Residential Forced Air and Condenser Wiring

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krist003

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A friend of mine is installing a new forced air unit and condenser at his house after demoing the old units. I told him I would help him to understand the work that needs to be done in order to get it up and running. He has a 50A breaker and what looks like #4awg wire that was going to the original unit. Can he (or an electrician for that matter) just extend that wire to the new unit and is #4awg overkill? What size wire then needs to from the unit to the disconnect and then the condenser? Should it all be the same size wire? Is there anything else he needs to expect?

Thanks
 
They key to all your answers should be on the factory nametag(s) Minimum Circuit Amps and maximum Over-Current Protection are the 1st step. Until you have those, its a guessing game.
 
Thanks Augie.

Just to clarify, he already has a dedicated 120V plug for the FAU in the attic. So he just needs to extend the 50A power to the condenser. I will have him get the info off the nameplate.
 
Sounds as if the FAU is gas. If so, the following from UL might prove interesting in regard to the plug:

(As far as the condenser, lets see what the nameplate shows._)

[FONT=&quot] The basic standard used to investigate the refrigeration portion of the products in this category is [/FONT][FONT=&quot][FONT=&quot]ANSI/UL 1995[/FONT][/FONT][FONT=&quot]. The basic standard used to investigate the gas heating portion of the products in this category is ANSI Z21.47/CSA 2.3, "Gas-Fired Central Furnaces."[/FONT][FONT=&quot]

[/FONT][FONT=&quot]For gas furnaces, UL has few listings, and those are part of rooftop units - permanent supply connection is employed. CSA has most of the listings, as well as ETL. [/FONT][FONT=&quot]

[/FONT][FONT=&quot]The applicable standard, ANSI Z21.47/CSA 2.3, Gas-Fired Central Furnaces, is quite vague on requirements for supply connection. This is typical of the electrical requirements of all CSA gas standards.[/FONT][FONT=&quot]

[/FONT][FONT=&quot]This is relative to permanent wiring connection.[/FONT][FONT=&quot]

[/FONT][FONT=&quot]1.18.14[/FONT][FONT=&quot]
[/FONT][FONT=&quot]A service junction box having provisions to accommodate fittings for metal-clad cable or conduit shall be[/FONT][FONT=&quot]
[/FONT][FONT=&quot]provided for the permanent connection of the line-voltage supply circuit. This box shall be located on a[/FONT][FONT=&quot]
[/FONT][FONT=&quot]part of the assembly that does not require movement for normal servicing and adjustment.[/FONT][FONT=&quot]

[/FONT][FONT=&quot]There is mention of strain relief of a power supply cord:[/FONT][FONT=&quot]

[/FONT][FONT=&quot]1.18.20[/FONT][FONT=&quot]
[/FONT][FONT=&quot]Strain relief shall be provided for all conductors leaving an enclosure. For low-voltage wiring, strain relief at[/FONT][FONT=&quot]
[/FONT][FONT=&quot]the point of exit from an enclosure is not necessary if, by wire location or support, protection is provided[/FONT][FONT=&quot]
[/FONT][FONT=&quot]against accidental strain. If the furnace is supplied with an external power supply cord handling greater[/FONT][FONT=&quot]
[/FONT][FONT=&quot]than 50 Volts then a pull test shall be conducted.[/FONT][FONT=&quot]

[/FONT][FONT=&quot]Generally speaking, gas furnaces should have permanent power supply connection. But if you see otherwise, check for the certification/listing mark that is applied, and contact that agency as it likely will not be UL. Based on the vague information contained in ANSI Z21.47/CSA 2.3, "Gas-Fired Central Furnaces," if there is a statement as you indicated that a Central Furnace is not ?readily removable for repair and maintenance.? then I would concur that an AHJ could state that this type of equipment was not in compliance with NEC section 400.7(A)(8). Based on the language used in 400.7(A), this is an all inclusive list unless 400.7(A)(10) can be used such as in 422.16. With that said, even 422.16(A) based on the language would not permit this type of equipment to be connected by cord and plug since again, CSA stated a Central Furnace is not ?readily removable for repair and maintenance.?[/FONT][FONT=&quot]

[/FONT]
 
I'm going to say if it is 4AWG it is overkill - unless it is a long run and was necessary because of voltage drop.

If 50 amp breaker was on the old unit - and assuming it never had trip problems during starting that old unit probably had a MCA of less then 30 amps, or you had a FPE breaker that doesn't trip;)

Unless you are replacing the old unit with a higher capacity unit, the new one will draw less then the old one - you can't find one that has a lower efficiency rating then even one that is only 10 years or so old. I guess I shouldn't say never - but this is true most all the time.
 
kwired said:
I'm going to say if it is 4AWG it is overkill - unless it is a long run and was necessary because of voltage drop.

If 50 amp breaker was on the old unit - and assuming it never had trip problems during starting that old unit probably had a MCA of less then 30 amps, or you had a FPE breaker that doesn't trip;)

Unless you are replacing the old unit with a higher capacity unit, the new one will draw less then the old one - you can't find one that has a lower efficiency rating then even one that is only 10 years or so old. I guess I shouldn't say never - but this is true most all the time.
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If the new unit has a Maximum OCPD less than 50A you will have to use a smaller breaker or add a fused disconnect.
 
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