sub-panel for water heater

[RustyShackleford]

I installed a new water heater awhile back, and thought I might also someday want another for a distant point-of-use. So I installed a sub-panel, protected with a 50-amp double-pole breaker in the main panel (and wired with 6 gauge Romex). I wired the water heater, spec'd with non-simultaneous 4500 watt elements, with 10 gauge and protected by a 30 amp breaker in the sub-panel (as per the water heater instructions).

The thought was that I might someday add a small 20 amp storage water heater for the distant sink. However, I now want to put a tankless electric water heater at that use point. To get the capacity I want, I need one that is spec'd for a 30 amp breaker (the specific one uses 5700 watts). I'm wondering if it's ok to drive that one off the same sub-panel (again with a 30 amp breaker and 10 gauge wire).

Each section of wiring is of sufficient gauge for the breaker protecting it (50amp/6ga for the feed from the main panel to the sub-panel, and 30amp/10ga for the feed from the sub-panel to each water heater). So there should be no safety issue. But 2 * 30amps is obviously more than 50 amps; seems like that's ok too, because it's normal for all the breakers in a given panel to exceed the ampacity of the breaker protecting that panel - IOW, if I add up all the breakers in my main panel, it's well over 500 amps, yet the main breakers (for the feed from the meter) are something like 150 amps.

So I think the main issue is whether or not the 50 amp breaker will trip, if I engage the tankless unit while the main storage water heater is also active. The total wattage would be 5700+4500 = 10.2kw, or 42.5 amps (at 240v). That's cutting it a bit close on a 50 amp breaker. There's also code of course (which I want to obey even though this won't be inspected); I'm probably good on that, since each wire gauge is sufficient for the corresponding breaker, but maybe not.

Appreciating any helpful thoughts (and yes, I know electricity is dangerous ).

 

[begreen]

Is the 6 ga wire copper and with high temp insulation (THW/THWN)? If so it can handle more amperage. Check it's insulation rating and the length of the run. If all is ok, switch the breaker out to 60amps. If the 6 ga wire is NM insulated then maybe use a small 4 gal, point of use, hw heater at the remote location. It would draw less amperage and plug into a 120v outlet.
http://www.homedepot.com/p/Bosch-4-Gal-Electric-Point-of-Use-Mini-Tank-Water-Heater-ES-4/206393135

 

[RustyShackleford]

Is the 6 ga wire copper and with high temp insulation (THW/THWN)? If so it can handle more amperage. Check it's insulation rating and the length of the run. If all is ok, switch the breaker out to 60amps. If the 6 ga wire is NM insulated then maybe use a small 4 gal, point of use, hw heater at the remote location. That would draw less amperage. These plug into a 120v outlet.
http://www.homedepot.com/p/Bosch-4-Gal-Electric-Point-of-Use-Mini-Tank-Water-Heater-ES-4/206393135

No, the 6ga from the main to the sub-panel is NM ("Romex"); so can't increase the breaker size. Nice thought though. I could retrofit, but I've never worked with THW; I guess you have to enclose the separate conductors in some kind of conduit ? I guess I could hope for the best (since I'm covered on safety, I believe), and if the 50amp trips, retrofit THW and a 60amp breaker in the main panel.

The small storage water heater is a good idea. In fact, that's what I had in mind when I sized the sub-panel (so maybe a 10gal unit with a 3000 watt element, protected by a 20amp breaker in the sub-panel). But I hate to add more storage losses (with another conventional unit), though the modern electric tank heaters are extremely efficient - over 90%, and IIRC that includes losses in piping (for the hot water left in the pipes after faucet turned off) for some specified regime of hot-water draws.

 

[RustyShackleford]

I am told elsewhere that I should add 125% of the continuous load and 100% of the non-continuous load to get the total rating needed for the breaker feeding the sub-panel (with the existing storage water heater and new tankless heater being considered continuous and non-continuous, respectively). So I could use a tannkless unit of 6000 watts or more and be ok with the 50amp feed to the sub-panel (6000watts + 4500watts*125%, divided by 240v).

 

[Highbeam]

I am told elsewhere that I should add 125% of the continuous load and 100% of the non-continuous load to get the total rating needed for the breaker feeding the sub-panel (with the existing storage water heater and new tankless heater being considered continuous and non-continuous, respectively). So I could use a tannkless unit of 6000 watts or more and be ok with the 50amp feed to the sub-panel (6000watts + 4500watts*125%, divided by 240v).

Okay now you're getting somewhere. Both loads are what I would consider continuous so the 125% applies to the sum. They're both hardwired, resistive, and known.

Tankless of any sort is only a good idea if you REALLY need the space or REALLY need an endless supply of heated water. Storage heaters are much wiser most of the time. I have one of the small, square, 5 gallon, 120volt, water heaters under my shop sink on a 20 amp circuit with a switch so I turn it on only when I plan to use hot water at that sink. It then takes about 30 minutes to finish fully heating that water. I have plenty of hot water to wash a full grown black pug dog in the sink plus clean up.

You can use a 60 amp breaker on 6 gauge NM. Yes, the chart says 55 but you are allowed to round up to the next common breaker size. I have two 60 amp circuits in my panel that are wired with 6 gauge NM and it was inspected.

http://www.cerrowire.com/ampacity-charts

 

[moey]

Are the panels close to each other? If they are move two circuits from the main panel to the sub panel preferably non kitchen or bathroom ones. Think living room lights. Then put the new water heater in the main panel.

Your 50amp breaker will start to trip at about 40 amps of continuous use. So you can not add it to that panel without doing something.

 

[RustyShackleford]

Are the panels close to each other? If they are move two circuits from the main panel to the sub panel preferably non kitchen or bathroom ones. Think living room lights. Then put the new water heater in the main panel.

Oh, if it came to that, I'd probably just wire both water heaters to the main panel, making room with a space-saver breaker (two 30amp double pole breakers in one dual-size unit). The sub-panel was probably a dumb idea, but I thought I was running out of space in the main.

Your 50amp breaker will start to trip at about 40 amps of continuous use. So you can not add it to that panel without doing something.

I'm confused by @Highbeam and you both saying I have to use 125% of both water heaters. I've read elsewhere that a load is not considered "continuous" (and thus the 125% factor) unless it could be on for 3 hours constantly. The tankless is only likely to be on for a few minutes at a time, and even the storage one is unlikely to be on for 3 hours straight (if you do the math, my 50gal one can fully heat a full tank of cold water in less than 2 hours).

Tankless of any sort is only a good idea if you REALLY need the space or REALLY need an endless supply of heated water. Storage heaters are much wiser most of the time. I have one of the small, square, 5 gallon, 120volt, water heaters under my shop sink ...

I hear you. I actually resisted all the advice to put a tankless in for the main water heater, when I replaced it a few years back. The efficiency gain is minimal, since modern electric tank heaters are so well insulated (R16), and by far most of the loss is in the piping, especially the hot water stranded in the pipes when you turn off the faucet. I just thought a little tankless made more sense for this remote location (my kitchen is kinda far from the main water heater and bathrooms, and it's been a nag for years to wait a couple minutes for hot water in the kitchen sink).

If I did go with a small tank like you suggest, I'd probably try to find a 240v one anyhow, so I can connect it to the sub-panel (no neutral in it).

You can use a 60 amp breaker on 6 gauge NM. Yes, the chart says 55 but you are allowed to round up to the next common breaker size.

REALLY ? Do you have a link for that ?

 

[Highbeam]

REALLY ? Do you have a link for that ?

NEC. I thought everyone knew you could round up.

 

[RustyShackleford]

NEC. I thought everyone knew you could round up.

Slapping forehead - yes, it's article 240.4(B).

I still wonder why you say 125% rule should be applied to the tankless; am I wrong about the 3-hour thing for "continuous" ? Oh well, I guess I can just replace the breaker IF the 50amp one starts tripping.

I just realized a hidden agenda on the tankless is it's 60 feet from the crawlspace access door, and the tankless is way smaller and lighter (and cheaper). But I think you're right, I should consider maybe a 10gal storage unit. Even with a 30amp tankless unit (if I change breaker feeding sub-panel to 60 amps) I'd be struggling to have enough hot flow if the dishwasher is filling when somebody is using the sink, or if main sink and prep sink are in use simultaneously. (Apparently dishwashers want to be connected to hot water, even though they have internal heaters than increase temperature way above what any sane person would set their hot water heater to).

 

[Highbeam]

I still wonder why you say 125% rule should be applied to the tankless; am I wrong about the 3-hour thing for "continuous" ?

I'm not an electrician. I've done a good bit of electrical work though. Seems to me, common sense, logic, etc. that a tankless is every bit as continuous as a tank heater. I've never heard the 3 hour thing and really that makes no sense because you can choose to operate either appliance for 5 minutes or for 5 years continuously.

125% rule applies to all circuits except welders and motors as far as I know.

 

[RustyShackleford]

I'm not an electrician. I've done a good bit of electrical work though. Seems to me, common sense, logic, etc. that a tankless is every bit as continuous as a tank heater. I've never heard the 3 hour thing and really that makes no sense because you can choose to operate either appliance for 5 minutes or for 5 years continuously.

125% rule applies to all circuits except welders and motors as far as I know.

Tormenting myself with the NEC, yes, storage-type water heaters are definitely "continuous" per 422.13.
Article 100 states that continuous means the load is "expected" to go for 3 hours straight, not "can".

Mike Holt's forum seems to think tankless is NOT continuous (since not "expected" to be run 3 hours).

 

[Buzz Saw]

Have you considered a small circulation pump to constantly pump hot water to you sink, then loop back to the hot water heater? I would be instant heat.

I woyld guess if you insulated the lines fairly well standby loss would be minimal/comparable to other losses.

Goof or bad idea?

Sent from my SM-G900V using Tapatalk

 

[RustyShackleford]

Have you considered a small circulation pump to constantly pump hot water to you sink, then loop back to the hot water heater? I would be instant heat.

I woyld guess if you insulated the lines fairly well standby loss would be minimal/comparable to other losses.

Yeah, I've thought of it. If I've ruled out the tankless type (which I think I have), you're essentially trading the losses through the jacket of the 2nd (storage-type) water heater against the losses through the piping in the continuous circulation loop (and the energy consumption of the pump). Both are small enough that I think the added complication of the pump is not warranted.

Just looking at a 6gal Bradford-White unit with height of 16"and diameter of 14", the surface area of the heater is about 7 sq-ft (if I calculated correctly). So with R8 insulation and a 60 degree-F diff'al between stored hot water and sealed crawlspace ambient, that's about 53 btu/hr or 16 watts. Not much. I think 30ft of 1/2" diameter pipe comes out to about 4 sq-ft; plus the pump. (I'd love if it someone wanted to check my calculations ...)