Electrical Question - Sub Panel or Not

[Tidybuoy]

Got a question regarding the wiring for my outside AC Compressor which is being moved to a new location.

A few years ago, I wired for a laundry room 240v using 6-3 wire. Now I need to wire for the A/C compressor which is 20' from my laundry room.

I'm wondering if I could install a sub-panel in the laundry room that receives the 240v from the main panel and then branch out to the two locations. I'm thinking that the sub panel would have proper size circuit breakers (I think 40amp ea.) but the main panel would have to be increased to accommodate the two devices.

It would be much easier to install the sub panel rather than crawl thru my attic for a completely separate run. And, I have a sub-panel that's been sitting in my collection of stuff.

Thanks.....

[Zeke]

If you have to increase the main service to accommodate more breakers, I would first consider the sub panel. However, the main has to still have enough capacity for the loads. You can't keep adding 240v circuits forever.

The way I understand this is that you are just replacing the condenser unit, so the load isn't that much more. If it boils down to spaces for breakers only, by all means put a sub in. The nice thing about a sub in the same building is that you don't need new ground rods. Just unhook (or don't complete) the bond between the ground bus and the neutral bus.

Calculate the total load on the sub when choosing the breaker to feed it. Most say that the running amps is just enough, so you don't need 70 amps to feed a 40 and a 30 breaker. Hopefully your new condenser has a soft start anyway. But, given what size breakers are available for your main might be the deciding factor. In that case choose the larger. You may want to add another circuit some day. Like a car charger.

[Tidybuoy]

Originally Posted by Zeke

If you have to increase the main service to accommodate more breakers, I would first consider the sub panel. However, the main has to still have enough capacity for the loads. You can't keep adding 240v circuits forever.

The way I understand this is that you are just replacing the condenser unit, so the load isn't that much more. If it boils down to spaces for breakers only, by all means put a sub in. The nice thing about a sub in the same building is that you don't need new ground rods. Just unhook (or don't complete) the bond between the ground bus and the neutral bus.

Calculate the total load on the sub when choosing the breaker to feed it. Most say that the running amps is just enough, so you don't need 70 amps to feed a 40 and a 30 breaker. Hopefully your new condenser has a soft start anyway. But, given what size breakers are available for your main might be the deciding factor. In that case choose the larger. You may want to add another circuit some day. Like a car charger.

For Clarification:
My main panel is adequate and currently has 3 240v circuits; 1) Oven, 2) Dryer, 3) Condenser.

I just thought it would be easier to install a sub-panel at the end of the 240v dryer circuit, and then branch off to the new condenser. (rather than replace the current condenser line and climb thru my attic, run wire down the wall to the main box). But, I don't think the current breaker for the dryer is large enough for both dryer and condenser. I was thinking that would need to be the combination of both units but then they would each have their own breaker at the sub-panel.

Lastly, it is a condenser or a compressor? I've always thought it was a compressor.

[Zeke]

Won't work to divide the existing circuit. No can do. What you are essentially doing is supplying one receptacle for 2 cords and swapping them back and forth.. (You wouldn't wire a condenser with a service cord.) Run both at the same time and it will overload the breaker. Change the breaker size and change the wirze size accordingly.

With all due respect, hire an electrician, even if he just hooks up the final bits. Crawling is tough so find a small energetic person to pull the cable. No reason to pay jourman's wages for that.

Condenser is the whole unit that has a compressor, coils and a fan.

[908/930]

I'm not an electrician by trade, but your #6-3 would be rated for max 60A breaker at main panel, if running through attic you would derate that wire a little depending on attic temp. As Zeke mentioned you should run the AC direct to main panel or you could run new #3 wire and install 100A subpanel in your laundry room.

[john70t]

A/C is a huge draw. Worse when it's old and the system gummed up.
Don't mess around. Do it wrong and then..

I've got a separate meter and shutoff for A/C from the city/utility street transformer. (18 SEER I think which is mid-range efficiency)
That is probably code.

A future problem is more expensive than doing it right the first time.
If any city or buyer's inspector sees that...

[Zeke]

Originally Posted by 908/930

I'm not an electrician by trade, but your #6-3 would be rated for max 60A breaker at main panel, if running through attic you would derate that wire a little depending on attic temp. As Zeke mentioned you should run the AC direct to main panel or you could run new #3 wire and install 100A subpanel in your laundry room.

Well, 100A sub is overkill for even a home shop with a welder, compressor, etc. 100A will do a pretty big shop. I read the electrical forum at the Garage Journal (before the the admin banned me for life). I also read Mike Holt and another forum for pros only.

I was going to take the CA test and add it to my license, but I folded my license altogether. I didn't have a lot of the really high voltage knowledge either. I'm a residential electrician, so to speak. In CA that requires a separate cert which I could pass except the hours (over 4000) required working under a C10 electrician (licensed).

I THINK a 60 amp sub will do this. 30 and 40 add up to 70, but you are allowed to have 125% breakers (not a rule in stone) in a panel. No room for a car charger in there, but you might squeeze a lighting circuit w/o a problem.

So Tidybouy, either get on the non pro side of the Electricians Talk or if you wish, tell us about your main service, brand and capacity. There are a lot of ways to skin this cat.

Edit: The shop analogy assumes a one man shop where you can't possibly expect all equipment to run simultaneously. But it does assume you might be plasma cutting with the compressor running and a heater as well. Total demand under usual circumstances will work as an estimate of panel size. YMMV.

[john70t]

IIUC Tidyboy is talking about adding supply line inside his personal wood stick-built home.
One main breaker.
(ie primary residence. not a concrete shop)

Different rules would apply here..

[Tidybuoy]

Originally Posted by Zeke

So Tidybouy, either get on the non pro side of the Electricians Talk or if you wish, tell us about your main service, brand and capacity. There are a lot of ways to skin this cat.

Edit:

My house has a brand-new main service that is 200amp. This was installed in May 2023 as required for my solar. I don't know the brand but I think it is square-d. It replaced my old Zinzor box.

I am replacing my old AC condenser that is currently on a 40amp circuit breaker, with a new condenser unit that requires a 40amp circuit. However, the new condenser requires new wiring because it is in a new location. I will use the existing 40amp circuit for the new unit but with new wiring.

The only reason I brought up the sub-panel idea is because I found a new sub-panel box in my attic where I store crap. Where it came from...I don't remember. So, I was only thinking that it might be easier to mount the sub-panel and branch out from there to the new compressor and the laundry room vs running a line from the main panel to the condenser.

[Tidybuoy]

Originally Posted by john70t

IIUC Tidyboy is talking about adding supply line inside his personal wood stick-built home.
One main breaker.

Correct except the sub-panel box would have separate circuit breakers for the A/C Condenser and the dryer. Those would be the only circuits on the sub-panel.

[Zeke]

Originally Posted by Tidybuoy

My house has a brand-new main service that is 200amp. This was installed in May 2023 as required for my solar. I don't know the brand but I think it is square-d. It replaced my old Zinzor box.

I am replacing my old AC condenser that is currently on a 40amp circuit breaker, with a new condenser unit that requires a 40amp circuit. However, the new condenser requires new wiring because it is in a new location. I will use the existing 40amp circuit for the new unit but with new wiring.

The only reason I brought up the sub-panel idea is because I found a new sub-panel box in my attic where I store crap. Where it came from...I don't remember. So, I was only thinking that it might be easier to mount the sub-panel and branch out from there to the new compressor and the laundry room vs running a line from the main panel to the condenser.

You can use the new sub as a junction box if you wish. But there is no way you can use two breakers being fed from a 40 amp breaker in the main unless they are less than 40 amps each. As I said the total can add up to 125%. (Rule of thumb) If you study charger stations they have a higher continuous draw than average usage. There is more to this than you apparently know.

You're not getting something here. Be reasonable you're dealing with electricity. You may not make for a fire but you sure could burn up an A/C pump with low voltage.

[Rusty Heap]

I'm a retired EE and this has always worked for me...............Thank Me Later Okay! Just follow the code books and you'll be fine.








and more wiring advice from the 2023 NEC and NFPA 70 code books...


https://www.mez.co.uk/lucas.html

[Tidybuoy]

Thanks for all the advice.

I ended up running a line from the main utility box to the final destination. It was an easy project except for the crawling under the house. I was lucky to have an available conduit line from the main box to underneath the house. However, crawling under the house and stapling the line to the floor joists was....let's just say I'm still sore.

Pad & 240v shutoff box: not shown is the 120v outlet box to the left.


New platform in attic for new HVAC - all old ductwork is being removed and replaced with insulated duct:


Old HVAC: