Sub panel

[Dentaltec]

200amp service on the house with a 50amp breaker feeding this sub-panel in my garage with general lighting and 120 receptacles. Need to add 240 dedicated line for the new air compressor that requires a 40amp breaker. What gauge wire will I need and will new wire need to be run from main panel to this sub-panel?

Main panel is 75 feet from the sub panel, compressor will be 30 feet from sub-panel

Thanks

View media item 90419

 

[Bert_]

What size motor on the air compressor? Any other loads in the garage? What size wire is feeding this panel now?

Just by looking at what you have for breakers I would say it's enough. But we really need to know the size of the compressor.

 

[Dentaltec]

7.5 hp compressor, no real load in there now,

Is the gauge printed on the wire I thought the visual may be helpful in the pic but the wire had paint splatter. I can clean it up and check if necessary

 

[Bert_]

7.5 hp compressor, no real load in there now,

Is the gauge printed on the wire I thought the visual may be helpful in the pic but the wire had paint splatter. I can clean it up and check if necessary

That compressor is going to take a #8 THHN or #6 romex with a minimum 50-60A breaker, 100A max breaker. Running load will be around 30A for a modern motor.

 

[Dentaltec]

Since I like to learn I understand that I need to need that size wire FROM sub panel TO the compressor with that size breaker in my sub panel, what I am unsure of is if the EXISTING wire from the main power panel to this subpanel will need to be changed. Thank you for your guidance.

 

[pattenp]

You say 7.5 HP compressor. Is that the rated HP on the motor data plate or is the 7.5 HP printed on the label that is on the tank? The actual HP of the motor is what you go by, not the marketing HP of the compressor as a unit.

If the motor is a true 7.5 HP then it needs a 50A circuit and hardwired. If that's the case I'd upgrade the feed to the subpanel.

 

[Bert_]

If you feeder can support a 60A breaker it will probably run fine if you don't have much other stuff running at the same time, it's not leaving much extra.. The current 50A breaker will trip on startup if you have other stuff on. If you plan on adding anything else at all that uses electricity I would be planning to upgrade the feeder.

 

[wyliesdiesels]

Is this garage attached or detached?

 

[Dentaltec]

Is this garage attached or detached?

Attached

 

[Dentaltec]

You say 7.5 HP compressor. Is that the rated HP on the motor data plate or is the 7.5 HP printed on the label that is on the tank? The actual HP of the motor is what you go by, not the marketing HP of the compressor as a unit.

If the motor is a true 7.5 HP then it needs a 50A circuit and hardwired. If that's the case I'd upgrade the feed to the subpanel.

View media item 90423

 

[pattenp]

If the feed wire to the sub is #6 THHN in conduit from main panel to subpanel you can push the breaker in the main panel up to a 70A. If the feed wire is made up from #6 NM-b (Romex) along any section then the max breaker is 60A.

 

[Dentaltec]

If the feed wire to the sub is #6 THHN in conduit from main panel to subpanel you can push the breaker in the main panel up to a 70A. If the feed wire is made up from #6 NM-b (Romex) along any section then the max breaker is 60A.

Just to learn, how can you identify what the wire is in place?

 

[pattenp]

If individual THHN wires pulled through conduit the individual wires will be stamped with the type and size. If cable such as Romex the outer jacket will have the same stamped on it.

THHN will have "6 AWG" for #6 gauge, NM-b will have 6/3 meaning three #6 in the jacket.

 

[Dentaltec]

If individual THHN wires pulled through conduit the individual wires will be stamped with the type and size. If cable such as Romex the outer jacket will have the same stamped on it.

Its in a flex conduit as far as I can tell, the wire has no info on it when I looked at it this morning

 

[pattenp]

If it's MC (Metal Clad) the individual wires should have printing on the insulation.

 

[kipjanet]

Two pole 40 amp breaker #8 thhn/thwn with #10 ground copper wire will be fine Compressor only runs part time

 

[pattenp]

Two pole 40 amp breaker #8 thhn/thwn with #10 ground copper wire will be fine Compressor only runs part time

Your first post and you're suggesting to wire at the minimum based on amps plus being non code compliant. And what does running part time have to do with sizing the circuit correctly?

 

[wyliesdiesels]

Two pole 40 amp breaker #8 thhn/thwn with #10 ground copper wire will be fine Compressor only runs part time

That is not the proper way to size a motor circuit.

 

[Dentaltec]

After more research the co recommends a 50amp breaker

 

[Bert_]

After more research the co recommends a 50amp breaker

50a breaker is absolute minimum. Code allows up to a 100a though it would be overkill for this.

 

[Dentaltec]

So the electrician decided to make a home run back to the main 200amp panel and install the 50amp breaker there. What I am worried about is he used #8 romex after looking at the charts I thought it should use #6??? Am I wrong?

 

[pattenp]

So the electrician decided to make a home run back to the main 200amp panel and install the 50amp breaker there. What I am worried about is he used #8 romex after looking at the charts I thought it should use #6??? Am I wrong?

Well..yeah if a general circuit. If hardwire motor circuit it's okay.

 

[Dentaltec]

Well..yeah if a general circuit. If hardwire motor circuit it's okay.

Thank you for easing my mind, yes its a single hardwire straight back to the main, there is a disconnect wired right at the compressor as well.

 

[pattenp]

Thank you for easing my mind, yes its a single hardwire straight back to the main, there is a disconnect wired right at the compressor as well.

You posted the HP was 7.5 on the motor. The breaker is not the issue, the #8 is okay for up to 5HP. The wire should have been #6 for 7.5HP to be NEC compliant.

Personally I wouldn't worry about it.

I'll add.. if you paid the electrician to do this then he/she needs to come back and do it to code. Tell him/her NEC 430.6 says to use table 430.248 to size conductors by motor HP.

7.5HP is 40A, so 125%(NEC 430.22) is 50A or #6 NM(Romex) or #8 THHN in conduit.


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[Bert_]

It's a technicality really. The NEC requires you to size the wire based on the FLC tables, I haven't seen a motor is a lot of years that draws anywhere near the FLC tables. Per NEC the wire should be sized for 40a with a 7.5 hp motor. Modern 7.5 hp motors are drawing 30-32A.

With any modern motor the wire will be fine. If someone manages to find and install a 50 year old motor that draws 40A then you will be over the wires rating.

 

[wyliesdiesels]

So the electrician decided to make a home run back to the main 200amp panel and install the 50amp breaker there. What I am worried about is he used #8 romex after looking at the charts I thought it should use #6??? Am I wrong?

He should be doing to code.

So the wire is too small.