Considering a sub-panel: reasons for 4-wire 240VAC

[JHZR2]

Hi,

My old home has upgraded 200A service. Becasue a lot of the circuits were highly branched knob and tube, as they got pulled, a good number of new circuits were made. We have an electric range, a 240V AC, etc., so we dont have a ton of space in our main box.

At the same time, on the opposite side of the house is where we are considering putting new AC equipment, be it split ductless or something else.

So Im toying with the idea of a sub-panel on the opposite end of my house (essentially opposite end of my basement) to support 240V loads.

Our through wall AC has a three-prong 240V plug (NEMA 6-15, IIRC). The ductless systems we have considered have wiring diagrams like this:

So, is there any good reason to run 4-wire 240VAC service (Im thinking of making the subpanel fed from a 60 or 80A breaker in the main panel, so the wiring will be hefty and $$$)?

To me, the only reason is IF I was ever to want to run a 120V branch off of the subpanel.

But, the reason I want the subpanel is so that I can pull the current 240 breaker from the main, use that as the subpanel feed, and then retain freed up open slots in the main panel to feed 120V circuits if they are needed.

So if Im putting in the subpanel to serve as isolation for a few 240V devices only, is there a good reason to go 4-wire 240V vs 3-wire 240V?

Thanks!

 

[AandPDan]

As you said, if you ever want to put in a 120 vac circuit you will need that 4th wire for a neutral.

Have you considered aluminum for your wires? It is perfectly acceptable as a feed to a subpanel and in larger sizes is very cost effective. Most likely your main service is aluminum.

Just make sure the terminals are torqued correctly and that you use an anti-corrosion paste on the terminals.

 

[JHZR2]

My main lines in are Al!

The whole house is Cu. Good point.

 

[The_Jeff]

For one thing it would future proof your AC, so if you got a newer AC that used 120V for remote controls or whatever you'd already be set up.

 

[JHZR2]

Originally Posted By: eljefino
For one thing it would future proof your AC, so if you got a newer AC that used 120V for remote controls or whatever you'd already be set up.


Well thats part of my thinking... Everything is going throttlable inverter DC tech, it seems.

So the need for 120 may be more and more moot...

Didnt think about the Al option, Ill bet it is cheaper to run 4-strand Al than 3-strand Cu for similar ampacity. Probably worth the application, especially if I can run oversized gauge and really reduce the voltage drop.

 

[HerrStig]

Be sure to use the proper cu/al connectors and the proper cu/al "no ox" type paste.

 

[JHZR2]

Agreed. Need to be sure that the bussbar in the main panel allows this, the breaker itself allows this, and that the terminals on the subpanel also allow this.

I assume that all will be labeled if Al is OK?

May just stick with Cu too, who knows... Need to look at $$$ at HD.

 

[757guy]

Do a 100 amp subpanel using Al 2-2-2-4. Great flexibility for future wiring needs. Don't bond the ground with the neutral in the sub panel (main panel already handles that).

Good luck!

 

[JHZR2]

Originally Posted By: 757guy
Do a 100 amp subpanel using Al 2-2-2-4. Great flexibility for future wiring needs. Don't bond the ground with the neutral in the sub panel (main panel already handles that).

Good luck!


Thanks. Yeah that's a good idea. A friend had the ground and neutral bonded and was getting all kinds of funny situations with it. Learned that one from him but its a great reminder.

Not 100% clear on why you wouldnt - its all the same circuit in the main box, but it is a lesson learned and worth repeating. Thanks!

 

[JHZR2]

Also, does anyone know the max AWG for connecting to the OEM busbars in a mis-2000s square D 200A residential main panel?

Will that 2-2-2-4 wit in the holes, or do I need to mod it somehow?

 

[AandPDan]

Assuming you mean for the neutral and ground, there are taps available that will handle whatever size wire you need. They basically take 2 of the lugs for the ground/neutral busses and make it into one big one.

Something like this: Neutral lug

The circuit breaker feeding your subpanel will specify what size wire it can handle.

 

[tom slick]

Originally Posted By: JHZR2
Originally Posted By: 757guy
Do a 100 amp subpanel using Al 2-2-2-4. Great flexibility for future wiring needs. Don't bond the ground with the neutral in the sub panel (main panel already handles that).

Good luck!


Thanks. Yeah that's a good idea. A friend had the ground and neutral bonded and was getting all kinds of funny situations with it. Learned that one from him but its a great reminder.

Not 100% clear on why you wouldnt - its all the same circuit in the main box, but it is a lesson learned and worth repeating. Thanks!


The neutral and ground are only bonded at the main panel so a neutral can't back feed into the ground/subpanel if the neutral is broken before it returns the the main... Make sense?

 

[Win]

I would go with the fourth wire - my experience has been it's always cheaper to do it once and be done with it. Then it will be there if you need it.

The power company around here uses aluminum wire to the rain head because it's light weight and can get convective cooling in free air, but personally, I would not use aluminum anywhere, and I remove it wherever I see it, even if the codes permit it.

You may have difficulty sourcing a U.S. made panel. I know the Siemens panels that I have been installing in my latest project have been marked as assembled in Mexico. The large four meter base ( Milbank Manufacturing? ) I just had set was also marked assembled in Mexico. MC cable, conduit, boxes, copper wire and the other small bits still look to be U.S. made. The smart meters have no place of origin marked, so I presume that means they are U.S. made.

You think finding a U.S. made small hvac is hard? Wait until you try to find decent looking commercial light fixtures that are U.S. made and are not 5X or more than their competition, pricewise.

Very disappointing.

 

[JHZR2]

I found that Eaton CH line is us made boxes. I dont see any breakers that are US made.

QO from square D which is what I have in my home was us made but the latest breakers are Mexico.

Do you think it is any different that the 2-2-2-4 wire that Id run would not be in a wall or insulated space? Just running along a joist in my cool basement. I share the reservations about Al... But didnt even see 2-2-2-4 Cu at HD last night!

thanks!

 

[Win]

If it's a sub panel, why do you need number 2?

That seems like a lot. Would 6 or 4 be sufficient? It seems like the largest I have seen four wire solid copper romex in is #8, but it's been a while since I looked at it.

When copper wire goes into conduit, it will be derated to 80%, I think, and the conduit has to be big enough for the wire to stay cool.

You might want to visit with an electrician about this.

 

[JHZR2]

60A circuits (total, really 3x20) are needed.

I was going to go with a 100A breaker off my main - room for the future.

So thus the heavier line.

 

[andyd]

plan for the future, I'm with Win on copper only inside the house. Here on the coast, AL has issues In MA. most electric driers have a 4 wire cord. Also 15 amp circuits now use 14/2

 

[JHZR2]

??? I have a gas dryer, just needs 120V. I generally wire with 12.2, even if using 15A breakers. My house is old so I dont have any dedicated lighting circuits, which is why I do that.

 

[andyd]

OOPS I meant 12/2 DOH