adding more circuits to my electrical panel ...

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RustyShackleford

Minister of Fire
Jan 6, 2009
1,331
NC
This is follow-up to another post (or two) of mine - needing a new thread. Skip the next two paragraphs if you like ...


BACKGROUND

I'm looking to replace my HVAC system. Currently I have a dual-fuel heat split system heat pump (including a propane furnace), as well as a minsplit I put in an addition. I need to replace this limping 26-year old system, and I want to put propane in my rearview. The current heat pump and minisplit run off 30- and 20-amp breakers in a 50-amp subpanel. (The romex to the subpanel is actually 6awg, so could support 60 amps, as it did when the house was built).

It a previous thread it was decided that my overall system could support a heat pump with electric-resistance "heat strips". But I'm now leaning towards a unit that adapts the variable-speed inverter technology of minisplits to a centrally-ducted system: https://www.alpinehomeair.com/produ...r-handler-complete-systems/blueridge/bmah3618 This unit appears capable of handling my needs without employing any supplemental heating (although it can accept heat strips if I decide I need them). But it comes at the price of a 41-amp minimum circuit ampacity (MCA). Since the minisplit has an MCA of 20 amps, I'm out of luck, since 41 + 20 > 60 (especially if I end up needing heat strips).

THE ISSUE

So I need another circuit feeding the HVAC area. Currently, my 20-space/40-circuit Siemens load center is almost completely full; there is a space-saver breaker in every position. There is one single-pole circuit available, and perhaps I could open up another pole by combining two of my existing 120vac circuits. But I'm thinking it's time for somthing else. Maybe the right thing to do is to replace the load center with a larger one; Siemens has a 42-space/64-circuit one with 225-amp bus bars, which appeals. But a sub-panel is more practical.

Here's a picture of my current setup:

PXL_20230913_025502044.jpg


Since only 120vac circuits (13 of them) are coming in from above, I thinking to bust those out into a subpanel. It probably needs to go on the wall to the left of the corner, since the central vacuum is dominating the area to the right, and the only possible location is above code-compliant height. But there's no way those 13 runs of 12- and 14-awg romex will reach to that location (since most or all of them head off towards the main part of the house, to the right in the photo.

I understand I could install a junction box where that romex comes through the ceiling, or maybe in the attic just above there. In fact, I could run THHN in conduit from that j-box to the new subpanel (in addition to the feed from the main panel to the subpanel). And I could put a ground bar in the j-box, and only need to run a single EGC down to the subpanel (at least as large as the largest one coming in, which is 12awg). Question: can I also put a neutral bar in the j-box, so I only need to run a single ground and a single neutral, and the 13 hots, into the subpanel ? Obviously that neutral would need to be large, handling all the neutral currents; but does it need to be only as large as the feeder from the main panel to the subpanel ? Any other thoughts or suggestions ?
 
you can run a single ground wire of #4 to the j box but you can't do that for neutral. if you go over 24 inch pipe to all the wires in the attic you will have to de rate the circuits depending on how many wires there is.

as far as your question about the neutral no and if you did it anyway it would work but the size of the pipe would have to be big. if you ran plastic romex connectors they are rated for two per connector as the metal ones are rated for one. or you can run a three wire romex to the attic j boxes and each three wire romex would take care of 2 circuits and the neutral can be shared as long as the circuit are on different phases and how that works is if you have 10 amps running on one circuit and 8 amps on the other the neutral sees 2 amps. one phase cancels the other phase when the circuits are on opposite phases. if not opposite phases the neutral would see 18 amps. the neutral never sees more than the breaker running the circuits if on opposite phases
 
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No, don't want to do any MWBCs. And I'll forget about doing anything else fancy with the neutrals.

Forgot about the de-rating and the 24". I think I can keep the conduit run from j-box(es) to subpanel less than 24". But it's gotta be at least 1", maybe 1-1/4", just on conduit fill it looks like (for Sch40 PVC).

I thought the combined ground only had to be as big as the biggest ground wire ?
 
Two questions. Is the air handler variable speed? Is the the compressor variable speed? I didn’t find any sensible/latent capacity charts. Have you seen them? You don’t want humidity issues.

will the air handler and compressor be on different circuits? Could the air handler be run to main panel and compressor off the sub. My 3 ton package unit (compressor and air handler) is on a single 40 amp

I would be tempted to just take the easiest path fastest path for my current needs. Are your double pole breakers space savers?

Is upgrading the 60A sub panel an option?
Will you be installing new afci and or gfci breakers according to code if you go with a new panel? That get pricey fast.

I really think the biggest decision here is a HVAC equipment. I didn’t look into the unit that much but. I read the install manual quickly but didn’t find the current rating for the outside unit.
 
Just spitballing here, but could an outside load center be placed that fed the existing panel as the sub-panel?
 
Two questions. Is the air handler variable speed? Is the the compressor variable speed? I didn’t find any sensible/latent capacity charts. Have you seen them? You don’t want humidity issues.
Yes, both are variable speed. They are basically applying minisplit technology to a centrally-ducted situation. As far as humidity, the project page has this rosy language: These models are designed to run consistently at low output levels to maintain optimal temperatures in your space but can ramp up their output as needed. This type of operation also enhances airflow through your structure, greatly improves efficiency, optimizes air filtration (especially if paired with a media or electronic air cleaner), and does a better job dehumidifying when in cooling mode than nearly any other type of cooling system.
will the air handler and compressor be on different circuits? Could the air handler be run to main panel and compressor off the sub.
Yes, compressor is 240vac, air handler is 120vac/15amps. That's what's already happening with the old system: there's a dedicated 120vac circuit going to the airhandler location.
I would be tempted to just take the easiest path fastest path for my current needs. Are your double pole breakers space savers?
Yeah, everything is space-saver. That sucker is FULL. Except for the one single-pole location I mentioned.
Is upgrading the 60A sub panel an option?
Not unless I want to pull 4awg romex. The 6awg is good for 55 amps MCA (protected by a 60-amp breaker per the "one step up" rule). But I suppose pulling 4awg romex might be easier than adding this subpanel.

But there's other reasons to add a subpanel. My surge protector is just connected directly to the busbars (I didn't install it) - which isn't really kosher, should be on a breaker. Also, I'm super constricted on locations in the ground/neutral bar; there was already some of the forbidden each neutral not having its own dedicated screw, and I had to pigtail some grounds to fix that. I really feel like the universe is telling me to either add this subpanel, or replace the main load center entirely. :)

Will you be installing new afci and or gfci breakers according to code if you go with a new panel? That get pricey fast.
No. Our AHJ doesn't require that for this kind of incremental change.
I really think the biggest decision here is a HVAC equipment. I didn’t look into the unit that much but. I read the install manual quickly but didn’t find the current rating for the outside unit.
It says on the main page (link in OP): MCA is 41, MOCP is 50.
 
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Just spitballing here, but could an outside load center be placed that fed the existing panel as the sub-panel?
Maybe, but I don't think it'd be any easier. I'm not much of a fan of outside load centers anyhow.
 
if you go over 24 inch pipe to all the wires in the attic you will have to de rate the circuits depending on how many wires there is.
If I go with this plan, I'd be running THWN from the j-box to the new subpanel, which I believe has higher ratings than NM-B. But I believe I can keep the run to 24" anyhow.

On thing I've gotta puzzle out: what should the ampacity of the subpanel be ? Those circuits are all lighting and outlets, none of the big 240vac loads, but maybe some of the kitchen loads.
 
Yes, both are variable speed. They are basically applying minisplit technology to a centrally-ducted situation. As far as humidity, the project page has this rosy language: These models are designed to run consistently at low output levels to maintain optimal temperatures in your space but can ramp up their output as needed. This type of operation also enhances airflow through your structure, greatly improves efficiency, optimizes air filtration (especially if paired with a media or electronic air cleaner), and does a better job dehumidifying when in cooling mode than nearly any other type of cooling system.

Yes, compressor is 240vac, air handler is 120vac/15amps. That's what's already happening with the old system: there's a dedicated 120vac circuit going to the airhandler location.

Not unless I want to pull 4awg romex. The 6awg is good for 55 amps MCA (protected by a 60-amp breaker per the "one step up" rule). But I suppose pulling 4awg romex might be easier than adding this subpanel.

But there's other reasons to add a subpanel. My surge protector is just connected directly to the busbars (I didn't install it) - which isn't really kosher, should be on a breaker.
Well I think you’ve done your homework. Last question are these communicating units? If not how does the ramping work? It has to be on some time to satisfy or temp delta.

The price gives me pause. The are cheaply priced units. American standard single stage fixed speed ECM 3.5 Ton package unit with 10k strips cost my dad 8k installed 3 years ago.

Go big I guess. Do it right so you can add heat strips. Find some latent/sensible data. Test your static pressure..

Some surge protectors can be mounted to bus bars. No breaker needed. I guess
 
Well I think you’ve done your homework. Last question are these communicating units? If not how does the ramping work? It has to be on some time to satisfy or temp delta.
Yes, there's 18/2 between them. I was hoping it'd be 18/4 and the indoor unit would be powered from the outside unit, like in a minisplit - but no.
The price gives me pause. The are cheaply priced units. American standard single stage fixed speed ECM 3.5 Ton package unit with 10k strips cost my dad 8k installed 3 years ago.
Well, wasn't about half of that labor ?
Do it right so you can add heat strips.
Is there anything else I need to "do right" for possible heat strips, besides having a plan to get the amperage there if need be ?
Find some latent/sensible data. Test your static pressure..
The calculator I'm using (https://hvac.betterbuiltnw.com/) says I need about 4000 btu/hr of latent cooling (and a total of about 40K). Yeah, the mfg'er doesn't break out sensible & latent (even if this big chart attached below), but it does say 45K btu/hr (at 95 degrees outdoors and 75 indoors) so I imagine it can handle the 4K latent. Not sure how to think about that though ...
 

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Yes, there's 18/2 between them. I was hoping it'd be 18/4 and the indoor unit would be powered from the outside unit, like in a minisplit - but no.

Well, wasn't about half of that labor ?

Is there anything else I need to "do right" for possible heat strips, besides having a plan to get the amperage there if need be ?

The calculator I'm using (https://hvac.betterbuiltnw.com/) says I need about 4000 btu/hr of latent cooling (and a total of about 40K). Yeah, the mfg'er doesn't break out sensible & latent (even if this big chart attached below), but it does say 45K btu/hr (at 95 degrees outdoors and 75 indoors) so I imagine it can handle the 4K latent. Not sure how to think about that though ...
The thing with communicating units they can run the coil just above dew point to hit some efficiency number. Rule of thumb for a single speed compressor/ blower is 70/30 sensible/latent ratio.

Really we want to think about pints per day because you don’t run the AC all the time. 4000 buts= about 4 pints so 100 pints per day. They have moisture load calcs that even give the time of year
 
Moving circuits is a pain. You don't need to move all the circuits to the subpanel. Just pick 4 of them to move and extend them by putting j-boxes above the existing panel. That will buy you enough space for the breaker to feed the subpanel.
Run 2-2-2-4 aluminum SER to a 100A main lug panel and put it on a 90A breaker. Then add your HVAC loads to the subpanel.

edit: alternatively, is there one 240V circuit you could easily move to the subpanel? that would save you from having to touch any of the smaller circuits.
 
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edit: alternatively, is there one 240V circuit you could easily move to the subpanel? that would save you from having to touch any of the smaller circuits.
Yeah, the EVSE. Would still need a space-saver feeding the subpanel though, but I guess that's ok. Hate to use aluminum. Can probably manage to run short piece of conduit (maybe liquid-tight) from main to subpanel, and put heavy THWN in it.
 
Really we want to think about pints per day because you don’t run the AC all the time. 4000 buts= about 4 pints so 100 pints per day. They have moisture load calcs that even give the time of year
How does the big dehumdifier in my crawlspace factor into things, if at all ? I guess most of the moisture the heat pump would need to remove comes in through infiltration through the walls, not from the crawlspace.
 
WHY do you hate to use aluminum? Is your main service not wired in aluminum?
No, copper. What's the advantage of SER ? Different protection requirements ? I recall someone saying I should've just run it for the EVSE (in mountain house) instead of THWN in conduit.
 
No, copper. What's the advantage of SER ? Different protection requirements ? I recall someone saying I should've just run it for the EVSE (in mountain house) instead of THWN in conduit.

A SER cable is like NM-b, but without the 60C limitation and you can run it outdoors.
It's much easier to install than conduit. In your case the conduit is probably not a big deal for such a short run, so do whatever you want.

The main advantage of aluminum wiring for larger gauges is cost. For such a short distance it's probably less significant, but lots of people shell out unnecessarily for copper wiring because they think it's top shelf so it must be better. In reality aluminum has no downsides except you need a slightly larger gauge for a given ampacity. The concerns about oxidation and failed terminations stemmed from an issue that was specific to small wire gauges and a particular alloy that is no longer available in production. For peace of mind, they do make an anti-oxidant paste you can put on the aluminum terminations.

I won't tell you what to do but in my house I would either use aluminum SER or put #2 aluminum THWN in a conduit which is good for 90A. If you're anal about getting the full 100A you can bump it up to #1. Both will be much less cost than the equivalent ampacity of copper.

BTW, my 200A service and every one that I've seen in this area is 4/0 AL. Overhead POCO drops are #2 AL because they have different rules.
 
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How does the big dehumdifier in my crawlspace factor into things, if at all ? I guess most of the moisture the heat pump would need to remove comes in through infiltration through the walls, not from the crawlspace.
It should have minimal impacts if you have a half decent vapor barrier. Most moisture is coming in with the air.

Thoughts… you get 3 pint per hour for each ton of AC (fixed speed). But the thing is it highest indoor humidity will occur when you have minimal or part cooling load. Hottest day of the year 12 hour run time at 3 tons. That’s 108 pints per day and covers your 100 pint per day load. AC may run longer but with variable speed what capacity is it running at?? It all depends on coil temp and air flow. Adding a whole house dehumidifier is not cheap. You’ve probably done more work correctly sizing your equipment than most contractors. Make sure you have adequate duct sizing and returns.

If you have less than 1000 sq ft per ton cooling you are oversized for cooling. But again what does this really mean for variable speed equipment? I think we will find a lot of moisture issues for homes that go with non communicating variable speed equipment in the future. I just don’t get the sense that (and the lack of latent data for yours is probably the norm) these newer import units will be sized or configurable to mange moisture well. That said 30 years ago no one cared.
 
You’ve probably done more work correctly sizing your equipment than most contractors.
Yeah, my last guy just looked at house size and said "2.5 tons" - for cooling, plus a 72K btu/hr (input) furnace. Sharp guy though, moonlighting cryogenic tech, and the system has worked great for 26 years and counting (notwithstanding going through contactors pretty frequently).
Make sure you have adequate duct sizing and returns.
Well, my duct system is awful. From the original system, which was a "gas pack" (A/C plus propane furnace). MBR is 10 degrees cooler than rest of house; which is good in summertime. And since we heat with wood, it doesn't matter in wintertime. But I'd like the system to behave properly without wood heat, and having the MBR 10 degrees hotter than the house ain't gonna cut it. So I'm not sure what to do. Maybe just dampers on the MBR registers, except that might raise the overall static pressure too high ?
If you have less than 1000 sq ft per ton cooling you are oversized for cooling ...
Well, this is the hot humid southeast (where I see you also reside). That 2.5-ton unit is barely big enough for our 1700 sq-ft, so ... Which is why I'm up-sizing to 3-tons, so hopefully the heat will work withoht auxiliary. And with the variable speed, hopefully the cooling oversize won't hurt.
 
Yeah, my last guy just looked at house size and said "2.5 tons" - for cooling, plus a 72K btu/hr (input) furnace. Sharp guy though, moonlighting cryogenic tech, and the system has worked great for 26 years and counting (notwithstanding going through contactors pretty frequently).

Well, my duct system is awful. From the original system, which was a "gas pack" (A/C plus propane furnace). MBR is 10 degrees cooler than rest of house; which is good in summertime. And since we heat with wood, it doesn't matter in wintertime. But I'd like the system to behave properly without wood heat, and having the MBR 10 degrees hotter than the house ain't gonna cut it. So I'm not sure what to do. Maybe just dampers on the MBR registers, except that might raise the overall static pressure too high ?

Well, this is the hot humid southeast (where I see you also reside). That 2.5-ton unit is barely big enough for our 1700 sq-ft, so ... Which is why I'm up-sizing to 3-tons, so hopefully the heat will work withoht auxiliary. And with the variable speed, hopefully the cooling oversize won't hurt.
I’m cooling 2000sq ft upstairs with 1000 sq ft basement with 17’ glass garage door with 2 tons. Original 1968 insulation. Brick veneer. One big shade tree. only get direct sun in two windows. My definition of big enough. it keeps temps within 1 degree of set point even if it has to runs non stop from 10 am-10 pm. Upsizing variable speed equipment is ok. I think. And upsize 1/2 ton just makes sense with global warming.

As for ductwork. Adding another run or three is always better than a damper ( to a point). You now have a variable speed blower so if your static pressure is ok to start with (it’s probably not. You need a 20x30 and another return for 3 tons) their may not be much air coming out the last vents at the end of the trunk line when it’s on low. All this said closing bedroom and bathroom doors does more to affect static pressure than almost anything else. Other than closing several vents off.

My new 4 ton ductwork running at 750 cfm barely gets air to the end registers. It’s ok. Lots of air near the unit. Installer did not like it and boosted blower up 150 cfm but… I didn’t like it. Air was not cold enough so I turned it back to down.
 
You now have a variable speed blower so if your static pressure is ok to start with (it’s probably not. You need a 20x30 and another return for 3 tons)
I need to find a caculator and try to compute it, I guess. I think the problem is, the rectangular cross-section main trunk gets narrower, plus the pieces of 6" flex coming off it are pretty long.
their may not be much air coming out the last vents at the end of the trunk line when it’s on low.
That's exactly what I'm worried about.
 
I need to find a caculator and try to compute it, I guess. I think the problem is, the rectangular cross-section main trunk gets narrower, plus the pieces of 6" flex coming off it are pretty long.

That's exactly what I'm worried about.
Need to know the air flow of the unit when it’s on low heating/cooling. 19 registers in hour house 735 cfm is ok for an oversized duct system with low static pressure. So 39 cfm per register. 50-75 cfm is more what you want to shoot for. Don’t go lower than 39.
 
Need to know the air flow of the unit when it’s on low heating/cooling. 19 registers in hour house 735 cfm is ok for an oversized duct system with low static pressure. So 39 cfm per register. 50-75 cfm is more what you want to shoot for. Don’t go lower than 39.
I guess I should first just measure the static pressure of the existing system. Then see what it measures with the registers in the MBR (the room that gets way too much air) closed. That would give an idea if simply putting dampers on those ducts would solve my problem, or if it'd make the static pressure too high.
 
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It turns out the MCA of the existing minisplit is pnly 16 (the MOCP is 20) and the MCA of the new heat pump is 41. I'm still unclear if that means the MCA of the existing subpanel needs to be 57, or if it can be lower.

I did find out that even though you're allowed to use a 60amp breaker with 6awg wire (the next-step-up rule), the ampacity of the circuit is only 55 amps. Oh, and the wire going to my existing subpanel is XHHW, not NM-B; I don't think that changes anything.
 
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