I'm running 70 feet from the breaker box in my garage, through the top
of the basement wall and along the basement ceiling into a new furnace
with 3/4 HP blower. Could someone tell me what kind of cable and gauge
I need? I'll be stapling it to the floor joists and wall.
Also do I need a 15 or 20 amp breaker?
My guess, mind you it is a guess at this point, would be 12 gauge wire, and
a 15 amp breaker. 12 gauge because of the length, (It maybe over kill, but
larger will not hurt), 15 amp should be adequate for the load of the furnace
We are talking 120 volt, correct?
Give us volts and amps of the furnace to be sure.
When does NEC require a jump in wire size for length?
I looked at some blower motors too and saw a bunch at less than 10 amps. The
15 amp breaker will most likely be large enough, but then we really don't
know with the info he provided! Good chance the furnace tag may show more
than a 10 amp draw though.
Still not enough info. If the furnace is a factory model, you need to
provide what is stated on the furnace nameplate, not the motor
nameplate. If it is a DIY furnace, then you need to go by the motor
nameplate amps. If the motor nameplate amps are not stated on the
motor nameplate, then you have to use the current draw from NEC Table
430.148. For a 3/4 HP single phase 120 volt motor that's 13.8 amps.
13.8 x 125% = 17.25 amps, which would require a 12-2 with ground romex.
A 20 amp breaker would be sufficient. A disconnect at the furnace is
also required. A 20 amp switch should be OK as long as it is at least
3/4 HP rated. If the switch is HP rated it will be stamped on the
If the furnace is a factory model, if the nameplate says to use "fuse
only", or similar statement, then the disconnect needs to be a combo
switch/fuse, then use the fuse size required by the nameplate.
The 12-2 w ground romex needs to be protected from damage with "running
boards" in the basement if not ran on the sides of the joists. 3/4"
Furring strips will suffice. Also romex is usually sleeved with conduit
when ran exposed down a wall.
This is Turtle.
Awwwwwwwwww, Greg they do have to go to larger wire sizes to keep the voltage to
not drop more than 2% on a circuit. you may have a motor that will draw 10 FLA
amps at 10 feet with #14 wire and then have the motor moved 3 miles from the
breaker and you will have to use # 4/0 wire to keep the 2% drop from taking
place. every foot of wire used to the circuit is a resister and drops the
voltage some with every foot it is extended.
They have charts for this and if you would like i can send you one to see.
You've got a lot of dancing for a simple question.
You can go to the below and put in the information
and get a real answer.
Your 3/4 HP probably used 9 A, (based on my 1 hp
saw motor that says it is 12.2 A) and the
calculator says use #12 wire. So you need a 15 A
breaker, 15 A socket if it plugs in.
If you have #12 wire, use it. If you have to buy
wire buy #10 because you might want to run
something else over there. And buy a 20A breaker.
That means besides the fan you have nearly 9 A of
capacity for other stuff.
Consider safety. If the system requires a
circulating fan to switch on to keep the heat box
(plenum if you will) from burning up, then you
should use a dedicated wire; #12 with a 15A
breaker is plenty.
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