air conditioner wiring insanity

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  #1  
Old 12-11-05, 01:07 AM
jerry814
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air conditioner wiring insanity

i've been working on rewiring a 50+ year-old house (mine), and the goofy (and by goofy i mostly mean plain stupid) things i've seen just keep on stacking up. the latest ... here's the background, before the questions:

my air conditioner/electric heat (central air ... it's a Goodman PCK048-1 Self-Contained Air Conditioner) was being powered via a subpanel (if you could call it that), that was fed directly off the main disconnect (housed seperately from the main service panel). the two hot lugs on the main disconnect (after the breaker) each had two wires (looked to be 6 AWG, or maybe 8 ... i'll measure it tomorrow to be safe) in addition to the expected wires to feed the main service panel. those four 6 AWG wires, along with two neutrals of the same size which were under the neutral lug on the main, powered the air conditioner's sub panel (it's about 15 feet from the main). the sub panel had two wires (the same as were under each of the hot lugs on the main disconnect) under each of the hot lugs. then, there was a 70 amp double-pole breaker on one side and a 60 amp double pole on the other. then, each of those had the hot/hot/ground wires leaving the subpanel, headed to the a/c through conduit (so six wires in total heading to the a/c). the 60 amp breaker has been off since i moved into this house, so unless there's something very strange going on that i've never heard about, i'm guessing that half of those wires aren't doing anything. the air conditioner info tag says that it needs to be protected by a circuit breaker of 50A max, so i know there's a problem there already.

and the questions:

why would that extra set of wires be running to the a/c unit (unfortunately i can't remove the fitting where the conduit with wires enters the a/c unit -- it's stuck on quite tightly)? a/c is wired like any other 220V appliance -- two hots and a ground -- right?

if i measure the wire and it's 6 gauge (which can carry 55A, if i remember correctly), i can safely put that wire into a new 50A double-pole breaker in the new main breaker box i just installed, right? and then just cut out the sub panel and the extra (i think) set of wires running to the a/c? or do i need to have an emergency cutoff right by the a/c, even if it is only 15 feet away and on the same wall of the house as the main service panel?

and in a not-so-urgent sort of way, a few other questions: it's crazy to run two wires from one hot lug to another hot lug, right? the current will always travel on the path of least resistance, so won't it pick whichever wire is in better shape and travel along that, and leave less to no current in the other wire? this would be okay in the case of only have on 50A a/c unit (assuming a 50A breaker were installed), but having it with a 70A and 60A breaker could send 130A down one 6 gauge wire, right?

thanks for the info, especially on such a long-winded post
 
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  #2  
Old 12-11-05, 01:36 AM
jerry814
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i just had another thought ... even if the wire proves to be 8 AWG, i should still be okay, right? i know #8 wire is rated to carry 40A, but since there are two hots, each hot will only be carrying half the load, so 25A before the breaker trips, right? or am i wrong about that?

thanks
 
  #3  
Old 12-11-05, 10:43 AM
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Originally Posted by jerry814
i just had another thought ... even if the wire proves to be 8 AWG, i should still be okay, right? i know #8 wire is rated to carry 40A, but since there are two hots, each hot will only be carrying half the load, so 25A before the breaker trips, right? or am i wrong about that?

thanks

I won't even touch the first post (I'll leave that to the electricians) but I can answer this one.

A fourty amp double pole breaker allows up to fourty amps on EACH pole. So, potentially, BOTH wires connected to the breaker can have fourty amps on them. Suppose one pole has a red wire connected to it, and the other has a black wire. That red wire could potentially draw 40 amps @ 120 volts without tripping the breaker. The black could do the same. Together, the red and black can draw 40 amps @ 240 volts. If this is feeding a pure 240 volt device, the draw across the two wires will be equal, but cannot exceed 40 amps (@ 240 volts) for a sustained period of time without tripping the breaker.


So...if #8 wire is only rated for 40A, it can only be used on a 40A or smaller breaker. It makes no difference whether it is a single or double pole breaker.


Now, on the ampacity of #8 wire. NEC table 310.16 rates #8 wire at 40A in the 60 column, at 50A in the 75 column, and at 55A in the 90 column. If you are talking individual THWN/THHN conductors in conduit, you can use the ampacity rating in the 75 column, and thus connect the #8 wire to a 50A breaker, IF *ALL* the terminations (breakers, lugs, etc.) are rated for at least 75.
 
  #4  
Old 12-11-05, 07:04 PM
jerry814
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thanks for the response chirkware. and my apologies to all who read through that long, rambling post. the questions were genuine, of course, but i had had such a long, frustrating day prior to posting it that i don't think i made a lot of sense. what you said about the 50A double pole breaker makes perfect sense, chirkware. i would like to think that i would have remembered that myself in better circumstances

the good news is, i got the a/c / heater rewired today, and it works perfectly with all of those extra wires removed. and the wire was 6 gauge after all, so no worries there either.

thanks again.
 
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