When to run a subpanel

[doojus]

Hi all,

I'm running a compressor and a welder with two branch circuits off my main panel right now and I'm trying to decide if I should just run a sub panel.

My compressor is 5HP and my main panel is more than 50' away so I need to add a disconnect near the compressor to get it up to code, so I know at very least I need to spend some money to get a disconnect, but I'm thinking it may just be a better solution to run a subpanel to my garage near my compressor. Then I wouldn't need a disconnect and I could have both of my circuits more neatly consolidated.


My main question on this is what type of wire do I need to run from my main panel to be up to code? The compressor is 5HP and it says it consumes 20.6 amps on the nameplate. The welder uses a max of 30 amps and it uses a standard 50 amp 6-50p plug. Do I need 6-3 romex with a 55 amp breaker for this or will 8-3 work since they obviously won't be pulling full load all of the time? Obviously the two will practically never be running at the same time but I want to be up to code.

 

[pattenp]

I'd do the sub-panel for a matter of convenience and flexibility. Use 2-2-2-4 Al SER for the feeder. Use a 60A ( I think 60A is the smallest breaker to fit #2) up to a 90A feeding breaker in the main. As long as the SER is not run within insulation it's good up to 90A. If the majority of the SER is run within insulation it's limited to 75A.

Edit: Actually after looking I'm seeing that some breakers down to 40A will take #2 max.

 

[doojus]

Yeah after I posted I realized I might need 4/3 NM-B for what I'm doing which doesn't seem to be readily available. My main panel is super congested so I need to use the smallest gauge conductor that's allowable, which means I need to use copper. I can't find 4/3 copper SER if it even exists. Would this stuff be allowable? It's 3 strands 4 gauge THHN and an 8 gauge ground in a PVC sheath: http://www.wireandcabletogo.com/4-3C-THHN-PVC-Tray-Cable-with-Ground.html. Says it's allowable for open-air installation.

 

[pattenp]

Where are you located? Do you have a local electrical supply? You should be able to get 4/3 NM-b or 4/3 Cu-SER. I don't see why the wire you linked to can't be used.

Edit: If you think you only need 60A to the sub-panel you can use #6 NM-b. Even though it's rated at 55A you can go to the next size standard breaker which is 60A.

Edit: The cable you linked to will need the blue conductor to be wrapped with white tape at the terminations to identify it as the neutral. This re-marking can only be done on #4 or larger wire.

 

[doojus]

I'm located in florida. You're right though, I could find 4/3 NM or SER locally, I just meant I couldn't find it online easily, seems like most places cut off at 6/3.

This is the sub panel I was going to use, any problems with it?

http://www.homedepot.com/p/Square-D...with-Cover-HOM612L100SCP/100190554?quantity=1

Also last question, since this is an attached garage, I assume that I don't need to drive a separate 8' ground rod for this box?

 

[pattenp]

That load center is okay. No problem if you know it provides enough space/circuits. You don't need any ground rods for the additional panel. My suggestion is to use this panel if money is not an issue, http://www.homedepot.com/p/Square-D...OM2040M100PCVP/204836397?N=5yc1vZbm30Z1z11za5

 

[doojus]

alright, thank you sir!

I was looking for the smallest panel that would fit my needs since space is at a premium in my garage, otherwise I'd use that bigger one.

 

[doojus]

Alright, I got the subpanel installed, but I have a question about the ground:

I made sure there wasn't a jumper bonding the neutral to the panel, so the neutral bar is completely isolated. I ran the ground from the ground buss in the main service panel to the ground buss bar in the new subpanel, which is also bonded to the subpanel case via screws. Wouldn't that mean if there were a short in one of the devices connected to the subpanel, and someone happened to be touching the subpanel at that time, that they'd get shocked?

I'm fairly sure I've done this all according to code, just trying to wrap my head around what would happen in this situation.

 

[sberry]

No shock, its grounded. If it wasn't it would be floating, you come by and complete a path. Its sposed to clear the fault by passing current to trip the breaker.

 

[sberry]

Grounding it/everything all together puts it all at the same potential, virtually no difference between the equipment you touch and the ground you stand on, or any 2 pieces of equipment on the same service.

 

[wyliesdiesels]

Alright, I got the subpanel installed, but I have a question about the ground:

I made sure there wasn't a jumper bonding the neutral to the panel, so the neutral bar is completely isolated. I ran the ground from the ground buss in the main service panel to the ground buss bar in the new subpanel, which is also bonded to the subpanel case via screws. Wouldn't that mean if there were a short in one of the devices connected to the subpanel, and someone happened to be touching the subpanel at that time, that they'd get shocked?

I'm fairly sure I've done this all according to code, just trying to wrap my head around what would happen in this situation.

no. In order to get shocked, a person would need to be in the middle of the circuit which means touching the panel in one hand and a hot conductor in the other...

 

[doojus]

Alright, makes sense, thanks fellas.

Hopefully last question, I've searched on this and couldn't find an answer, maybe I searched for the wrong thing. I'm running EMT conduit from the sub panel through the ceiling into the attic. How do I terminate the conduit in the attic properly? Just poke a hole in the ceiling and stick one of those plastic insulating collars in the end and call it done?

 

[wyliesdiesels]

Yes u need a conduit bushing to protect the wire...

 

[doojus]

Good deal, that's what I ended up doing. Finally got it all squared away this weekend, thanks for all the help.