
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Yeah I have run my 2.5 ton at my old house on an 8kw. But the lights dimmed a bit when the compressor started. And you could see the governor take the throttle wide open foe a few seconds
 
The rule of thumb for motors is three times the running amps ? Portable generators have a hard time meeting that based on the basic size rating.
 
Joethemechanic said:
View attachment 2567199
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Ah, the good old suicide cord. Not that code matters, but NFPA 70, section 406.7(B) states, "Connection of Attachment Plugs. Attachment plugs shall be installed so that their prongs, blades, or pins are not energized unless inserted into an energized receptacle or cord connectors. No receptacle shall be installed so as to require the insertion of an energized attachment plug as its source of supply." So, I guess TECHNICALLY that's a code infraction.
 
Tip DS said:
Ah, the good old suicide cord. Not that code matters, but NFPA 70, section 406.7(B) states, "Connection of Attachment Plugs. Attachment plugs shall be installed so that their prongs, blades, or pins are not energized unless inserted into an energized receptacle or cord connectors. No receptacle shall be installed so as to require the insertion of an energized attachment plug as its source of supply." So, I guess TECHNICALLY that's a code infraction.
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Yeah but they make them now so people who don't have the ability to install a cord cap can backfeed their panel

Nema-5-15p-To-5-15p-Extension-Cords-Adapter-For-Transfer-Switch-H9w6.jpg_Q90.jpg_.jpg
 
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