Residential Load Calcs - Standard vs. Alternate Methods

[JeremyWhaling]

Breaking off a topic from the tankless water heaters topic over in the environment section of this forum. This isn't Tesla energy specific exactly but I wanted to catch the attention of the master electricians roaming this little section of TMC.

Anyway, I was sharing how my panel was nearly maxed out capacity wise - plenty of spaces in a new panel but only 125A service (which would be mega bucks to upgrade) and I was concerned I wouldn't be able to stuff a water heater circuit (240V/30A or 15A depending on model) if I eventually wanted to get a heat pump hot water heater (house is nearly all gas appliances). I showed the standard load calc which put me at 7A (corrected later to ~9A) under the 125A limit, and then showed my worksheet to prove how few amps I had left.

I then did a rough "alternate method" calc which showed I had 28A to spare and would be ok even if I did add the 30A circuit using the alternate method.

I'm quite surprised by the difference these two methods calculate. Is this just the way things are? Is the alternate method nearly always the way to go?

Here's the posts with the load calcs themselves:
Tankless water heaters are terrible....

As an aside, I find the replies further down the thread that "I just need to switch to LED lights" particularly humorous. I of course have already done so, but that doesn't change code.

 

[Ostrichsak]

Breaking off a topic from the tankless water heaters topic over in the environment section of this forum. This isn't Tesla energy specific exactly but I wanted to catch the attention of the master electricians roaming this little section of TMC.

Anyway, I was sharing how my panel was nearly maxed out capacity wise - plenty of spaces in a new panel but only 125A service (which would be mega bucks to upgrade) and I was concerned I wouldn't be able to stuff a water heater circuit (240V/30A or 15A depending on model) if I eventually wanted to get a heat pump hot water heater (house is nearly all gas appliances). I showed the standard load calc which put me at 7A (corrected later to ~9A) under the 125A limit, and then showed my worksheet to prove how few amps I had left.

I then did a rough "alternate method" calc which showed I had 28A to spare and would be ok even if I did add the 30A circuit using the alternate method.

I'm quite surprised by the difference these two methods calculate. Is this just the way things are? Is the alternate method nearly always the way to go?

Here's the posts with the load calcs themselves:
Tankless water heaters are terrible....

As an aside, I find the replies further down the thread that "I just need to switch to LED lights" particularly humorous. I of course have already done so, but that doesn't change code.

Actually, you may make the problem worse as an LED has lower running wattage (the number that marketing departments key on) but at initial startup they use way more wattage than incandescent. This suggestion won't have much of a difference in your load as you need to factor for these spikes at startup to avoid overtaxing the system.

 

[BrettS]

Actually, you may make the problem worse as an LED has lower running wattage (the number that marketing departments key on) but at initial startup they use way more wattage than incandescent. This suggestion won't have much of a difference in your load as you need to factor for these spikes at startup to avoid overtaxing the system.

I don’t believe that this is the case and I just spent a bit of time googling and didn’t really find anything one way or the other. Do you have any data to back up this claim?

 

[Ostrichsak]

I don’t believe that this is the case and I just spent a bit of time googling and didn’t really find anything one way or the other. Do you have any data to back up this claim?

It's called inrush current & it affects LEDs at startup & can trip breakers at a lower wattage rating than their higher wattage rated incandescent counterparts. This is why you'll often see dimmer switches that are compatible with LEDs that have a rating of 500 watts for incandescent & only 150 watts for LEDs. It's not a frequent occurrence as parameters need to be just right but the overarching point I was making as switching to LEDs won't resolve your load dilemma & in some cases can actually make things worse.

 

[BrettS]

It's called inrush current & it affects LEDs at startup & can trip breakers at a lower wattage rating than their higher wattage rated incandescent counterparts. This is why you'll often see dimmer switches that are compatible with LEDs that have a rating of 500 watts for incandescent & only 150 watts for LEDs. It's not a frequent occurrence as parameters need to be just right but the overarching point I was making as switching to LEDs won't resolve your load dilemma & in some cases can actually make things worse.

Very interesting. I was able to find this article that gives some additional details...

High Inrush Current With LED Lights - How to Fix? - LampHQ

 

[wwhitney]

I'm quite surprised by the difference these two methods calculate. Is this just the way things are? Is the alternate method nearly always the way to go?

Yes, and yes.

Cheers, Wayne

 

[Ostrichsak]

Very interesting. I was able to find this article that gives some additional details...

High Inrush Current With LED Lights - How to Fix? - LampHQ

I first discovered this years ago when I was trying to add a smart switch to my garage lights. I've got a smart home automation system with nearly triple digits worth of devices so it was news to me when I saw a dimmer switch say that it could run 500-watts of incandescent or 150-watts of LED lights max. I thought it was a typo because I assumed my new garage upgrade from about a dozen old school fluorescent fixtures to half that many LED flat panels that used a fraction of the previous wattage and put off double the light in all temperature conditions was going to make this easier. I thought for sure I was all set to add a Z-Wave switch to the mix to get scene control over my primary garage lighting. So when I saw that rating disparity I was a bit surprised and contacted Zooz to point out the typo. I was quickly schooled on inrush and prior to that I really had no idea. The fact that I had no idea is what leads me to the previous statement of it being an infrequent occurrence and parameters needing to be just right but it is possible and as such we need to account for it in our load engineering plans.

 

[jrweiss98020]

I was concerned I wouldn't be able to stuff a water heater circuit (240V/30A or 15A depending on model) if I eventually wanted to get a heat pump hot water heater (house is nearly all gas appliances).

Why not get a gas-fired tankless water heater? I have one, and love it!

 

[Ostrichsak]

Why not get a gas-fired tankless water heater? I have one, and love it!

This is the only way to go IMHO. We chose that for our house when we built it 15-years ago and it's been absolutely flawless.

 

[JeremyWhaling]

Yes, gas tankless is possible, and we might go that way, but I do like the idea of reducing our emissions further. We'd be down to just two gas consumers - the stove/oven (wife will not budge on this), and our clothes dryer (new - pushing for another 30A circuit isn't likely).

We have a house project coming up soon where there will be a potential path from the attic to the garage (townhouse, three levels) which would allow running conduit for future use. Current 40 gallon gas water heater in the attic is only a year old, so I've got some time to decide.

 

[Ostrichsak]

Yes, gas tankless is possible, and we might go that way, but I do like the idea of reducing our emissions further. We'd be down to just two gas consumers - the stove/oven (wife will not budge on this), and our clothes dryer (new - pushing for another 30A circuit isn't likely).

We have a house project coming up soon where there will be a potential path from the attic to the garage (townhouse, three levels) which would allow running conduit for future use. Current 40 gallon gas water heater in the attic is only a year old, so I've got some time to decide.

We have a electric range and dryer so for us, the instant hot water heater is quite literally the only gas/oil consuming device we own. IMO it's the best solution for that particular need and until I can get an electric one that performs as well it will continue to be the last NG device we use. The fact that NG is ridiculously cheap in Colorado helps me get over the fact that one of our devices we use regularly can't be powered by solar (once we get it) which we don't currently have anyway. If they can get the GPH rate up on those electric unit I'd switch though.

 

[Vines]

I then did a rough "alternate method" calc which showed I had 28A to spare and would be ok even if I did add the 30A circuit using the alternate method.

I'm quite surprised by the difference these two methods calculate. Is this just the way things are? Is the alternate method nearly always the way to go?

Think of it this way, one load calculation is conservative and one is more aggressive. I often see new good quality homes use the more conservative calc, allowing extra loads to be added later.

As to this application, I'd 100% use a gas fed tankless heater if you are bumping into the limits of your 125A service. Don't make the problem worse imo especially if you are an electric car family.

 

[miimura]

Think of it this way, one load calculation is conservative and one is more aggressive. I often see new good quality homes use the more conservative calc, allowing extra loads to be added later.

As to this application, I'd 100% use a gas fed tankless heater if you are bumping into the limits of your 125A service. Don't make the problem worse imo especially if you are an electric car family.

I also use a gas tankless water heater. However, if you have a traditional tank gas water heater, it may have a small gas line running to it. The tankless water heaters use a larger burst of heating, so it may require a larger gas line. Just something to think about. I had the house built with this in mind. I also had it plumbed for a nat-gas backup generator, so that also has a large gas line and it necessitated a larger than normal gas meter so that it wouldn't have a pressure drop when everything was running.

 

[Vines]

I also use a gas tankless water heater. However, if you have a traditional tank gas water heater, it may have a small gas line running to it. The tankless water heaters use a larger burst of heating, so it may require a larger gas line. Just something to think about. I had the house built with this in mind. I also had it plumbed for a nat-gas backup generator, so that also has a large gas line and it necessitated a larger than normal gas meter so that it wouldn't have a pressure drop when everything was running.

I have a tankless heater, and the original piping, we did not upgrade the LP gas line. Not sure if its limiting my output, but it sure is very hot water to 2 faucets.

I did see the warning in my installation manual to use larger line (1" I think) but I am only like 5' from the propane tank, might be helping my case.

 

[JeremyWhaling]

I had a large gas line running in the attic for a furnace anyway. No issues running a tankless here.

I did some more research and notice that my particular 40 gallon gas water heater has a fairly high recovery ratio (41 GPH) and first hour rating (71 GPH) for its size. The Rheem is 27 GPH and 60 GPH respectively. I am not sure this matters, but considering the overall size of the unit... it probably does.

SCE looks to be gearing up for a $1,000 rebate which would make it very inexpensive. But in our case I don't think so... Gas seems to be better at recovery even in tank form.

 

[Patrick66]

While I generally prefer the idea of the heat pump water heater in a climate that does not have a lot of heating degree-days, with your 125A service sticking with gas makes sense.

I would have an electrician price out an upgrade though, 125A isn’t much to work with in the future. Just know a precise cost and plan for it at some point in the future.

 

[JeremyWhaling]

I would have an electrician price out an upgrade though, 125A isn’t much to work with in the future. Just know a precise cost and plan for it at some point in the future.

It's probably tens of thousands of dollars. Seriously an avalanche of things:
New meter cans for me and my connected unit, current meter can is rated for 125A. The wiring feeding it is only sized for this as well (I want to say 1/0 aluminum, it's pretty small), and it's fed underground to a splice box which then shares a weather head with our other complex building, so probably need to upgrade all of that (well except that building's feed from the splice box). Probably then need to upgrade the secondary wire on the pole which goes ~30 feet before joining the other neighbors' secondary feeds. Not sure how undersized the upstream secondary is from there, but the secondary goes another 50-75 feet to the end of the block where there's a 50kVA transformer feeding something like 5 multi-family buildings (3-4 units in each) in total. Nearly everything is undersized or basically at the limit. I think it's because as far as I can tell nobody has A/C except us, and so cal is typically nearly all gas appliances for heat related things so the actual demands can be quite low. The townhome I'm in originally had 60A service. Originally a 20 kVA transformer fed the block until ~2009 according to google maps.

I did have SCE out but the gentleman said "I highly recommend you just upgrade to 125A". I was happy with being able to do that at the time, so I didn't press for costs.

But that said, I don't think it's all bad. At the end of the day it's a townhouse that doesn't need a heck of a lot of power. We can have A/C, could do a heat pump hot water heater if we really wanted to, or have an electric dryer (but probably not both). Electric stove and oven are out of the question, but my wife would want gas anyway. The 125A service doesn't currently limit our EV charging needs ether, especially since the gen 3 wall connector will do power sharing by the time we need it for the second Tesla, so ether car could charge at full power.

If by some reason the utility decides to bring the primary feed down the block and add another transformer, maybe it would be cheaper. But, highly unlikely.

 

[Vines]

We see this all the time with service upgrades especially underground feeds. Its rarely less than 20k if PGE has to come out and dig new underground lines.

The work and equipment at the typical home is about $5k in this part of the country, just to have the new 200A service physically installed and loads landed to the bus. Its even worse if your current service location is not at least 36" from the gas meter vent, and the whole thing has to be moved.

If its an overhead service drop you may think you are safe, except that all changes to the service in your jurisdiction must be replaced with an underground service (Portola Valley, Los Altos Hills, and some others) Otherwise overhead drops are usually near free for the PGE work to string a new aerial from the pole or line to your service. In some cases your service drop tap is right to the open span of the service feed, but they wont allow that connection today with the larger 200A wire, so you have to either go to the pole, or underground.