70 Amp Electric Furnace

[hanklazard]

Some of my customers, against my advice, are switching from gas furnaces to electric furnaces.
They purchased a 70 amp electric furnace and it will be located in the attic.
Wouldn't 2-2-2-4 Service entrance cable aluminum be suitable for this or does the attic temperature require an ampacity adjustment.
What wire are you guys using for these installations like these?
Luckily the service is 200 amp.

 

[James L]

Even with an ambient temperature adjustment a #2 aluminum is plenty.

However, you don't need a neutral for the furnace.

What you can do is feed a small subpanel with 2-2-2-4, and then run furnace circuit from there. That would allow you to run a service receptacle and attic lights right outbof that panel

 

[letgomywago]

Most furnaces have a copper conductor spec so you'll want to transition somehow and the panel is a cheap way to do it.

 

[jmellc]

Most furnaces have a copper conductor spec so you'll want to transition somehow and the panel is a cheap way to do it.

I got snagged on that once. Inspector let me use cu/al splice blocks and add a foot of copper wires onto the AL.

 

[kwired]

As mentioned your unit likely doesn't require a neutral.

Also attic temp - if there is a call for heating chances are the attic temp is low anyway. If this unit doubles as the air handler for cooling then the conductor is well over adjusted for the load at that time.

 

[letgomywago]

I got snagged on that once. Inspector let me use cu/al splice blocks and add a foot of copper wires onto the AL.

The ones that use combinations of 60a breakers to terminate on I've used an AC disconnect next to it since it's cheaper than the splice blocks but what's funny is those breakers if in a panel would say AL/CU on them.

 

[hanklazard]

Thanks everyone for being so helpful. I was hoping the service entrance cable was a solution since it's the main thing that's weather proof and readily available and more affordable than most copper solutions, which no one seems to be stocking anyhow.
Even 4-4 copper seemed a bit undersized since it would be continuous use.

 

[letgomywago]

Thanks everyone for being so helpful. I was hoping the service entrance cable was a solution since it's the main thing that's weather proof and readily available and more affordable than most copper solutions, which no one seems to be stocking anyhow.
Even 4-4 copper seemed a bit undersized since it would be continuous use.

446 copper se cable is a 90° cable the termination is what limits it but look at the specs they'll have what the requirements are. Now alot of furnaces go in multiples of 60a or less if it's a 30 and 40 amp circuit you'll want to hit something like a panel or use the tap rules. Might even be worth it to just pull the 2 smaller cables just for ease to work with.

 

[hbiss]

Might even be worth it to just pull the 2 smaller cables just for ease to work with.

Ummm, are you allowed to parallel those cables?

-Hal

 

[letgomywago]

Ummm, are you allowed to parallel those cables?

-Hal

There often separate circuits to separate heaters in the furnaces

 

[letgomywago]

Ummm, are you allowed to parallel those cables?

-Hal

Here's a picture off the internet of what the 2 circuit furnaces look like inside. I don't run my thermostat cable through the line volt side though.

 

[hbiss]

Must have missed where he said it was two circuit.

-Hal

 

[letgomywago]

Must have missed where he said it was two circuit.

-Hal

Almost never ends up being bigger than 60amps for residential furnaces. The ones that say 70a are actually a 30 and 40. Same with insta hot water heaters.

 

[hanklazard]

Many combo A/C and Furnace units will have a 30 amp ac circuit and a 50-60 amp electric furnace, so it's feasible to run 2 seperate lines.
The Furnace company I was working with was not clear and said they needed a 70 amp circuit.
Home Depot is selling a 112 amp instant tank water heater and it hooks up to 3 2 pole breakers.

Rheem Performance 27 kw Self-Modulating 5.27 GPM Tankless Electric Water Heater RETEX-27 - The Home Depot

Save energy and costs with this Rheem Performance Tankless Electric Water Heater. This compact heater can be installed under a cabinet or sink with no venting required. It uses a 3 x 49 amp double pole

www.homedepot.com

 

[augie47]

I can't count the number of jobs that get rejected based on verbal info from HVAC sales people.
So often the 60° rating on NM is overlooked.
Best to get an actual cut sheet on your unit or wait until you can see the nameplate for # of circuits and MOCP, MCA.

 

[Little Bill]

I can't count the number of jobs that get rejected based on verbal info from HVAC sales people.
So often the 60° rating on NM is overlooked.
Best to get an actual cut sheet on your unit or wait until you can see the nameplate for # of circuits and MOCP, MCA.

Good luck finding an installer that marks the aux heat package on heat pumps!

 

[kwired]

Must have missed where he said it was two circuit.

-Hal

I was thinking of questioning that as these units usually subjected to a maximum of 60 amps per sub circuit and figured if he needs more than 60 amp supply that it likely could be split into multiple supply circuits.

 

[kwired]

Good luck finding an installer that marks the aux heat package on heat pumps!

You just have to look harder and figure it out yourself when they don't do that. Probably not a bad idea to make sure they marked it correct anyway.

Some EI's are adamant that it be marked, and I suppose NEC maybe does say it should be done, though in reality nobody really cares once initial install has been done.

 

[augie47]

Good luck finding an installer that marks the aux heat package on heat pumps!

In this area they are either marked or rejected
Often when unmarked the data sticker is available to check.
The real pain is when the unit has a "adder" sticker supplied with the heat strip and said sticker is in the trash,.
A good 25% of the 10kw installs i inspected had inadequate wiring.

 

[petersonra]

Almost never ends up being bigger than 60amps for residential furnaces. The ones that say 70a are actually a 30 and 40. Same with insta hot water heaters.

the reason for this is likely 424.22 (B)