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Old 03-18-2008, 07:02 AM   #1
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Tankless Water Heater / Rewiring


I just read someone else's thread on tankless water heaters.
Electricity for water heater

I've used tankless in Japan and Europe and love it. There is some argument whether it raises your elec bill or lowers it. I've committed to it, so that's water under the bridge.

My 2nd floor bedroom and bath had a dedicated small tank type heater that leaked and I got rid of it. It was too heavy on the beams and 1st floor heater is too far with no runs near. So I bought a Bosch (max 17.25KW) with two 240v circuits. Installation called for two runs of 8/3 copper wire to subpanel to two 40a dble pole breakers. The 2nd fl has its own subpanel which also has 3-15a breakers for lights/outlets/outdoor lights. It HAD a 240v double pole 40a breaker for the old water heater + 240v double pole 40a breaker for a broken high BTU A/C. I thought, too simplistically that I could just detach the A/C and old water heater and use the same two 40a dble pole breakers and the load would be the same. But part of me said "hey, this tankless draws 80a and the house service is 100a so how can that be?" I have read more and understand more since then. At the time, I also didn't look at the Watts on the old A/C and H20 heater. Anyway, the 2nd Flr panel is fed from another subpanel on 1st flr outside right next to the main panel. This 1st flr subpanel has 3 slots, 1 empty and 2 hold a dble pole 60a breaker. I keep tripping this. I bet they tripped it before the A/C unit quit too but my tankless is more of a hog even though they were using the same amp breakers. Anyway, the 1st flr subbox is rated for only 70a. I guess that I need a dble pole 100a here. (I want to have capacity for a 2nd flr split air too. Seems they draw 20a). (Q1)Is 100a for the 1st flr subpanel enough?
I will have to replace the subbox for a 100a one and also buy a 100a dble pole breaker. Yes, I am getting a 200a service installed too, but want to do all the work past the meter / main breaker myself.
Next, I checked on the wiring between the 1st & 2nd flr sub panels. Turns out to be aluminum (1980's 2nd flr addition) with no markings. I used calipers and measure the dia and checked a online table for alum which says it is 14 awg. Shock. Even before the tankless this would have been dangerous. So I need to replace this cable. Its about 80 ft and runs thru a tall crawl space where I put the tankless in so I'm comfortable working there. It exits to outside at 2nd flr and runs down thru 1" or so metal conduit which then runs underground for about 20' and then back up to the subbox. I can replace this with plastic conduit. (Q2) Should I use 3 awg copper? (Q3) Should I use wire with "W" in it for wet, inside the conduit (what dia)? (Q4) Should I use cable in plastic sheath without conduit underground? (easy to dig, just behind a bush with earth there.) I mainly want confirmation on all I have said, and would love many details on what wire/cable/conduit to use besides just gauge because the THHW, THHN or USE etc. codes are greek to me. BTW, this is Hawaii so metal rusts, plastic is great. I love learning this stuff. You guys are great.


Last edited by kailuakeith; 03-18-2008 at 07:13 AM.
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Old 03-18-2008, 09:39 PM   #2
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Tankless Water Heater / Rewiring


Quote:
the 2nd Flr panel is fed from another subpanel on 1st flr outside right next to the main panel. Anyway, the 1st flr subbox is rated for only 70a. I guess that I need a dble pole 100a here. (I want to have capacity for a 2nd flr split air too. Seems they draw 20a). (Q1)Is 100a for the 1st flr subpanel enough?
You said that the sub is right next to the main. Use the sub exclusively for the tankless and route the additional circuits to the main. Use a junction box or gutter if you have to.

Quote:
So I need to replace this cable. Its about 80 ft and runs thru a tall crawl space where I put the tankless in so I'm comfortable working there. It exits to outside at 2nd flr and runs down thru 1" or so metal conduit which then runs underground for about 20' and then back up to the subbox. I can replace this with plastic conduit. (Q2) Should I use 3 awg copper? (Q3) Should I use wire with "W" in it for wet, inside the conduit (what dia)?
Yes you can use #3 wire with a #8 ground. MOST but not all THHN is also THWN rated. If it were me, I'd put in 1 1/4" but I believe NEC allows 1".

Quote:
(Q4) Should I use cable in plastic sheath without conduit underground? (easy to dig, just behind a bush with earth there.)
Anytime I have to fix a problem in a detached building it is ALWAYS wiring without conduit. Put it in pipe.

I hope this is informative. I'm still not 100% sure I understand the setup fully, but I think I got my head around it enough to give an educated answer.

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Old 03-19-2008, 08:25 AM   #3
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Tankless Water Heater / Rewiring


Thanks for helping. You said to route the additional circuits to the main. I wasn't planning on working on the main, how safe working on it if I only switch off the main? There is still danger of accidentally completing a circuit even though the main breaker is off, right? Is there some other safety tip when doing this because at some point I may need to upgrade the wiring from the sub directly next to the main anyway. I guess you just stay away from the hot leads by a mile.

I checked out a elec supply house and Home Depot today. On island no one carries No #3 or other odd numbers, just even. So I would have to get THWN#2. Can I confirm with you? It sounds like I would run two THWN #2's for hot feeds and a single THWN#2 for neutral and then a single THWN (not bare)#8 for ground all in a 1 1/4" conduit (plastic rules here).


It's about $1.70/ft here for THWN #2 so the whole thing will be over $600 just for wiring before conduit, breaker, new subpanel (old one is rusted out anyway). I'll try to supply a picture next time. At least it will be done right.
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Old 03-19-2008, 05:05 PM   #4
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Tankless Water Heater / Rewiring


When I said to route the circuits to the main, I was referring to the main panel. The new 200 A panel you had mentioned was going in. You should be warned that swapping this out could be dangerous. The wires that come from the meter to the main breaker are always hot. Even if you kill the main breaker in the panel, they will still be hot. I would definitely recommend getting a licensed electrician to upgrade your service.

As for the wire, sorry but you'll have to step up to #2. Funny, but in my area #4 is slightly more expensive than #3 (demand I guess). You've got the rest right, two hots, one neutral (all#2) and a #8 ground.
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Old 03-19-2008, 06:04 PM   #5
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Tankless Water Heater / Rewiring


Thanks. I am definitely getting a pro for the meter/main box, breaker and service upgrade.

The main panel is the longest possible run from 2nd floor. 1st flr is "H" shaped. That must be why they put the smallest possible sub-panel near the meter/main instead. Still, there might be a way.

Thanks for the confirmation on the wires.
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Old 03-19-2008, 07:40 PM   #6
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Tankless Water Heater / Rewiring


Keith, post some pics of the unit when you get it installed. It sounds like a fairly large one. It's a little hard to judge since most all heating appliances in my area are gas. Take care.
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Old 03-21-2008, 09:33 PM   #7
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Tankless Water Heater / Rewiring


Hi Goose,
until I get my camera out here is a link to the model.
http://www.boschhotwater.com/StartPa...5/Default.aspx

It is less than 3' tall x same wide x 4 inches deep. Its small. I have the AE 115 installed. The AE125 wasn't sold around here but the box is the same size. I have had it open. There is just another circuit in the box. Sorry the pic in the link is not too good. Its light too. Small and light is perfect for the crawl space I have. There used to be a 30 gal tank there but the beam under it was groaning and then it sprung a leak. So I'll be happy when the 200a service is upgraded so this works like it should.

Can I ask another question?
When I first examined the existing aluminum cable to the 2nd flr subpanel, I didn't know how to identify it. There were no markings. So I got my calipers out and measured the diameter of the alum wire on the end where the insulation has been stripped (about 5 times to be sure). It was 6 cm = 60 millimeters or about 1/4 inch. This chart said that was 14 awg
http://www.interfacebus.com/Aluminum_Wire_AWG_Size.html
Which made me think this was way to substandard for the load on it before or after the tankless heater. But now I am thinking this process to identify it was flawed somehow (I'd love to know why).
The 2nd floor panel is fed by a plastic sheathed cable with 2 black hots aluminum 6 strand (1/4" inside diameter, 5/16" outside diameter) a white neutral of same size plus a fat bare ground which size I didn't measure. The whole cable with sheath has an outside diameter of 3/4". I don't know the gauge and load capacity of this cable. Do you know?

I read some chatroom talk about URD and SE (service entrance) cables. I am guessing SE cable always has 4 wires.
I found this link for aluminum SE cable with 4 wires. This could be my cable based on description.
http://customcable.thomasnet.com/vie...dry?&forward=1
Since mine measures 3/4" it seems to be between 4awg and 6awg. Why is this result so different from the other way of measuring it?

Next I found this site:
http://www.cerrowire.com/default.aspx?id=46
This chart is useful. But I am not sure which of the two columns for aluminum I should use. I am not sure what to make of the temp differences, and I don't know which of the kind of wire (THWN, SE, XHHW-2, etc...) is mine. Could you explain any of this? Its greek to me.

Any help on this identification would be great. Thank you

Last edited by kailuakeith; 03-22-2008 at 12:06 AM.
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Old 03-22-2008, 05:51 AM   #8
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Tankless Water Heater / Rewiring


When you used your calipers to measure, were you measuring the entire conductor, or one strands in the bundle? I'm not sure that calipers would be the easiest way to tackle this. You'd need a wire gauge. It sounds like you have SE cable but you'd have to check on the jacket of the cable for its marking. Manufacturers are required to print type of insulation, voltage rating, size of wire, and type of material.

If you can find the writing on the jacket ANYWHERE in the run, you should be able to tell all with a glance.

The table you refer to is also in the code book om section 310. They come in various forms for conductors in air, triplexed conductors, in earth, in raceway. The purpose of these tables is to give a reference point for ampacities. There are certain insulations that are rated at 75deg C. This is the table you come to find out how many amps it can hold. Now the 90deg col. is only used for derating purposes. These are the factors one uses to correct for ambient temperature. Anyway, that is what the table is all about. Aluminum wire will always need to be of a larger size than copper to carry equivalent current. Copper is a better conductor.

Don't feel bad about asking questions. I gotta go to work. We're tearing out 3000 A service and firing up the 600A temp. Have a good weekend
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Old 03-23-2008, 02:48 PM   #9
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Tankless Water Heater / Rewiring


I used the calipers to measure the dia of the twisted strand by trying it at several points around the perimeter. This wire was installed with the 2nd flr addition about 1985. I've looked on the alum wire, on the sheath in the crawlspace, several times. I know it should be marked somewhere, but can't find it. I gotta invest in a wire gauge.

Still, its hard to imagine my measuring so much less then the actual dia. to arrive at 14awg instead of 6awg.

Where can I read to understand the meaning of abbreviations like THHN or THWN, XHHW-2, etc., etc.?

The subpanel on the 1st floor which feeds the 2nd flr has been tripping lately under normal usage (lights, etc.). The SE alum wire that I have been planning to replace, runs between these two panels. So, I turned off the meter-main panel, opened up the 1st floor subpanel to take a peek. I found the existing breaker is fried and part of the insulation on the wire was melted on the wire coming from the main which is copper and clearly marker THWN 4 awg. Besides being melted for a couple inches near the breaker, it was cracked in several spots. I think I dodged a bullet (fire).
No telling if the alum SE cable that runs to the 2nd flr is fried too. It doesn't look it, but its covered by sheath. Since I have to replace it anyway when I upgrade to 200a, I just won't use that circuit until its been rewired.
Since I can't rewire from the meter to the adjacent 1st flr subpanel, I will hire a pro to do it. Then, I can get rid of this subpanel and just replace the service meter box with one that has a couple of slots to accomodate the 2nd flr circuit. If I hire a pro for the meter box and service upgrade, do you think I can find a pro who will cooperate and let me install the 1st flr to 2nd flr SE cable myself but just let the electrician hook it up? I am not sure how many pros would reject this.

Hope your 3000A service changeover went smoothly.

Last edited by kailuakeith; 03-24-2008 at 04:54 AM.
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Old 03-24-2008, 05:52 AM   #10
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Quote:
Where can I read to understand the meaning of abbreviations like THHN or THWN, XHHW-2, etc., etc.?
If you'd like a general description of the uses of the various types of insulation, there is a table in the code book that gives general type applications for insulation. Sounds like you got lucky with that sub panel. It could be that you have #4 copper feeding smaller aluminum. Definitely replace the whole run.

Quote:
If I hire a pro for the meter box and service upgrade, do you think I can find a pro who will cooperate and let me install the 1st flr to 2nd flr SE cable myself but just let the electrician hook it up? I am not sure how many pros would reject this.
Part of the problem with this plan is that a licensed contractor is going to be liable for everything he hooks up. While I'm quite sure that you have every intention of getting it right, whoever shows up may not share my enthusiasm. The exposure to liability is too great. Imagine if you were the pro that had to hook up the current system you have that was run by the homeowner.

That said, all you can do is ask the contractor involved. Ultimately, it is their decision.

Service switch went well. Now it's time to harvest the copper.

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