Central Air wiring gone bad

Reply

  #1  
Old 05-10-04, 04:32 PM
Member
Thread Starter
Join Date: Apr 2003
Location: USA
Posts: 122
Central Air wiring gone bad

We had an HVAC technician come out and look at our central air outdoor unit because it was not blowing cold air. He said the wiring to the unit is black from overheating, and that we'd need to have it re-wired. I've done quite a bit of re-wiring in the house, already and consider myself at least knowledgable enough about code requirements to make safe connections. Therefore, I should be able to at least run the wire myself (even if I need to call out an electrician to make the actual connections).

What I'm not sure of is what gauge wire is required for a central air unit? I could probably tell by looking at the existing wire, but I'd like to pick the appropriate wire up on my way home from work tonight.

Thank you in advance for your assistance.
 
Sponsored Links
  #2  
Old 05-10-04, 07:24 PM
hotarc
Visiting Guest
Posts: n/a
Look at the nameplate on the outdoor unit. It should list the maximum fuse or breaker size. Use this # to size your new wire. You will probably need either 8/2 or 10/2. Or 8 or 10 THHN if this is a conduit installation.
 
  #3  
Old 05-10-04, 07:51 PM
Member
Join Date: Sep 2002
Location: welland ontario
Posts: 5,678
Don't go by the old wire size. It sounds like it was not correct.
 
  #4  
Old 05-10-04, 08:39 PM
Member
Thread Starter
Join Date: Apr 2003
Location: USA
Posts: 122
Central Air Wiring Gone Bad

Thanks for your replies.

I was able to assess the situation when I got home this evening. The wire insulation is almost completely melted away at the point the wire enters the Central air unit. The installer used 10/2 wire from a 30 amp circuit breaker, which from my understanding is correct wire size. Now, there isn't a disconnect by the unit, so the wire was running from the house wall to the unit, unprotected. We purchased the house a year ago from someone who is now deceased, so I don't know if I could find out how old the unit is, but I assume it's been wired this way for 10+ years. It may be dumb luck that the house hasn't burned down before this!

The panel on the side of the unit is hard to read (worn away), but I could make out the following:
"MIN. SUPPLY AMPACITY 35.0 @ 208 V 31.0 @ 230 V"

Does this mean the 30 amp breaker was too weak? Do you think I need to get a higher amp breaker and use 8/2?

I'm planning on installing a disconnect box near the unit, so I'll be running a short distant of flexible conduit to run the wire from the disconnect to the unit.

Also, I've done quite abit of normal household wiring (lights, recepticles, small appliance, etc...), but have yet to wire anything on a 220 circuit, which is what I assume this is. Is it appropriate to use 10/2 wire on this circuit, or should I use 10/3? Hopefully, this isn't too dumb of a question.

Thanks!
 
  #5  
Old 05-12-04, 08:24 AM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
To determine the rating of the Branch-Circuit breaker, multiply the Full-Load Current by 250%.

To determine the ampacity of the Branch-Circuit conductors, multiply the F-L-C by 125%.

If the compressor motor is single-phase, it a 2-wire 220 volt Branch-Circuit. If it's 3-phase ( possible, but not "probable"), it's a 3-wire 208-volt B-C.

The Wiring Method must provide an approved Equiptment Grounding Conductor.

Good Luck & Enjoy the Experience!!!!!!!!!!
 
  #6  
Old 05-12-04, 11:23 AM
Member
Thread Starter
Join Date: Apr 2003
Location: USA
Posts: 122
Originally Posted by PATTBAA
To determine the rating of the Branch-Circuit breaker, multiply the Full-Load Current by 250%.

To determine the ampacity of the Branch-Circuit conductors, multiply the F-L-C by 125%.
Could you please define what Full-Load Current is? I have a general idea, but obviously I don't want to screw around with this a mess something up.

Thanks.
 
  #7  
Old 05-13-04, 06:04 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Electric motors consume different levels of electric power depending upon the mechanical load applied to them. The full load current is the current which would flow through the load when the maximum rated mechanical load (full load) is applied.

I disagree with PATTBAA's recommendation in this case.

For air conditioning equipment (NEC 2002 Article 440), you are permitted to use a circuit breaker than is _up to_ 175% of the FLA of the compressor motor, and if that is not sufficient to start the motor, you can go up to 250%. This is where PATTBAA's numbers come from.

However 1) we do not know the FLA of your compressor. Instead we have the nameplate minimum circuit capacity. This manufacturer's instruction is supposed to include the necessary 'headroom' for motor overloads. 2) we know that this compressor has been reliably starting using a 30A breaker.

1) You need _conductors_ with 35A ampacity. Normally #8 wire has a 60C ampacity of 40A, and should be fine, however this is subject to temperature de-rating. Is any part of the wire run in a particularly hot location (eg an attic?)

2) I would recommend protecting the conductors at their rated ampacity, rather than using the higher rated breakers permitted by code for this application. If the compressor has been starting on a 30A breaker, then it should start with no problems on a 40A breaker. The only situations in which this won't be the case are if the 30A breaker were bad, or if the voltage drop in the wire significantly changed the starting current of the motor.

Point 2) leads me to ask one additional question: How long is the run from your panel to the unit. Motors consume much more current on startup, and this increases voltage drop in any supply conductors. The lower the voltage, the longer they spend starting, and the more current they draw. Having too much voltage drop during startup put increased stress on the motor and conductors. If you are replacing the wire, now is a good time to correct this potential problem.

-Jon
 
  #8  
Old 05-13-04, 08:06 AM
Member
Thread Starter
Join Date: Apr 2003
Location: USA
Posts: 122
Winnie:

Thanks for clearing some of this up for me. In regard to your question in point 2, above:

There are two conduit lines running out of the electric meter on the side of the house. Of course, one of them is running to the main panel, while the other one runs about one and a half to two feet to a disconnect box (mounted on the side of the house) with a 30 amp breaker in it. From this box there is 10/2 wire running from the box to the central air unit. The wire enters the house just below the disconnect box, and runs about 30 feet inside the house (basement, so it's a cool area), before it exits the house again, to run about 2 to 3 feet outside to the unit. So, the total length of the run is about 35 feet.

The technician who came out to look at the unit told my wife that we needed to have a disconnect box within three feet of the unit. Therefore, my plan is that when I run the new wire (which will most likely be 8/2), I will put a disconnect box at the point the wire exits the house, and then run 8/2 through flexible conduit for the 2 to 3 feet between the new disconnect box and the unit. I'll end up with this run on two disconnect boxes, but I figured it's better to have that situation, and be in code compliance, than not in code compliance at all.
 
  #9  
Old 05-13-04, 10:56 AM
hornetd's Avatar
Member
Join Date: Aug 2001
Location: Maryland
Posts: 695
Thumbs down Something is not right!

Originally Posted by claassen
Winnie:

Thanks for clearing some of this up for me. In regard to your question in point 2, above:

There are two conduit lines running out of the electric meter on the side of the house. Of course, one of them is running to the main panel, while the other one runs about one and a half to two feet to a disconnect box (mounted on the side of the house) with a 30 amp breaker in it. From this box there is 10/2 wire running from the box to the central air unit. The wire enters the house just below the disconnect box, and runs about 30 feet inside the house (basement, so it's a cool area), before it exits the house again, to run about 2 to 3 feet outside to the unit. So, the total length of the run is about 35 feet.
It takes special lugs in the meter enclosure for the meter to provide current to two separate disconnects. I think you should get the installation checked by a fully qualified electrician. I suspect that the AC disconnect was bootlegged onto the meter by inserting it's supply conductors into the single lugs that were already supplying the house panel. Because the two conductors that may be sharing a lug in your meter can will expand and contract at different times the connection may gradually loosen and fail. There are enough things wrong here that you need to get the installation checked out.

If you have sufficient space in your existing panel than you can install a forty ampere breaker in the panel to supply the AC compressor. The conductors would need to be number eight american wire gauge copper if they are run in type NM cable. If you use type SE cable then they would need to be number six aluminum. Either cable need only be two insulated conductors with an uninsulated Equipment Grounding Conductor.
The technician who came out to look at the unit told my wife that we needed to have a disconnect box within three feet of the unit. Therefore, my plan is that when I run the new wire (which will most likely be 8/2), I will put a disconnect box at the point the wire exits the house, and then run 8/2 through flexible conduit for the 2 to 3 feet between the new disconnect box and the unit. I'll end up with this run on two disconnect boxes, but I figured it's better to have that situation, and be in code compliance, than not in code compliance at all.
I don't know were he is getting that three foot figure but it is not from the US National Electric Code. The disconnect need only be "within sight of" the equipment it serves. Within sight of is defined as "Where this Code specifies that one equipment shall be “in sight from,” “within sight from,” or “within sight,” and so forth, of another equipment, the specified equipment is to be visible and not more than 15 m (50 ft) distant from the other."
--
Tom Horne
 
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