Service Panel Calculations

[brazosdog02]

Hello, I ended up here after a bit of research. I am a homeowner with some electrical experience. I have a question concerning calculations Im making, but they are not for me to perform the work. They are for me to make reasonably certain my electrician is doing the job correctly (ive gotten burned with a few of them) for the safety of my family. Is this forum meant for people like me? Can I even be here? I wanted to check before going through a relatively long post and getting banned.

 

[charlie b]

You would not be banned for asking a question. If a person who is not an electrician posts a question that reveals an intent to perform their own electrical installation work, the post will be closed by a moderator. This forum was created for the benefit of electricians, electrical contractors, electrical inspectors, electrical engineers, and other members of the electrical industry. If you post a question related to a calculation, and if there is a clear intent that the question will assist you with your contract with your electrician, then the question should be allowed. Please understand that this forum is entirely voluntary. So I can't promise that you will get any answers.

 

[brazosdog02]

Yes, I understand. Just didn't want to be out of line. Here is the deal. I have two electrical panels. Each is 200A, and on their own meters. Yes, one residence, two meters, two feeds. One Panel is pretty much all lights and outlets, and another is pretty much all 'heavy loads'. It has a 5 ton AC (heat pump with electric backup), 2.5 ton AC (conventional), dryer, oven, etc and THREE tankless heaters (POU types). These heaters all pull ABOUT 15,000 VA (all require dual 40a circuits) a piece. Actually, one is 17kw, one is 12kw, and one is 14kw. The manual says that is MAX...they probably dont operate at max. All of this is on one 200A panel. It has been seen by electricians and a house inspector several years ago and no one balked at it. One of those heaters is running a single 40A circuit, but actually should be using two but it serves a sink and washing machine. Second circuit is now available for use but not hooked up yet. Another heater is actually rated at 12,000 VA and using a single 40A circuit, but its old, and when it gets replaced, it will also be a dual 40A circuit most likely because thats just whats on the market. So, Im trying to plan accordingly. Upon checking the electricians work and calculating loads (to the best of my ability) I suspect this layout is not the best. My 'lighter load' panel is on a generator, so thats why all the heavy stuff is elsewhere...it is deemed non-necessary for emergencies.

The long and short of this is that my electrician is coming out to see what his workers have done, and hopefully offer suggestions/fix. I basically want to know if its possible to safely (to code) re-arrange some things in these two panels so that they can function in some combination on my two 200A panels. The killer on the power is the three 15,000 VA heaters (actual numbers above). Now, as it is wired, I have never had a breaker trip, but that might not be the case in the future when the one goes into full service. Ive used the load calculator here and determined that YES, it is possible to arrange these on my panels. But, I want a pro to give their insight. I have had more than my fair share of trouble with contractors and electricians...even though they are licensed. Basically, it would require having them move two heaters to the lighter panel since its purely lights, outlets, fridge, etc.

Can it be done, or does it require a service upgrade? My calculations are telling me with the way I have outlined, 200A service will suffice legally (175 min on one, 200 min on other), but I am NOT an electrician and I have no experience with running load calculations. It was using the 'Residential Standard Calculation'. Im just not sure because I do not know the code and how tankless should be accounted for since they are not on at the same time, though the potential is there.

 

[charlie b]

You are in over your head, a common circumstance for people who don?t do this for their living and who try to get involved in the process. It is the primary reason the owner of this forum does not want us to assist do-it-yourselfers. He doesn?t want you to get an answer to the question you ask, but fail to get answers to the many other questions you needed to ask, but that you were unaware that you needed to ask.

For your situation, what is needed is a formal load calculation that takes into account the entire house loads, and that separately accounts for the loads that will be placed on the generator (a subset of the total loads). That calculation will address more than the things you mention. It will include the required circuits for such things as the kitchen and the laundry, as well as any specific appliance loads. There is a way to properly take all this into account. But if you try to do it with some type of calculation software without understanding how that software is going about its calculation methodology, you are likely to get a wrong answer.

As to the tankless heaters, yes they can each draw a lot of power, but they don?t operate for more than a matter of tens of seconds at a time (not for hours, like a light might do). And they are not likely to all three be running at full load at the same time as the stove, the washing machine, and the dryer. There is a proper way to account for non-coincidental loads, and one does not learn such things through the Internet.

You might find that the situation is more difficult than you imagine. For example, I don?t know how it came to pass that you have two metered services to the same building. But the code only allows that under limited conditions. If your house does not meet those conditions, the local authorities might be willing to let it stand the way it is, given that they have no basis for requiring you to change an existing situation. But if you (or rather your electrician) apply for a permit to do any significant revisions to the existing configuration, they authority having jurisdiction can use that as the basis to force you to bring the existing installation up to the current code requirements. That might mean replacing the two services with one service, and changing out the two existing panels to one larger panel.

My suggestion is that you throw the load calculation and the design options into the hands of the electrician who will be performing your installation.

 

[JDBrown]

I'm afraid I read that whole thing twice and I'm still not sure what your question is. :?

The first thing to come to mind as I read was NEC 230.2, which generally prohibits 2 services for the same structure. It's not an absolute rule -- there are some pretty clearly defined exceptions -- but I wouldn't expect to find any of the exceptions in a residence, unless it's a RESIDENCE. I suppose you could technically have one service supplying two meters... Otherwise, this in itself is probably a violation (couldn't say for sure without more information, though).

I'm confused by the use of "single 40A circuit" and "dual 40A circuit". So you know, a typical residential service in the US is 120/240V, single phase. The voltage from either "hot" to neutral is 120V, while the voltage between the two "hots" is 240V. When you say "single 40A circuit" I think you mean the circuit is connected to a 40A, 1-pole breaker. This would be a 40A, 120V circuit. When you say "dual 40A circuit" I think you mean the circuit is connected to a 40A, 2-pole breaker. This would be a 40A, 240V circuit.

If that's the case, then you need to understand that most equipment is rated for either 120V or 240V -- you can't just switch between the two. If you have a 120V water heater, it needs to be connected to a 1-pole breaker. If you have a 240V water heater, it needs to be connected to a 2-pole breaker.

I assume there's something I'm not getting here, because it sounds like 2 of your water heaters (the 12kW and the 14kW maybe?) are being fed from 40A, 1-pole breakers (120V). The thing is, 12kW at 120V is 100A, and 14kW at 120V is 117A. Even if all of the water heaters are actually connected to 2-pole breakers (240V), 12kW at 240V is still 50A, and that's your smallest heater! This all seems a bit unlikely, given that you say you haven't had a breaker trip.

So something's not right here, but it's hard to tell exactly what. Of course, it's entirely possible that the only real problem is that I totally misunderstood you.

P.S. I just saw Charlie's last post, and he's right. There's a lot more that goes into load calculations than you would imagine, and throwing a generator into the mix only complicates things further. When I do load calcs, I often spend a week or so chasing down and collecting information from various manufacturers before I ever start to do an actual calculation. People take special classes just to learn how to do NEC compliant load calcs, and it's not because they're bad at math. There are just too many nuances to explore and explain in a forum setting like this.

Now, if you had posted an actual load calculation and asked, "Do you see any issues with this?" that might have gotten you some more useful responses.

P.P.S. If your city has an electrical plans review division, they're usually pretty good about checking things like load calculations. That seems to only exist in larger cities, though. If your city does have one, I'd recommend worrying about it even less. If the electrician is able to get the calculations approved by the electrical plans review division, I seriously doubt you'll have problems with things like overloaded breakers or panels.

 

[suemarkp]

I think by dual he means these heaters have stages of 40A each. Typically, electric tankless units have from one to three circuits that feed them, and a common value for each circuit is 40A at 240V (probably a 32A heating element so about 7.5KW of heating for each 40A circuit).

Note that there are two different calculation types in the code book -- the standard and optional. The optional typically yields a smaller value than the standard, so that's what I'd use as long as this is a residence. As Charlie said, there are many things in there you need to count that were not mentioned. HVAC loads are also handled in their own special way. But once you're past a certain threshold of equipment and the HVAC, everything on the list gets a 40% factor applied. So those three tankless heaters will end up adding about 70 amps to your service calculation because of that 40% factor. And that is without any special factor for non-coincident loads.

So it may well be possible that a 200A panel is not overloaded with the 3 tankless units, a heat pump, and other stuff. Your clue to an overload panel is when its main breaker trips. If that is happeniong, you definitely have too much load on the panel unless there is a bad breaker or wiring problem.

 

[brazosdog02]

Thank you for all of the responses. Yes, I am aware I am over my head here, but I wanted to make sure. That's why I ended up here where real professionals can give insight. I appreciate it. To be clear, you folks may be aware that finding a legitimate licensed electrician can have WILDLY varying results as far as quality is concerned. My home is not 'in the city'. As such, there are a LOT of things that can be done that may be practically OK, but not code compliant...that's what Im trying to discern here and my hope is to gain enough insight to not be a homeowner completely at the mercy of which straw I draw when i call a licensed electrician. I figured there was more to calculation that just plugging in numbers.

Here are some more clarifications:

The water heaters are on for more than a few seconds, they are point of use, but that point of use is usually a washing machine or shower.

My terminology was wrong. What I mean was some of my water heaters require single double pole 40A circuits, and some require dual double pole 40A circuits. The manuals clearly state they pull max of 30A, and that of course depends on the inlet temp and the temperature I set the outgoing water to be. I doubt im pulling max load on any of them with a single faucet or shower. It is entirely possible and likely that all three heaters on three separate showers will be running concurrently.

It is a residence. There are two feeders to two meters that go to two 200A disconnects right under the meter, then outside the house, there are two additional disconnects of 200A a piece...from there, the service goes to their respective 'breaker panels'. The original intent of the home was to basically have an apartment in it with its own meter. Obviously, that never happened.

In short, it sounds like there is POTENTIAL for these units to all be on one panel because of the load calc, and have it be totally OK and legal. That might be why no one ever complained about it and it passed inspection. That's why I asked....that way, if a contractor comes out and says "we need to add more panels, and more wire, and more service, and itll take a week's worth of labor" I can have a clue as to whether he is full of it or not. So far, it has paid off big time to be involved/informed with every trade. I don't need to be an electrician, framer, plumber but it helps to have some clue about what is and is not correct. Plus its educational, I learn something and sleep better at night knowing I am convinced its safe. For instance, my current contractor wired a bathroom with a bedroom and tied all the outlets for everythign together on a single 15A breaker.....not a problem really, itll probably work fine and never give an issue, but its a code violation for the bathroom to be done like that. I know there are two acceptable means of doing it, but that is not one of them. It makes me feel better, especially when one looks me in the eyes and tells me "its good to go".

I appreciate all of the info!!