# Using existing two wire cable for new furnace.



## Hanford43 (Mar 5, 2017)

I am being a little lazy today so I thought I would ask you guys about this before I grabbed the code book. Alot of the work I do for my current company is subcontracting for an HVAC company when they do furnace/AC swaps etc. Often I get to these jobs with no prior knowledge of what I am getting into: panel type, existing wire size, etc. Obviously we only have short time to get things running, and in a way that passes inspection.

Anyways, today I ran into something and I wanted some other opinions. An old single circuit furnace was being swapped out with a new one with a 15kw heat strip (2 circuits 60 amp and 25amp). The old furnace was fed with #2 copper SE cable. 2 wire with a bare ground. With these jobs my employers policy is to get things done quickly and safely but avoiding altering the existing wiring as much as possible to avoid issues with the inspector. 

We charge the HVAC company time and material, so sometimes they will use a different heat strip instead of having to pay us to run a new circuit etc. After discussing it with the HVAC estimator guy, he decided to go with a single circuit heat strip for the new furnace. I guess I technically could have used this old SE cable to feed a 240 volt sub panel with no neutral and then run two circuits off that so they could have used the original heat strip, right? In this case would this be considered an actual sub panel or just a disconnect that served two circuits of the same furnace?

I felt I would be opening a can of worms if I tried doing that especially with this ancient SE cable with brittle insulation and no neutral. Obviously it would be ideal to just run new circuits for everything but sometimes that is not possible.


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## drspec (Sep 29, 2012)

Hanford43 said:


> I am being a little lazy today so I thought I would ask you guys about this before I grabbed the code book. Alot of the work I do for my current company is subcontracting for an HVAC company when they do furnace/AC swaps etc. Often I get to these jobs with no prior knowledge of what I am getting into: panel type, existing wire size, etc. Obviously we only have short time to get things running, and in a way that passes inspection.
> 
> Anyways, today I ran into something and I wanted some other opinions. An old single circuit furnace was being swapped out with a new one with a 15kw heat strip (2 circuits 60 amp and 25amp). The old furnace was fed with #2 copper SE cable. 2 wire with a bare ground. With these jobs my employers policy is to get things done quickly and safely but avoiding altering the existing wiring as much as possible to avoid issues with the inspector.
> 
> ...


setting a 4 circuit disconnect is what we have always done in a case like this. the HVAC equipment doesn't need a neutral so why wouldn't you do it that way?


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## Hanford43 (Mar 5, 2017)

drspec said:


> setting a 4 circuit disconnect is what we have always done in a case like this. the HVAC equipment doesn't need a neutral so why wouldn't you do it that way?


Right. I wasnt really concerned about the lack of a neutral. Its just once you start adding extra disconnects the inspector gets more picky about working space and the furnace is usually in a cramped basement corner or broom closet.


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## MechanicalDVR (Dec 29, 2007)

First off let's use the correct terminology. A furnace doesn't have strip heaters, that would be an air handler and they are normally 240v and need no neutral.

A sub panel with two circuits (1-15 amp that powers the blower and 1-40 or 60 amp for the rack heat)is common for an air handler.


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## wendon (Sep 27, 2010)

MechanicalDVR said:


> First off let's use the correct terminology. A furnace doesn't have strip heaters, that would be an air handler and they are normally 240v and need no neutral.
> 
> A sub panel with two circuits (1-15 amp that powers the blower and 1-40 or 60 amp for the rack heat)is common for an air handler.


You've never hooked up a furnace with strip heaters? Happens all the time. Some HVAC guys use them as supplimental heat with a heat pump. Commercial jobs use them to preheat the fresh air intake. 

Sent from my Samsung with a cracked screen protector


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## Dennis Alwon (May 9, 2009)

wendon said:


> You've never hooked up a furnace with strip heaters? Happens all the time. Some HVAC guys use them as supplimental heat with a heat pump. Commercial jobs use them to preheat the fresh air intake.
> 
> Sent from my Samsung with a cracked screen protector


I believe his comment was to distinguish between a furnace and an air handler. A furnace is usually referring to as a fuel fired unit.

Most people do, however, call an air handler with heat strips a furnace.


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## sbrn33 (Mar 15, 2007)

I would tell the HVAC guy to get a single point entry. Then just use the existing se cable. Most air handlers have breakers installed anyway so you shouldn't even need a sub.


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## MechanicalDVR (Dec 29, 2007)

wendon said:


> You've never hooked up a furnace with strip heaters? Happens all the time. Some HVAC guys use them as supplimental heat with a heat pump. Commercial jobs use them to preheat the fresh air intake.
> 
> Sent from my Samsung with a cracked screen protector


Point being a furnace heats air, if there is a heat pump coil and a furnace the furnace is the supplemental heat. 

Have I installed air handlers and supplemental rack heat, 1000's of times.

Have I installed a furnace (nat gas, propane, or oil fired) and added supplemental rack heat, NO! Never seen that done.

I've also installed plenty of heat for fresh air make up and for countless VAV/VVT reheat units.


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## MechanicalDVR (Dec 29, 2007)

sbrn33 said:


> I would tell the HVAC guy to get a single point entry. Then just use the existing se cable. Most air handlers have breakers installed anyway so you shouldn't even need a sub.



Agreed!

But it sounded like this equipment was already installed when they arrived to wire it all up to me.

Breakers in an air handler are normally optional when ordering and not standard equipment with many brands.


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## MechanicalDVR (Dec 29, 2007)

Dennis Alwon said:


> I believe his comment was to distinguish between a furnace and an air handler. A furnace is usually referring to as a fuel fired unit.
> 
> Most people do, however, call an air handler with heat strips a furnace.


Exactly!

Air handler = metal box with a fan that blows air.

Furnace = metal box with a heat source not always containing a blower ie: a duct furnace.

An air handler containing heat strips/ rack heat becomes and electric furnace.


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