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Looking for validation or correction on my understanding of requirements for wiring 5 ton Trane air handler with 20kw heat strips. Of course, I read the label and identified that there are (2) 240v circuits required. Minimum circuit ampacities are 57 and 50 amps respectively. MOPs are accordingly 60 and 50. The heat strips are designated 40a per circuit. It's indoors in basement and wire runs from panel are about 25 feet.

The heating system was recently installed and HVAC contractor reused existing 2/0 AL cable connected in my panel to a single 4 pole 125a breaker. He installed a single feed kit that serves the (2) 60a disconnects in the unit. Concerns: 1) the single feed kit is labeled for use only up to 100a, 2) the air handler is labeled for use only with copper wire.

I therefore would like to have new wiring run to the heater. My options, as I understand them, are a) keep the single feed kit and run a single copper feed from the panel presumably with a 100a breaker replacing the 125a or b) remove the single feed kit and run two feeds, one for each of the circuits in the air handler, appropriately sized and connected to one 50a and one 60a breaker.

Is there a general preference of rule of thumb for which of these options is best? Will the single feed with 100a breaker be adequate or will it trip when running on full aux heat?

Using sizing calculators, tables and derating - appears that single 100a feed will require either 1 awg copper NM (SE) or 3 awg THHN in conduit. The air handler uses 2 conductors plus ground (no neutral) for each circuit and I have not seen or been able to find 2-conductor copper plus ground in 1 AWG or larger.

Separate feeds will require either 4 awg 2-conductor copper plus ground NM (also difficult to find) or 4 awg THHN in conduit (which would require 4 THHN conductors plus ground in a conduit - for which I derated the ampacity of the cable to 80% of the 75 degree value).

Hate to have to use conduit, especially since there are many turns (more than 360 degrees) and practically zero clearance for the bend required where cable enters the air handler.

Also wondering: if I run two separate circuits/lines, do I need to tie together to the (2) 240 volt 2-pole breakers in the panel so that they would trip together?

Sorry about the long message but wanted to be precise. Any suggestions?


UPDATE:

I want to get rid of the AL wiring because the old 125a breaker shows some possible heat damage (dark spot) and one leg of my main feed coming into the house partially melted down at the meter. Not sure if this was related to the old HVAC system but seems suspicious and I'd reather start fresh.

Everyone seems to agree that 6/2 would be adequate for the 60 amp circuit, even though I thought I read that maximum ampacity of 6 awg NM wire is only 55a (and the max current listed on the heater is 57!)

By the way, the way the air handler is configured, each of the 2 circuits has one 9.6kw heat strip on it, plus one of them also includes the blower. Thus, my question on tying the breakers together in the load center.

  • Please don't post an "answer" to update your question. Instead, edit your question or use the comment feature. I have edited your additional information in o your question for you, and then updated my answer below. – longneck Mar 23 '15 at 12:24
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Unless I'm missing something in your explanation, using the 2/0 AL wire doesn't seem to be a problem, except that it's AL and the unit does not specify AL. What will probably be simpler than running all new wire is to just get AL-to-copper splice connectors.

And in response to:

if I run two separate circuits/lines, do I need to tie together to the (2) 240 volt 2-pole breakers

No, you don't. If the heater circuit trips, you'll just lose your heat. And if the air handler breaker trips, it will shut off the heater strips. So no problem there.


In response to your update:

If the breaker is visibly damaged, then replace the breaker. If you decide to keep the AL wire, then make sure the breaker is rated for AL. Again, replacing the breaker will be cheaper than running all new copper.

A partially melted lug is a bad sign. You need to get an evaluation from a professional.

Having two breakers is still not a problem. If breaker for the lone heater strip trips, you get reduced heating capacity. If the other breaker trips, the air handler will shut off the other heater strip automatically.

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Your question is not long at all, much better too much information than not enough.

If it were mine:

  • Remove everything starting with the 125 amp breaker until you hit the disconnects on the HVAC stuff.

6/2 Romex - copper is good for 60 amps (NEC 240.4 (B) & 334.80 -60 degree) A couple of breakers, romex connectors and big staples Should be less than $200 in parts.

If they are separate disconnects, no need to tie them together.

Sell the Al and left over Cu to the local recycle guy.

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