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  #1  
Old 09-27-2012, 07:12 PM
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LAMPSguy LAMPSguy is offline
 
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Default Electrical question (Shop)

I am having a 240V outlet with a 60A breaker put in for my wife's pottery kiln. Contractor recommended a separate outlet and breaker (at an additional $100) for the compressor. I will never run them at the same time, so I figured I would save money and just plug in what I want to use, but he says the breaker is too high for the compressor????

I thought the breakers protect the wiring from excess current, not the device plugged in.

He says IF there was a problem with the compressor it would pull to much current before breaker tripped and screw up my comp motor?

Any contractors/EE's here to confirm or deny what I need?
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  #2  
Old 09-27-2012, 07:23 PM
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I've done the exact same thing you are suggesting many times. This afternoon I did exactly that for my new house...welder/air compressor circuit.

Your toaster doesn't pull 15 amps...but it's on a 15 amp circuit. Your TV doesn't pull 15 amps either. Neither does your computer, or most anything else in your house, but they are on oversized breakers. Circuit breakers protect the house wiring, not the unit plugged in. Find another electrician....yours is trying to take you for a ride.

PS. I am a contractor, though it wouldn't take a tradesman to tell you the above.
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Last edited by ColoRv : 09-27-2012 at 07:31 PM.
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  #3  
Old 09-27-2012, 07:24 PM
David Z David Z is offline
 
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Plug it in and don't worry about it.
If your compressor motor sucks back that much power, the motor is already fried.
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  #4  
Old 09-27-2012, 07:28 PM
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Electrical panel breakers are to protect the wires from turning into magma and burning down your house with you and your loved ones in it. The electrical code could give a rodent's arse about protecting the devices you plug in.
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  #5  
Old 09-27-2012, 07:35 PM
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Yeah, thats what I thought...best to double check though.

Can't really get another company, and the price quoted was $50 less than what I paid in my last house for the same (but didnt have compressor). I am in base housing and in order to do this I have to use pre-selected contractor, bids, etc, etc military money ****...then I have to pay to have it removed when I leave!

BUT, a few hundred $ so my wife will stop squawking about the kiln...priceless!
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  #6  
Old 09-27-2012, 08:12 PM
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Base housing. Understood. Just tell him you're not going to use the compressor after all. Adios.

Thanks for your service. Semper Fi.
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  #7  
Old 09-27-2012, 10:34 PM
Coolerfixer Coolerfixer is offline
 
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Sorry, I have to agree with the electrician. I have my sparky license and would never knowingly do as you asked.
You need to to look at the data plate on your compressor/motor. It should have maximum fuse/breaker size listed on it. You can also calculate the correct circuit size using Ohm's Law if the rating is in watts (Watts/Volts=Amps). A common misconception is that the breaker is ONLY there to protect the conductors (wires) in the building. Would you plug your toaster into a 200 amp protected circuit? I sure wouldn't!
The point of all circuit protection is to prevent fires and personal injury,including death. If what you are plugging into that outlet is not rated for 60 amps, it can very easily cause a fire. 60 amps will make 10 gauge wire glow red hot long before it would trip a breaker. 10 gauge or smaller is what your cord is if your compressor is 7hp or smaller.
There is an overload in some compressor motors. I wouldn't bet my life or anybody else's on a 3 cent part from China. Your compressor motor may have an LRA (locked rotor amperage) rating listed. I have seen LRA's over 150 amps which would trip a 60 amp breaker obviously, but depending on what size motor you have, your LRA is probably less than 60.
The NEC most certainly does address appliance wiring external to the building wiring, anyone who says otherwise knows not of what they speak. If your compressor didn't come with a pre-installed cord, it was meant to be hard wired. The instructions that came with it may cover that.
Sorry if this comes across as a rambling rant, but I see this kind of DIY/handyman electrical far too often, and it can and will kill you.
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  #8  
Old 09-28-2012, 01:22 AM
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I've never seen an air compressor without its own thermal protection. Regardless, the NEC allows for oversize on breakers to allow for startup draw unless something drastic has changed since last I had this discussion with one of our engineers, which I doubt.

Still, I was curious so I did a web search and found this discussion between electricians about breaker size for a 5hp air compressor install.

Quote:
"The breaker can be sized at up to 350% of the full load current to allow starting without tripping the breaker.
That means you can use a breaker as large as 98 amps (actually 100 would be allowed).
Quote:
The OP has the correct idea that the motor protects itself against overload with built in thermal protection and thus cannot overload the feed, and all the breaker is doing is protecting the feed conductors against short circuit damage.

125% of full load motor current for the wires, and up to 350% of the full load current for the breaker.

So yes, you can see breakers significantly larger than what is typical for general use branch circuits based solely on wire size.
I'll not lose any sleep over my compressor running on my welding circuit anymore than I would about my iPhone charging on a 15a circuit, but as in all things we each must live with our own decisions.
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Last edited by ColoRv : 09-28-2012 at 01:27 AM.
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  #9  
Old 09-28-2012, 04:40 AM
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Quote:
Originally Posted by LAMPSguy View Post
I thought the breakers protect the wiring from excess current, not the device plugged in.
True,

How do you plan on terminating the #6 60amp wires in the motor housing?

If you trim them down, you will have say... #12's fused at sixty amps.

Hmmmm... that doesn't sound good at all!

As an electrician, THAT is your violation. It used to be table 310.16 but they just changed it this past code to something like 310.15.b.16 or something like that. Same table with a few changes. I can cite the new one for you if you really need it.

Aside from that, yes, the motor has it's own thermal protection and if you can keep the wires FULL size to the motor, use the 60 ampere male configuration on the compressor and do everything else like supporting, disconnecting means and not violate any UL listings by modifying equipment (that's gonna be a tuffy here once you try to get those big wires to the compressor's pressure switch... see article 110.3b) you will be legal.

I would like to see how you plan on getting 60 amp wires in there, though...

Let me know what you come up with! I am glad to help out!

CJ
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  #10  
Old 09-28-2012, 07:32 AM
Wayne Gillispie Wayne Gillispie is offline
 
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I have seen oversized breakers cause fires inside heat pumps, condensers and electric furnaces and yes all of them had thermal overloads.

Read the data plate or installation instructions on most any larger electrical appliance in your home. It will provide you with "Max fuse or breaker size".

Contrary to what many learn building their plane, it is not just to protect the feed wiring.

Talk to a fire marshall, most house fires don't start in the properly sized feed circuits, usually in the appliances.

Install a smoke detector near equipment.
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