Tankless water heater wiring dilemma

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Old 11-24-12, 05:07 PM
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Tankless water heater wiring dilemma

So I am remodeling these small condos (~600sf). The hot water for which was supplied by these 40 gallon pony water heaters occupying space under the kitchen cabinets. I had a brilliant idea to replace these with tankless water heaters…not only more efficient, but also making more cabinet space available. Backgroud info…(1)the pony water heaters are 240V wired from the panel on a double-pole 30 amp breaker with 2 #10 wires through ¾” EMT conduit; (2) the tankless are 240V 27kW units with max draw of 112 amps; (3) the panel receives its power from the meter via 2 #4 wires on a double-pole 80 amp breaker. Now that i have come to installation, my brilliance has given way to the reality of electricity limitations…

The tankless has 2 pigtails each requiring 240V connections. According to manufacturer’s instructions, the tankless calls for 2 double-pole 60 amp breakers run to the unit via #6 wire. Is it possible to pull 4 #6 wire through ¾” EMT? Is allowed by code? Finally, given the feed to my panel, will support that level of a draw from my tankless?

So I got to thinking…maybe a subpanel? Since my panel is fed by 2 #4 from a double-pole 80 amp breaker, I would assume there would no point in exceeding that mark. From looking at the wire diameters, it seems that pulling 2 #4 through ¾” EMT would be easier than 4 #6. Assuming I can, could I establish a subpanel feed from a double-pole 80 amp breakers via 2 #4’s, given the “parent” panel is itself fed from the meter through a double-pole 80 amp breakers via 2 #4’s…sounds like I’d be violating some electrical rule.

I thank you all ahead of time for your input on my thoughts or ANY new insight or ideas you may have. Looking forward to reclaiming some of my brilliance.

Until again, SmartRookie
 
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Old 11-24-12, 05:14 PM
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Now that i have come to installation, my brilliance has given way to the reality of electricity limitations…

I'll say......your new water heater draws more than the service can deliver. Your service is limited to around 20 kw. (20,000 watts) max
 
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Old 11-24-12, 05:20 PM
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For you to think they are more efficient....not. Gas units are somewhat more efficient, but the electric ones are, IMO, horrible, as you are finding out with all the wiring necessary.
 
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Old 11-24-12, 05:38 PM
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okay, PJmax. Am a correct that you established this from the meter feed (80 amps x 240 V= 19200 watts)? If so, the meter not far from the panel...how about replacing the meter breakers to a double-pole 120 amp breaker and #2 wire to the panel inside? does this create new possibilities?
 
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Old 11-24-12, 05:43 PM
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Chandler, I found and read so much info on both sides of the fence. Ultimately, I was sway by the fact these condos are only occupied at most half the nights of the year. so i felt that the cost of maintaining constant hot water could be mitigated. at this point, i own the tankless, and have done te cabinet rework on the first condo, so i'm pot commited so to speak with the first one.
 
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Old 11-24-12, 05:44 PM
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You would have to apply for a permit to upgrade service. You may only have a 100 amp service. The service drop, meter pan, main panel all figure into the mix. You can not just increase the size of the wire in the middle of a system.

You need to know what size service cable you currently have.


Yes......V x A = W
 
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Old 11-24-12, 05:50 PM
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Damn it! how would determine the amp level of my service? also, could you share some insight on my situation assuming my service is set to provide enough amps? that way, i can weigh the possibility of having my service upgraded if its not enough amps.
 
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Old 11-24-12, 05:53 PM
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determining my max amp service isn't as simple as taking my multimeter to the meter panel, is it?
 
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Old 11-24-12, 06:03 PM
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You would need a 150 amp service at the minimum and between 150 amp and 200 amp is not much of an increase. You have to allow enough service for the electric water heater, electric cooking, A/C runs on electric.

Do you know any electricians..... we recognize service size based visually on outside wiring. It's normal to run the size of the service cable as the same size as the main breaker....which means you are probably at 100 amps.
 
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Old 11-24-12, 06:26 PM
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the condos are individually metered...6 meters in the meter bank, fed by 3 large wires directly off the pole/transformer, so my gut says the amps is likely easily available. so once we've cleared the power to the panel issue, any thoughts?
 
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Old 11-24-12, 06:33 PM
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BTW...the subpanel idea is based entirely on the possible ease of pulling 2 #4's instead of 4 #6...there would be nothing else on the subpanel.
 
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Old 11-24-12, 07:43 PM
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.6 meters in the meter bank, fed by 3 large wires directly off the pole/transformer, so my gut says the amps is likely easily available.
And my gut says no. It is unlikely the contractor would have sized the wire any larger then necessary because of the cost of the wire. An even bigger problem if you do all six of the condos may be the power companies service. They may well charge extra if they have to replace the transformer and there could be an extra monthly charge for more then normal residential use. Of course that is assuming their service can be upgraded. In rural areas especially sometimes it isn't possible.
 
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Old 11-24-12, 08:07 PM
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Fair enough...i'll consult with an electrician regarding the current level of service and its upgradeability. that being said the possible pre-panel modifications (upgrading the service and meters) will need to taken into consideration with the post-panel modifcations (pulling new wires? sub-panel? etc.). So assuming the panel can be provided the requisite amps, then what? i read that 4 #6 wires are allowable in 3/4" EMT, as are 2 #4's. If i am correct, a sub-panel established from a double-pole 80 amp breaker via 2 #4's from the "parent" panel would not be enough for the two double-pole 60 amp breakers that the manufacturer calls for. am i correct in my analysis?
 
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Old 11-24-12, 09:32 PM
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The best way to supply it might be to mount a 150 amp subpanel at the water heater supplied by a 125 amp breaker in the main panel. You could then install two 60 amp breakers in the subpanel for the water heater. That would need at least #1 copper or 2/0 aluminum.
 
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Old 11-24-12, 09:49 PM
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I agree with Ray. Without seeing the drop from the street or your incoming service I'll bet you'll find a 200 amp drop coming in. A 400 amp service drop would require like 500mcm wiring. That is like 1-1/4" in diameter.

You can fit four # 6 thhn wires in 3/4" EMT (pipe) and that would give you approx 60 amps per leg. For a total of roughly 120 amps.

You can fit two # 4 thhn wires in a 3/4" EMT (pipe) BUT that would only give you 85 total amps.

So the two # 4 idea is out. Technically you should also be running a ground wire in that EMT.
 
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Old 11-25-12, 08:04 AM
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the condos are individually metered...6 meters in the meter bank, fed by 3 large wires directly off the pole/transformer
6 gang meter sockets are pretty popular for apartments and condos and they are readily available is 100 amps and 200 amps per position; yours are most likely 100 amps per position since the units are now fed with only 80 amps. Increasing load by approximately 22 KW per unit X 6 units is a total 132 KW increase. I can assure you the power company will have work to do in this major electrical upgrade. Whether the service lateral is owned and maintained by the power company or the owner is up to your power company's rules. Even dividing the cost by the number of units (6), this is an expensive upgrade no matter how you divide the cost.

Is this overhead or underground service.

Without seeing the drop from the street or your incoming service I'll bet you'll find a 200 amp drop coming in.
Of course we cannot tell without seeing the actual overhead service drop (or underground service lateral), but I'd lean more toward a 300 amp feed from the power company if the lateral was power company installed. Assuming it's an underground feed, if the power companies rules required the owner to furnish and install the underground, the feeder sizing would be determined by the NEC and probably would be larger based upon the size of the 6 services. If it is a power company furnished and installed lateral, it would be sized upon load calculations per the NESC and would likely be smaller.
 
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Old 11-25-12, 10:08 AM
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Code requires heating loads to be broken down to 60 amps max. You must run 2 60 amp circuits and 4 - #6 copper wires.

Are doing a tankless per each unit?
 
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Old 11-25-12, 10:59 AM
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We have not yet discussed the fact the tenants may not be happy with reduced flow rates to get water that is hot enough or that too many faucets open at the same time may overwhelm the heaters ability to adequately heat the water.
 
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Old 11-26-12, 11:49 AM
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Thank you all...you've given me lots of great information to digest. I was able to talk to an eletrician today in a much more educated fashion, than i would have had i not had your input.

He told me to look at the transformer feeding the meter bank, and there would be a large number sticker on it with some multiple of 25. the sticker was 50 itself. he said that this is a 50 kW service, and therefore approximately 200 amp service.

Each of the tankless water heaters could potentially draw 112 amps, so
needless say to my "seemingly brilliant" tankless water heater was not so brilliant when met with reality.

but it does leave me with a question...each of the meters has an double pole 80 amp breaker, and therefore a max draw of approximately 20kW per unit. so why would there only be a 50 kW service feeding this meter bank? is it just assumed that not every unit will be running at full tilt? and what happens when they are? does everyone just get less power until folks start turning items off?

thank you all once again.
 
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Old 11-26-12, 12:57 PM
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is it just assumed that not every unit will be running at full tilt
Yes. The kearney fuses on the primary might blow if the load was sustained long enough but it just isn't likely to happen because there probably just isn't that much load to be on.
 
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Old 11-26-12, 01:18 PM
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and correct me if i'm wrong, but that is the problem with the tankless in this case...since it is a single piece of equipment with such a heavy draw, that if any single unit were using both showers and maybe the kitchen sink as well that the tankless by itself would account for the entire meter bank's service amp capacity.
 
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Old 11-26-12, 02:28 PM
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With multiple tankless heaters yes. Oh and if you did blow the transformer without getting permission for the unexpected load guess who pays parts and labor for replacement of the transformer?
 
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Old 11-26-12, 08:00 PM
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He told me to look at the transformer feeding the meter bank, and there would be a large number sticker on it with some multiple of 25. the sticker was 50 itself. he said that this is a 50 kW service, and therefore approximately 200 amp service.
That would be a 50 KVA transformer and sized by the utility according to the calculated load submitted to the utility by the contractor/engineer when the building was built. My opinion is that most utility companies undersize transformers, feeders and other critical equipment, but I rarely see them changing and upsizing their equipment till a major upgrade occurs at the facility. Utility companies don't follow the National Electric Code, they follow the National Electric Safety Code.
 
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Old 11-26-12, 09:28 PM
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smart rookie..... I haven't looked into those electric tankless heaters but is there a smaller one you could use......less power consumption ? Did you just pick one to use or was it determined by some kind of useage formula ?



Utility companies don't follow the National Electric Code, they follow the National Electric Safety Code.
........and their own code too.......sometimes like nothing we've seen before.
 
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Old 11-27-12, 01:43 PM
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he did say KVA, but figured that KVA stood kilo-volt-amp, which i figured mathematically equivalent to a kilowatt? but you now have me curious...anyone care to enlighten me?

each unit has 2 full baths and a a kitchen sink...bathrooms aren't large, so the hot water being used in a vanity at the same time you have a shower going isn't terribly likely, so i just size it based on 2 showers and a sink in simulteneous usage. i feel like i could certainly get away with an ~18kW unit given the condos are mainly occupied in the warmer months and therefore the cold water coming in isn't all that cold, but even then most in that power range have a 75 amp max draw, which still seems like it is more than i should have running from a panel fed from by a double pole 80 amp breaker via #4 wires...not to mention that if 2 of the possible 6 tankless heaters were running at full draw, it wouldn't take much more to overwhelm the 50 KVA transformer feeding the meter bank...and there are ac's, stoves, microwaves, and fridges to consider.
 
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Old 11-27-12, 01:58 PM
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While both the volt-ampere (abbreviated VA) and the watt have the dimension of power (time rate of energy), they do not have the same meaning. Some devices, including Uninterruptible Power Supplies (UPSs), have ratings both for maximum volt-amperes and maximum watts.

The VA rating is limited by the maximum permissible current, and the watt rating by the power-handling capacity of the device. When a UPS powers equipment which presents a reactive load with a low power factor, neither limit may safely be exceeded.[3]Continued at: Volt-ampere - Wikipedia, the free encyclopedia
Adding 25 extra characters so it will post.
 
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Old 11-28-12, 12:28 PM
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a little more educated than before...
 
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