Wire gage needed for 68.5 amp 240vac?

[Bill C]

Hello,



Can someone please tell me what gage wire is required to service a 68.5 amp
240vac tankless water heater? The manufacturer recommends an 80 amp
breaker. I'd love to replace my now leaking water heater with a tankless!



I'd need to service it out of a 100 amp subpanel that has three * 3/4 "
conduits running to about the same area of the house 40' away for a water
heater, range and clothes drier. It looks like there are two 8 or 10 gage
hot wires through each conduit now. I haven't investigated where the
neutral is coming to them. I don't know if it's feasible to pull anything
through them now. This is a 30 year old house, and there are probably 3-4
90% bends in the conduits. (Two conduits are steel, one is PVC.)



Any recommendations?

Thank you

Bill

 

[William P. N. Smith]

I'd love to replace my now leaking water heater with a tankless!

I suspect you're going to be really unhappy with tankless electric,
unless you've really done your homework. How about tankless propane
or natural gas?

 

[danny burstein]

I suspect you're going to be really unhappy with tankless electric,
unless you've really done your homework. How about tankless propane
or natural gas?

68.5 amps and 240 vac = 16.4 kw. Call it 15 kw for an envelope back

That would give you 50,000 or so BTUs/hr, 850 btus/minute.

So... with water weighing (nominal) 8 pounds/gallon, that'll
let you pull 1 gallon/minute with a 100F rise. Or, in a more
realistic scenario looking for a 40F rise, that's a bit
over two gallons/minute.

 

[Bill C]

Thanks Danny & William.

Propane here in the Florida Keys is $2.88/gallon. One dealer came out,
looked at our setup and said, "save yourself the trouble and go to Home
Depot and grab and electric tank heater."

After reviewing the wiring from the subpanel, I've given up on the idea of
tankless too. I don't know whether I'm better off buying a $250 60ef
electric and attempting a solar feed to in from the roof (cement slab) or
spending a lot for a very high efficiency. Another research project...

Thanks again.

Bill

 

[danny burstein]

Thanks Danny & William.

Propane here in the Florida Keys is $2.88/gallon. One dealer came out,
looked at our setup and said, "save yourself the trouble and go to Home
Depot and grab and electric tank heater."

I don't have your original post on the server here. What's the final use
for the water? If it's a wash-up sink, then using a small ( two or five
gallon ) undersink electric heater could be a good choice.

 

[John P. Bengi]

That depends on the temperature of the insulation, what the conductor is
enclosed in, how many conductors and the material in the conductor.

You need to size wire for your overcurrent protection and not the current
draw.

 

[SQLit]

Hello,



Can someone please tell me what gage wire is required to service a 68.5 amp
240vac tankless water heater? The manufacturer recommends an 80 amp
breaker. I'd love to replace my now leaking water heater with a tankless!



I'd need to service it out of a 100 amp subpanel that has three * 3/4 "
conduits running to about the same area of the house 40' away for a water
heater, range and clothes drier. It looks like there are two 8 or 10 gage
hot wires through each conduit now. I haven't investigated where the
neutral is coming to them. I don't know if it's feasible to pull anything
through them now. This is a 30 year old house, and there are probably 3-4
90% bends in the conduits. (Two conduits are steel, one is PVC.)



Any recommendations?

Thank you

Bill

Because of the wording of the manufacture your looking at #4. #4 is 85 amps
at 75c and 95 at 90c
Conductors would be copper, for AL one size bigger.

I was going to replace my NG water heater with a tankless because of space
issues. Then I started looking at "what if" A conventional tank heater has
advantages on holiday weekends as the tankless ones parts are not stocked in
my city.
I bet you will find that an standard electric, with maybe a timer is the way
to go. expense wise

 

[Bill C]

Thanks Danny,

Because the use is whole house, I'd need to upgrade a 100amp subpanel fed by
the meter, get electric company to turn off and on the whole service or work
with the stuff live at deadly levels, trench and run 40' of new conduit and
pull wire or pay someone many hundreds to do it. I need to fall back to
just replacing the standard water heater with the same, look into preheating
it with rooftop solar later.

Thanks for the help anyway.
Bill

In <[email protected]> "Bill C"

Thanks Danny & William.

Propane here in the Florida Keys is $2.88/gallon. One dealer came out,
looked at our setup and said, "save yourself the trouble and go to Home
Depot and grab and electric tank heater."

I don't have your original post on the server here. What's the final use
for the water? If it's a wash-up sink, then using a small ( two or five
gallon ) undersink electric heater could be a good choice.


--
_____________________________________________________
Knowledge may be power, but communications is the key
[email protected]
[to foil spammers, my address has been double rot-13 encoded]

 

[Bill C]

I agree. Thanks.

Because of the wording of the manufacture your looking at #4. #4 is 85
amps
at 75c and 95 at 90c
Conductors would be copper, for AL one size bigger.

I was going to replace my NG water heater with a tankless because of space
issues. Then I started looking at "what if" A conventional tank heater has
advantages on holiday weekends as the tankless ones parts are not stocked
in
my city.
I bet you will find that an standard electric, with maybe a timer is the
way
to go. expense wise

 

[Bill C]

Thanks, I need to take another route.

That depends on the temperature of the insulation, what the conductor is
enclosed in, how many conductors and the material in the conductor.

You need to size wire for your overcurrent protection and not the current
draw.

 

[m II]

That depends on the temperature of the insulation, what the conductor is
enclosed in, how many conductors and the material in the conductor.

You need to size wire for your overcurrent protection and not the current
draw.

Wrong again, you top posting loon. You will kill somebody with your ignorance.

You HAVE to size the conductor to the current draw and allowable voltage drop.
240 volts lets you double the distance allowed for 120 volt circuits for a given
drop.

Generally, the conductor should NOT be loaded to more than eighty percent of
it's rated capacity in the conditions it's located in. Breaker size for that
wire capacity will vary with the type of load being connected to that conductor.
A motor will have different overcurrent protection than a dedicated lighting
circuit of the same current draw.. A residential general use circuit will have
different protection than a welder of the same current draw.


You've been giving out misinformation as long as I've followed your multiple
personality disorder across usenet. You are a menace to life and limb. I pity
anyone who takes your rotten advice and winds up dead.



Give it up, you fraud.



For the original poster:

The following derating factors should be taken into account:

http://www.okonite.com/engineering/ampacity-correction-factors.html

Check to see if local requirements are different.


Check these numbers againts your local code. This table shows a number four
conductor is minimum for equipment rated at one hundred percent load capacity.
If you need the eighty percent, then 68.5 amps / .8 = 85.63 minimum conductor
capacity, which would kick up the size to a number two, because you are slightly
over the allowed capacity of the number four. Any other derating factors would
be calculated the same way, usually, if not always, increasing the conductor
size in order to reduce heating effects.

http://www.realgoods.com/renew/tables/index.cfm/id/21/sd/1601/dp/1600

The overcurrent/shortcircuit protection device will be based on the calculated
(derated) ampacity of the wire you have. It is there to protect the conductor,
not what you have connected to it.

If you had, say a 115 amp wire at an eighty percent load limit, you'd base the
breaker size on a 92 amp number. (115 X .8) If that size is not available, you
may go to the next size up, which in this case, would be a hundred amp breaker.
As I mentioned earlier, different sorts of electric motors are calculated
differently, mainly because of startup current variations.




mike

 

[John P. Bengi]

There are standard tables in the electrical code books to determine this.
usually only a few different temperature insulation values are available in
most outletrs so the decision is narrowed down. See if you can find a local
electrician or electrical engineer to help you with the tables or getting
the tables.
Some will give you advice and you can do the grunt work and they can check
it over for a few bucks or get the inspector in and he will usually help a
poor guy out with some tips and corrections. They aren't your enemies.