Wiring for tankless water heater

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Old 05-28-10, 10:46 AM
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Wiring for tankless water heater

I have a tankless water heater that I need to wire up. It requires two 240V 50A circuits. I'm in industrial electrical engineering & design, but not residential. So the foundation is there, but some of the specifics are a little fuzzy.

My distribution panel is mounted on an outside brick wall and has a 200A main. I don't know what the panel itself is rated for, but I assume it's 200A.

My non-lighting/receptacle loads are HVAC, cooktop, oven, microwave, and washer/dryer. The tank-style water heater will come out when I put the new one in. I worked through NEC 220 and from what I can tell my total load with the new water heater is 160A, but I'll verify some loads & post back..

I plan to route the cable for the new circuits in 2" NM conduit up through my soffet (about 12"-16"), through my attic (no conduit) and down the wall in my garage (back in conduit) to the local disconnects for the water heater.

My main questions right now..

1) The install manual for the water heater suggests #8 cable, but #8 Romex is only good for 40A as it must be sized for 60C. An electrician friend told me that #8 SER Copper cable is rated for 50A, but the ampacity charts I've found say that this is also only true at 75C & 90C. At 60C, it's only rated for 45A and from what I've read that's what I have to use since the cable will be routed in my attic... So.. #8 SE? #6 Romex? Or could I run 4 #8 THHN + a ground in conduit all the way?

2) Is there anything in particular that I must do where I penetrate the soffit? Obviously, I can just hole-saw at the OD of my conduit & silicon or putty around it, but I'm not sure this is the best thing to do. I saw a "Hub Meter" fitting that looked like it might be helpful. Is there some kind of penetration fitting that I can or should use? Also, do I need any sort bushing or fitting where the cable exits the conduit?
 
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Old 05-28-10, 11:06 AM
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You said your new tankless calls for 2 50 amp circuits. I am concerned if your service will support the increased demand.

Under the 08 NEC SER ampacities are reduced to the 60 degree column.

You will probably have to derate your cable even further due to the ambient temp in the attic.
 
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Old 05-28-10, 11:08 AM
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Originally Posted by Spaceball1 View Post
1) The install manual for the water heater suggests #8 cable, but #8 Romex is only good for 40A as it must be sized for 60C. An electrician friend told me that #8 SER Copper cable is rated for 50A, but the ampacity charts I've found say that this is also only true at 75C & 90C. At 60C, it's only rated for 45A and from what I've read that's what I have to use since the cable will be routed in my attic... So.. #8 SE? #6 Romex? Or could I run 4 #8 THHN + a ground in conduit all the way?
IF it was me I would run conduit the entire way and pull in THHN wires. #8 THHN is fine for 50 amps. If this is not possible then you are correct that NM-b would need to be #6. As for the SER I belive you are reading the 310.16 chart wrong. I see #8 aluminum is rated at 45 amp on the 90 degree. I would stay with copper if you could.

Originally Posted by Spaceball1 View Post
2) Is there anything in particular that I must do where I penetrate the soffit? Obviously, I can just hole-saw at the OD of my conduit & silicon or putty around it, but I'm not sure this is the best thing to do. I saw a "Hub Meter" fitting that looked like it might be helpful. Is there some kind of penetration fitting that I can or should use? Also, do I need any sort bushing or fitting where the cable exits the conduit?
Where the cables come out of the conduit you will need a connector (male adapter) to bush the end. IF you wish you could also add a bushing.

You will need two disconnects at the water heater.
 
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Old 05-28-10, 11:22 AM
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Originally Posted by Spaceball1 View Post
my total load with the new water heater is 160A, but I'll verify some loads & post back..
That sounds okay for a residential 200A service. Residential services are actually designed for 80% of the named amperage, so you're just at the limit. If you ever get a spa or other big load you would need to upgrade.

With the tankless you'll also want to evaluate the service drop from the power company or notify them so their engineering staff can evaluate it. They size wires and transformers to actual connected load, not nameplate service size so you might need a drop upgrade to handle the added 70A.

At 60C, it's only rated for 45A and from what I've read that's what I have to use since the cable will be routed in my attic...
The 2008 code now requires that SE always be used at 60 so both SE and Romex #8 are limited to 40A. You would have to increase to #6 if you wanted to use a cable.

Or could I run 4 #8 THHN + a ground in conduit all the way?
Yes this is what I would do. THHN in conduit is allowed up to the 75 column, so you would be good to go with this method. This allows you to go with the smaller conductors. You only need 3/4" conduit for (4) #8 + (1) #10 THHN.

If the need for flexibility is a concern in the attic, you could use flexible pvc conduit (ENT, smurf tube). You can glue ENT directly into rigid PVC fittings and boxes. It would also be acceptable to use 3/4" EMT pipe for the rigid sections and flexible metal conduit for the flexible sections.

I can just hole-saw at the OD of my conduit & silicon or putty around it
That's all you need to do.
 
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Old 05-28-10, 04:13 PM
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Originally Posted by pcboss View Post
You said your new tankless calls for 2 50 amp circuits. I am concerned if your service will support the increased demand.
Me too. I have a load demand calc started, just need to backcheck my nameplates & such. I'll post here when that's done..

As I was driving to Lowe's to price stuff and see what options might be there, I started to think that THHN in conduit might be the best way to go no matter what. Yeah, it's a little more work, but the extra work is relatively easy. Plus, it seems more solid code-wise and more in line with my work experience (we design most of our stuff as THHN in RGS).

These comments...

Originally Posted by ibpooks View Post
THHN in conduit is allowed up to the 75 column, so you would be good to go with this method. This allows you to go with the smaller conductors. You only need 3/4" conduit for (4) #8 + (1) #10 THHN.
Originally Posted by Tolyn Ironhand View Post
IF it was me I would run conduit the entire way and pull in THHN wires. #8 THHN is fine for 50 amps.
...just clench it.

In fact, I'll probably use 1-1/2" conduit instead of 3/4" just to make the pull that much easier. It's only like another $10..

Originally Posted by ibpooks View Post
If the need for flexibility is a concern in the attic, you could use flexible pvc conduit (ENT, smurf tube). You can glue ENT directly into rigid PVC fittings and boxes. It would also be acceptable to use 3/4" EMT pipe for the rigid sections and flexible metal conduit for the flexible sections.
Nah, conduit routing isn't a problem except for right above my soffit penetration. My attic is roomy, but as with all attics, it's tight over there by the walls.

Dumb question here.. Do I have to glue the conduit together?
 
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Old 05-28-10, 04:24 PM
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Also, do I need any sort bushing or fitting where the cable exits the conduit?
Assuming it's PVC conduit, use a male threaded adapter (box connector) with a locknut to terminate the conduit in a knockout. No need for special bushings, just the adapter.
 
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Old 05-28-10, 04:28 PM
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Do I have to glue the conduit together?
Yes, use standard PVC cement.
 
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