Anonymous wrote:Thank you, electrician friend! So our current setup is hydronic, heat-only, and we have ducts and 3 separate air handlers. The air handlers will be replaced 1-for-1. The tankless water heater that heats the water for the hydronic system runs on gas.

The HVAC guys are proposing various things, but the least ridiculous proposal involves 35A + 30a + 30a for 84,000 BTU total across 3 units (adding AC coil for each air handler). So same 84,000 BTU as you’re calculating with a high SEER unit. Now I’m wondering if this design is also constrained by the 3 separate air handlers and duct setup. The house is 2-stories but mostly 1 room deep so it’s pretty stretched out. One of the guys was adamant that higher BTUs and less air handlers/AC units would not work and would be less efficient.

You’re welcome.

I would say that you’re definitely constrained by your ductwork and air handler layout to requiring three compressors. Just so you have it, the power consumption of an AC system is dependent on the thermal load rating in BTUs and the seasonal energy efficiency ratio (SEER) rating. The minimum allowable SEER rating is 14; higher is better.

An 84,000 BTU (14 SEER) system would draw 84,000 / 14 watts of power = 6 kilowatts. At 240V, that is a current draw of 6000 / 240 = 25A. This is the actual steady state current draw, but most AC units will have some safety buffer and accommodation for fans, electronics, and other non-compressor components. This is captured as Minimum Circuit Amps (MCA) which is usually a little under 2X the typical current draw. About 48A for an 84,000 BTU (14 SEER) system. This will determine wire gauge. Then, HVAC systems are special since they draw a tremendous amount of current when first starting up (locked rotor amps) and this translates to use of a larger sized breaker (typically ~50% greater than the MCA rating for the system). The actual breaker size used for an HVAC is specified by this and is indicated as MAX CKT BRK AMPS on your HVAC nameplate.

95A still seems a bit high, TBH, unless you’re getting really inefficient systems – should be more like 75A across three breakers. Definitely don’t even worry about upgrading to a 400A service though. Even if all three systems are running at the same time, you’re only going to pull about 30A maximum steady state. The real issue is when a particular AC system initially kicks on and draws massive current for a fraction of a second, but you have three systems operating independently, so likelihood of any 2 or 3 kicking on at the exact same time is close to zero.

An alternative to using a breaker with a higher amperage rating would be to use a Type C (5-10X rated current) or Type D (10-20X rated current) industrial breaker specifically designed to withstand a higher inrush current, as often seen with HVAC compressors and large motors. More than you wanted to know, OP.

Anonymous wrote:Not the electrician but also you should look into mini split heat pump if your ductwork has some kind of issue.

The ductwork is fine, but just has a long enough run that it requires the multiple handlers.

Not the electrician but also you should look into mini split heat pump if your ductwork has some kind of issue.

Thank you, electrician friend! So our current setup is hydronic, heat-only, and we have ducts and 3 separate air handlers. The air handlers will be replaced 1-for-1. The tankless water heater that heats the water for the hydronic system runs on gas.

The HVAC guys are proposing various things, but the least ridiculous proposal involves 35A + 30a + 30a for 84,000 BTU total across 3 units (adding AC coil for each air handler). So same 84,000 BTU as you’re calculating with a high SEER unit. Now I’m wondering if this design is also constrained by the 3 separate air handlers and duct setup. The house is 2-stories but mostly 1 room deep so it’s pretty stretched out. One of the guys was adamant that higher BTUs and less air handlers/AC units would not work and would be less efficient.

listen to this wise electrician.

I would also be very leery of these particular HVAC guys. Telling you that you need 95amps for an A/C compressor is suspicious. I'm putting an additional 3 and 5 ton inverter units in (Daikin) and they are 20 and 35 amps respectively (We're adding 4400 sq. ft). Anyway, be leery that they are just putting in a too large compressor unit

Anonymous wrote:

Anonymous wrote:Does your current box have capacity for 4 new breakers? That would be good. 200 should be fine and was for our prior house with 2 AC units.

To me it looks like yes, but my electrical knowledge stops at HS physics and taking notes on my parents’ box for a project.

Electrician 1 insists it has room but is going to run the numbers to be sure.

Electrician 2 wants to add a panel but I think it’s just to simplify his other work.

HVAC guy 1 wants an additional 95A.

HVAC guy 2 insists he needs >100 additional amps.

Contractor who is neither HVAC nor electrical insists that all of these calculations are wrong. We have more electricians coming out in 10 days so I hope to get clarity then, but we are also under time pressure because we need to move and list our other house soon and book work before the post-Christmas busy season starts.

None of this matters if the city can confirm that we have a conduit and not just a buried line, or that we already have upgraded service from a past renovation, but it takes 2 weeks to be assigned a rep and even more time to get them to do a site visit.

It’s suddenly very obvious why the previous homeowners didn’t do certain things, and frustrating that even a thorough inspection can’t provide insight into some of the problems we’ve stumbled into.

PP electrician again. Needing to add an additional 95-100A seems suspicious, BTW. Are the HVAC contractors suggesting your panel lacks this capacity already? Really unlikely based on your descriptions. If breaker space is lacking, you could run a new subpanel. Further, that’s a lot of power and amperage draw for an AC system. A 4000 sqft house needs an 80,000 BTU AC = two units smartly sized to achieve 80,000 BTUs total.

Carrier makes nice units and their Comfort™ Series Single-Stage Air Conditioners are good entry level systems. See page 9 on: https://www.shareddocs.com/hvac/docs/1009/Public/06/24SCA4-01PD.pdf

You could go with a 48,000 BTU unit on a 40A circuit plus a 36,000 BTU unit on a 25A circuit (which are hard to find, so say 30A).

Or, you could also consider upgrading to a very high efficiency AC such as the Carrier Infinity® Variable-Speed Air Conditioner with Greenspeed® Intelligence. This will consume less power and be more likely to fit within your load center’s capacity. See page 4 on: https://www.shareddocs.com/hvac/docs/1009/Public/0F/24VNA6-02PD.pdf

With a high SEER unit, you could go with a 48,000 BTU unit at 30A plus a 36,000 BTU unit at 20A. This is much less of a power draw, should easily fit within your existing load center’s capacity, and will save you lots of $$$ in the future on operating costs. Plus quieter, better for the environment, and still very powerful. Definitely a more expensive install up front…but not if you can avoid upgrading from 200A to 400A main service.

Assuming you already have ductwork installed and an existing air handler on a dedicated 15A or 20A 1P circuit, not that this adds much to total power draw.

Don’t let these shady contractors fool you, OP.

Anonymous wrote:So you have 200 Amp right now and some people are telling you to go up to 400Amp ?
If you’re only adding AC you might be okay.

Yes, at 200 and trying to figure out if we need 400. Only AC is going to be added.

OP here. Thank you, electrician PP! Your numbers helped me make sense of some of the load calcs I’ve been doing while I wait for better info from the electricians. I think that with the additional 95A required for the AC (I’ve gone through the specs and the units are unfortunately the right size for our space), we don’t need the service upgrade but are right at the very edge of capacity. So we would need to be intelligent about not running a dryer and turning on the oven and every light when it’s 95 degrees out.

Fortunately we live out of the dmv now and we really only have a few weeks per year that are truly too hot to not have AC.

I found out this afternoon that the buried line may be in conduit based on when the city buried it. If it’s in conduit, then it’s not a big deal to upgrade but it will just be a long wait of 4-6 months. If it’s not in conduit, DH and I decided that we will not dig and we’ll just be hot and grumpy but have some money left to spend to make ourselves feel better.

Anonymous wrote:We have a gas furnace and hot water heater and are in the process of getting central AC (yay!). We currently have 200 amp service and have been surprised that multiple HVAC companies want us to upgrade our service from the city to add 200 amps.

Meanwhile, every electrician is doing load calculations and saying that it’s not only unnecessary, but will take forever (3-9 months) and the city might say no since we have buried lines that go to an overhead line across a busy street. We can’t start necessary electric or HVAC work because no one agrees on what to do. Could we really need another 200 or are these guys just giving us lazy information?

House is 4000 sf, no big electrical things like a pool or hot tub, gas stove but other kitchen applicants are electric. No plans to add a car charger. We are a small family and we do sometimes entertain but would never have more than 4-5 people sleeping in the house, so it’s not like we have a 10 person electrical load.

An upgrade to a 400A service seems unnecessary.

Our house is 3600 sqft finished with a 200A main panel which includes a 60A 2P circuit for central AC, a 50A 2P circuit for a double wall oven, 20A and 50A 2P circuits for dual level 2 car charging, a 30A 2P circuit for an electric dryer, and a 40A 2P circuit for outdoor radiant heating. Then a bunch of 15A and 20A 1P circuits for lights, outlets, and other fixtures. We have a gas furnace, gas cooktop, gas hot water heater, and gas fireplaces. Never had any issues, but we’re pretty close to capacity. Though some stuff is unlikely to be used at the same time (e.g., AC + outdoor heating for screened porch). Plus many circuits won’t pull anything close to maximum capacity (e.g., 50A car charger circuit has 40A charging unit that is programmed to pull 32A [80%] max / 20A charger pulls 16A max / etc…).

I am an electrician. HVAC guys are just covering their butts.

We have an all electric house with a well and a car charger and 200 amps is fine for us.

So you have 200 Amp right now and some people are telling you to go up to 400Amp ?
If you’re only adding AC you might be okay.

Without seeing the box it's impossible to know.

It could be that the existing box is already overloaded.

Anonymous wrote:Does your current box have capacity for 4 new breakers? That would be good. 200 should be fine and was for our prior house with 2 AC units.

To me it looks like yes, but my electrical knowledge stops at HS physics and taking notes on my parents’ box for a project.

Electrician 1 insists it has room but is going to run the numbers to be sure.

Electrician 2 wants to add a panel but I think it’s just to simplify his other work.

HVAC guy 1 wants an additional 95A.

HVAC guy 2 insists he needs >100 additional amps.

Contractor who is neither HVAC nor electrical insists that all of these calculations are wrong. We have more electricians coming out in 10 days so I hope to get clarity then, but we are also under time pressure because we need to move and list our other house soon and book work before the post-Christmas busy season starts.

None of this matters if the city can confirm that we have a conduit and not just a buried line, or that we already have upgraded service from a past renovation, but it takes 2 weeks to be assigned a rep and even more time to get them to do a site visit.

It’s suddenly very obvious why the previous homeowners didn’t do certain things, and frustrating that even a thorough inspection can’t provide insight into some of the problems we’ve stumbled into.

Does your current box have capacity for 4 new breakers? That would be good. 200 should be fine and was for our prior house with 2 AC units.

We have a gas furnace and hot water heater and are in the process of getting central AC (yay!). We currently have 200 amp service and have been surprised that multiple HVAC companies want us to upgrade our service from the city to add 200 amps.

Meanwhile, every electrician is doing load calculations and saying that it’s not only unnecessary, but will take forever (3-9 months) and the city might say no since we have buried lines that go to an overhead line across a busy street. We can’t start necessary electric or HVAC work because no one agrees on what to do. Could we really need another 200 or are these guys just giving us lazy information?

House is 4000 sf, no big electrical things like a pool or hot tub, gas stove but other kitchen applicants are electric. No plans to add a car charger. We are a small family and we do sometimes entertain but would never have more than 4-5 people sleeping in the house, so it’s not like we have a 10 person electrical load.