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#1 Mynameisluka

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Posted 19 May 2020 - 12:19 PM

if you have an 8000 watt generator plugging into your 2 phase breaker panel at 240v and 30 amps, how many amps can you use in the house at 120v?


the electrician keeps saying i will only have 30 amps available in the house at 120v. i don't get it. i think it would be 60 amps. if i am wrong, what am i missing?


Edited by Mynameisluka, 19 May 2020 - 12:18 PM.

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#2 Mynameisluka

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Posted 19 May 2020 - 12:47 PM

i asked the electrician to explain it to me, but he just says "that's how electricity works". i just want to understand why that is the case if he is right. anybody?


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#3 Mynameisluka

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Posted 19 May 2020 - 12:57 PM

By the way, page 3 info...he is trying very hard to sell me a whole house generator when all I want is a transfer switch. I understand i won't be able to run my whole house with what I have...that isn't my objective.
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#4 djinkc

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Posted 19 May 2020 - 01:00 PM

watts = amps  X volts

 

but I don't know how that works out with common  core


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#5 BrewerGeorge

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Posted 19 May 2020 - 01:02 PM

Your generator is capable of producing 30 Amps, period.  Odds are that is only 30A when it's 240V, and you'll only be able to get 15A from two 120V circuits.

 

Residential power comes into your house at 240V, ungrounded.  To get 240V for dryers, water heaters, stoves, etc they use both "hot" legs.  For 120V circuits they split the transformer in the middle and make two out of phase signals.  Your generator is going to mimic that arrangement.

 

ETA:

What you have going into the panel is +120V on one leg and -120V on the other.  If you want to power a 240V load they add together (to 240V not zero) but you still only have 30 Amps max for those loads because that's what the generator is capable of supplying.  Your 120V circuits will still get to use up to 8000 Watts, but they won't draw more than 15A without blowing your typical breakers.

 

Honestly, the Amps are kind of irrelevant except that you won't be able to fully power 240V loads that might want more. 



#6 Mynameisluka

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Posted 19 May 2020 - 01:20 PM

ok...so, you're kind of along the same lines of where i started with the electrician. if the generator is 8000 watts, and you are loading that on 120v circuits, isn't that 60 amps? he is saying i will be lucky to run a fridge and freezer with what i have. if i recall correctly, the fridge is something like 650 watts, and the freezer is somewhere around 500 watts. where are the other ~7000 watts going? i bought the generator based on wattage, not amperage. to run what i want to run on it, i really only need about 7000 watts. when he started talking about amps, i was thrown off.

 

i know enough about electricity to be dangerous. i'm just trying to understand this, so your help is greatly appreciated.


Edited by Mynameisluka, 19 May 2020 - 01:27 PM.

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#7 BrewerGeorge

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Posted 19 May 2020 - 01:41 PM

Well, first the 8000 W rating is probably not steady state but surge to cover things like that fridge starting up.  Steady state max should be 7200 W.  Think of that as a resource pool to draw from.  Whenever you plug something into your panel (via your wall outlets) it uses up some of that capacity.  All you have to do is limit what you turn on so it doesn't go above that 7200 W pool.  When you have the fridge and freezer running, the left over 6000 W or so is available capacity that is not being used - and it will use less fuel providing 1000 W than 7200 W to reflect that.

 

You don't have to worry about current any more than you normally do by not trying to run a space heater and hair dryer or microwave on the same circuit at the same time, because those kinds of things are big enough by themselves to fully load a 15 A household circuit.  The generator available current is not limiting you in the 120 V circuits, your 15 A panel breakers are doing that.  But again, that's all exactly the same as when you're connected to municipal power.  (Municipal power is a limited reservoir, too, but it's determined by your breaker service and is big enough that you can never run up against its limit.  Instead of 7200 W from your genny, your municipal service will provide at least 12000 W and probably more like 24000 W.)

 

Realistically, the only time the lower Amperage of the generator might matter is if you have an electric stove.  An electric dryer will run on 30 A, but running it will use up half your available capacity from the generator.

 

Long story short, you can basically keep adding 120 V loads until you reach that 7200 W limit without worrying about current.



#8 Mynameisluka

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Posted 19 May 2020 - 01:46 PM

Well, first the 8000 W rating is probably not steady state but surge to cover things like that fridge starting up.  Steady state max should be 7200 W.  Think of that as a resource pool to draw from.  Whenever you plug something into your panel (via your wall outlets) it uses up some of that capacity.  All you have to do is limit what you turn on so it doesn't go above that 7200 W pool.  When you have the fridge and freezer running, the left over 6000 W or so is available capacity that is not being used - and it will use less fuel providing 1000 W than 7200 W to reflect that.

 

You don't have to worry about current any more than you normally do by not trying to run a space heater and hair dryer or microwave on the same circuit at the same time, because those kinds of things are big enough by themselves to fully load a 15 A household circuit.  The generator available current is not limiting you in the 120 V circuits, your 15 A panel breakers are doing that.  But again, that's all exactly the same as when you're connected to municipal power.  (Municipal power is a limited reservoir, too, but it's determined by your breaker service and is big enough that you can never run up against its limit.  Instead of 7200 W from your genny, your municipal service will provide at least 12000 W and probably more like 24000 W.)

 

Realistically, the only time the lower Amperage of the generator might matter is if you have an electric stove.  An electric dryer will run on 30 A, but running it will use up half your available capacity from the generator.

 

Long story short, you can basically keep adding 120 V loads until you reach that 7200 W limit without worrying about current.

 

no, it's 8k steady state, 10k surge. this is the generator i have: https://shop.cat.com...fense™-530-1234

 

thanks...you confirmed what i was initially thinking...i am good up to 8k watts, as long as no one thing consumes over 30 amps and i don't overload any circuits (right?). i still don't get what the electrician was saying, but i'm not going to use him anyway, so it doesn't really matter.


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#9 dagomike

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Posted 19 May 2020 - 01:49 PM

Post the make/model so we can talk down to you too. :P

 

The advertised watts is peak and probably requires huffing moon dust. Continuous amps is what I’d look at, or convert to watts if that makes more sense.


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#10 Mynameisluka

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Posted 19 May 2020 - 01:51 PM

Post the make/model so we can talk down to you too. :P

 

The advertised watts is peak and probably requires huffing moon dust. Continuous amps is what I’d look at, or convert to watts if that makes more sense.

 

it's 8k watts continuous, 10k watts max.

 

this is the generator:

 

https://shop.cat.com...fense™-530-1234


trust me, i at least knew enough to know the difference between continuous and surge. this is from the specs:

 

  • Running Power:8000 W
  • Starting Power:10000 W
  • Running Wattage:Over 8000 Watts
  • Starting Wattage:Over 10000 Watts

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#11 dondewey

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Posted 19 May 2020 - 01:55 PM

You gotta make sure you know which 120v circuits are on which legs of your 240v. If you do that, and plan correctly, then you can draw 30amps off each leg, for a total of 60 amps.

But, this is really hard for most people to figure out. Some kitchens are every other outlet on same circuit, some are one side circuit one and one side circuit two. And sometimes those two circuits are on the same leg of your 240v, and sometimes they are on opposite legs.
You can really #### up the generator if you try to draw like 6000w all from the same 240v leg.
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#12 Mynameisluka

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Posted 19 May 2020 - 01:55 PM

You gotta make sure you know which 120v circuits are on which legs of your 240v. If you do that, and plan correctly, then you can draw 30amps off each leg, for a total of 60 amps.

But, this is really hard for most people to figure out. Some kitchens are every other outlet on same circuit, some are one side circuit one and one side circuit two. And sometimes those two circuits are on the same leg of your 240v, and sometimes they are on opposite legs. my stove is gas, so not too worried about that.

 

 

see, when he started talking about amps, that's pretty much what i thought. my panel has 2 columns of breakers...each column is a leg. it is pretty well balanced between the 2 legs.


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#13 BrewerGeorge

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Posted 19 May 2020 - 01:56 PM

no, it's 8k steady state, 10k surge. this is the generator i have: https://shop.cat.com...fense™-530-1234

 

thanks...you confirmed what i was initially thinking...i am good up to 8k watts, as long as no one thing consumes over 30 amps and i don't overload any circuits (right?). i still don't get what the electrician was saying, but i'm not going to use him anyway, so it doesn't really matter.

Other than a short fault, there is no way for something to consume more amps than is provided.  If you try to attach a 50A stove, it will just run at lower power.



#14 Mynameisluka

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Posted 19 May 2020 - 01:57 PM

Other than a short fault, there is no way for something to consume more amps than is provided.  If you try to attach a 50A stove, it will just run at lower power.

 

 

good to know...see, this is where things start to get above my pay grade. i get what you're saying, and it makes sense, but i wouldn't have thought about it like that.


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#15 AspenLeif

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Posted 19 May 2020 - 01:58 PM

I had a honda 5000.  When I set my house in Maine, this ran particular light circuits, fridge, freezer, fan motor for the wood stove, and furnace.  All at once, and could add more as I turned on breakers if needed.  

 

You gotta make sure you know which 120v circuits are on which legs of your 240v. If you do that, and plan correctly, then you can draw 30amps off each leg, for a total of 60 amps.

But, this is really hard for most people to figure out. Some kitchens are every other outlet on same circuit, some are one side circuit one and one side circuit two. And sometimes those two circuits are on the same leg of your 240v, and sometimes they are on opposite legs.
You can really #### up the generator if you try to draw like 6000w all from the same 240v leg.

This is correct.  You need to balance the load.  

 

You mentioned that you had a 2 phase panel...you meant to say single phase, 2 leg.  

 

see, when he started talking about amps, that's pretty much what i thought. my panel has 2 columns of breakers...each column is a leg. it is pretty well balanced between the 2 legs.

Look again, they alternate in the same leg.  That's why a 240 breaker is next to each other, as it's grabbing 120 off each leg.  


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#16 dagomike

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Posted 19 May 2020 - 01:59 PM

Yeah, that says 8000 running 10,000 surge. So, I guess 66 amps across all outlets. 


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#17 AspenLeif

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Posted 19 May 2020 - 02:00 PM

siemens-individual-subpanels-s3040l1200-


every other breaker in the same column is on the same leg


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#18 BrewerGeorge

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Posted 19 May 2020 - 02:03 PM

You gotta make sure you know which 120v circuits are on which legs of your 240v. If you do that, and plan correctly, then you can draw 30amps off each leg, for a total of 60 amps.
...

That's not really how Amps add up, or rather, the local amps in a particular leg can be higher or lower than the 30 that the generator provides. We had long, complicated problems in ET school about this.  Bottom line is that is't Watts which matter.



#19 dondewey

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Posted 19 May 2020 - 02:03 PM

Exactly. I also figured right vs left until I opened up my panel.

And then you still have to assume they are all labeled correctly and nobody did something stupid downstream. I have two cheap multimeters. Left them plugged into two outlets in the kitchen which were on different legs and monitored voltage when I was really stressing my old generator and made sure I didn't get a deviation of more than a couple of volts.
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#20 Mynameisluka

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Posted 19 May 2020 - 02:03 PM

siemens-individual-subpanels-s3040l1200-


every other breaker in the same column is on the same leg

 

 

ok...even more reason not to trust the electrician from today. he's the one that told me that each column of breakers was it's own leg. 

 

you're correct...i said phase earlier, i did mean leg.


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