Electric furnace wire/breaker size

Reply

  #1  
Old 11-28-11, 12:38 PM
Member
Thread Starter
Join Date: Nov 2011
Location: Usa
Posts: 2
Electric furnace wire/breaker size

I am installing a used electric furnace (RUUD) htr. K.w. 10.8/14.4. Amps 55.3/63.4. Mca 69.2/79.3. Mop 70/80. It came with 2 60amp 2pole breakers connected by a jumper bar. I'm wondering what size wire to use and what breaker for the panel. Also do I need a neutral and if so would it just connect to the ground lugs. Thanks
 
Sponsored Links
  #2  
Old 11-28-11, 06:13 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,979
Since it is a 240 volt device you do not need a neutral. You do, however, need a ground which would be connected to the ground lug.

Wire size for 60 amps is #6 Copper. The ground can be #10 if you are not using a cable. (NM-b) ***Edit: See other post for amended information of wire size***

When you say there is a jumper are you talking about a handle tie or is there a set of wires between breaker one and breaker two? A picture might be helpful.
 

Last edited by Tolyn Ironhand; 11-29-11 at 03:06 PM.
  #3  
Old 11-28-11, 06:14 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,352
MCA stands for minimum circuit ampacity. This will determine your wire size.

Depending on your voltage you may need either 70 or 80 amp conductors.

You do not need a neutral.
 
  #4  
Old 11-28-11, 06:53 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,212
What is the voltage rating of the furnace? My assumption is that this is a 240 volt furnace and you have the typical residential service voltage of 120/240 volts, is this all correct to this point? Can this furnace be connected for either KW rating, 10.8 or 14.4 or is this a two circuit furnace with both KW ratings running at the same time? The thing that bothers me is that usually having two 60 amp breakers is indicative of a two circuit 20 KW furnace, but I think what you have is a two circuit 25 KW furnace. A 25 KW furnace could be either a two or three cicuit unit. Are these breakers mounted in the furnace as a disconnecting means with the handles exposed through the top door? Can you provide a model number of the furnace?

What I suspect is this, you have a 25 KW furnace that has had a 5 KW element disconnected and one breaker has been changed to run the unit as a two circuit 20 KW furnace, but knowing the model number would probably tell the story. Once we establish exactly what you have we can tell you what size wire to run to it. I doubt you'll need a neutral conductor, just two hot wires and one ground for each circuit.
 
  #5  
Old 11-28-11, 07:50 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,356
OR, those posted specifications may be for a 208/240 furnace. The "...2 60amp 2pole breakers connected by a jumper bar." could be a two-pole circuit breaker and the OP doesn't know the difference between that and two single-pole breakers.

At any rate, the NEC requires that fixed heating appliances have their branch circuit designed for 125% of full load current. That would point to #4 copper conductors and an 80 ampere branch circuit breaker as is also specified by the MCA and MOP numbers.
 
  #6  
Old 11-29-11, 08:06 AM
Member
Thread Starter
Join Date: Nov 2011
Location: Usa
Posts: 2
More details: RUUD Model # UBHA-17J15SUCA, Volts: 208/240vac
HTR. K.W. : Total=10.8/14.4, Mult. Suppl. Circuit Data=CKT#1-7.2/9.6, CKT#2-3.6/4.8
Amps: Total=55.3/63.4 Mult. Suppl. Circuit Data=CKT#1-38/43.4, CKT#2-17.3/20
MCA: Total=69.2/79.3 Mult. Suppl. Circuit Data=CKT#1-47.5/54.3, CKT#2-21.7/25
MOP: Total=70/80 Mult. Suppl. Circuit Data=CKT#1-50/60, CKT#2-25/25

The breakers are accesable as a shut-off on the panel door. They are connected by a bus bar with lugs to connect both breakers through one set of wires "single point wiring"
thanks for your help.
 

Last edited by Notaboutus; 11-29-11 at 09:27 AM.
  #7  
Old 11-29-11, 02:53 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,979
Originally Posted by Notaboutus View Post
The breakers are accesable as a shut-off on the panel door. They are connected by a bus bar with lugs to connect both breakers through one set of wires "single point wiring"
thanks for your help.
Ok, That makes more sense now. You need to wire it as the others have mentioned using the MCA and the MOP. This will require #4 copper and a maximum breaker or fuse of 80 amps @ 240 volt.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'