Advice needed for 84 amp load.

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  #1  
Old 02-12-14, 03:30 PM
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Advice needed for 84 amp load.

I'm trying to gather the supplies I'll need to install a 20KW heater. I can't seem to figure out for certain the size wire I'll need. I need to go 65 feet one way. I came up with #2 copper or 1/0 aluminum.

Do I need to consider a plenum heater as a continuous load, as with a water heater? So far I'm not.

I'm figuring 84x1.25 is 105, so I'll need a 125A breaker.

I'd like thoughts on minimum gage for both copper and aluminum.

Also I may be able to modify the unit to run as two 42A loads. So thoughts on minimum gage for that would be great too. I came up with #6 copper.

Thanks for any advice.

PS: Forgot to mention I'm in MN, USA.
 
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Old 02-12-14, 04:29 PM
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Welcome to the forums Paul, from a fellow Minnesotan!

Do you have the heater yet? Are you planning to run this heater on off peak rates?
IIRC - Code requires that a heat load is limited to 60 amp feeds. In your case, your load will be divided into multiple feeds. This should be confirmed.

The size of wire will depend on the wiring method. How are you planning to wire the heater? NM cable? Conduit and THHN wire?
 
  #3  
Old 02-12-14, 04:50 PM
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Tolyn:

A short 1 foot section will be in conduit on its way to and from the off-peak meter. The rest will be pulled through ceilings and fed through holes drilled in 2x10s. It will run along with a #10x3 and some bell wire for another application.

The plenum heater is designed to accept a single feed. I don't know if I can modify it to accept two. I know a different model accepts 2, but the one I bought has one pair of lugs.
 
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Old 02-12-14, 05:03 PM
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The rest will be pulled through ceilings and fed through holes drilled in 2x10s
The "rest" is going to be a cable correct? You can not just pull individual wires in a wall/floor joists. IS this a used heater and do you have a make/model number?
 
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Old 02-12-14, 05:34 PM
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That's the plan yes. I don't have the model info in front of me, but it is a unit produced around the late 80s that's never been installed before. A really nice unit built far better than the new ones. I noticed that the website for the new units shows that you can get optional bus bars to allow powering with a single pair. Is this proper in some areas but not MN?
 
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Old 02-12-14, 05:37 PM
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the one I bought has one pair of lugs.
Check the unit and lugs carefully, many electric heaters and furnaces specify copper only. 20 KW electric furnaces use to come as either a single circuit or two circuit. The single circuit furnaces would need #2 copper conductors with a 110 amp breaker (sometimes 125 amp breaker) and the two circuit furnaces used two 60 amp circuits with #6 conductors.
 
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Old 02-12-14, 05:44 PM
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This specifies #2 CU, but I'm thinking I could use appropriate adapters if I decided to use aluminum.
 
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Old 02-12-14, 05:51 PM
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This specifies #2 CU, but I'm thinking I could use an appropriate adapters if I decided to use aluminum.
You might be able to get away with using aluminum with pin connectors to convert it back to #2 copper for the terminations or even installing a splice box near the end of the circuit and change back to #2 copper.
 
  #9  
Old 02-12-14, 05:52 PM
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Unfortunately I can't seem to find anyone around here that sells sheathed #2 cables in copper.
 
  #10  
Old 02-12-14, 05:57 PM
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Unfortunately I can't seem to find anyone around here that sells sheathed #2 cables in copper.
Try looking for #2 copper SE Style U cable, commonly just called SEU. It'll have two insulated conductors and one bare for the ground.

Copper SE Cable

Or a big box store.

Southwire 500 ft. 2-2-2 CU SEU Service Entry Electrical Cable-13080705 at The Home Depot
 
  #11  
Old 02-12-14, 06:02 PM
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So is it definitely against code to hook this up unless I can modify it to accept 2 circuits? Or is this unit grandfathered in because it was manufactured to be wired with one circuit?
 
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Old 02-12-14, 06:06 PM
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I'd have to research that, can't answer off the top of my head. Seems like I have read that somewhere, but cannot think of a single reason why the code would require a maximum of 60 amps per circuit.
 
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Old 02-12-14, 06:10 PM
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That looks like exactly what I need but nobody around here sells it. If I put in my zip (55353) it shows none of the local HDs sell it. And of course I'd need it by the foot so as not to buy a 500 foot roll. I drove to 5 stores today including 2 supply shops for electricians.

By the way, can SEU be used in ceilings/walls?
 
  #14  
Old 02-12-14, 06:24 PM
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Any good supply house and probably Home Depot can order by the foot, you'd have to ask that question. Actually, I'd expect a supply house to be cheaper. I'd bet that Home Depot would probably find it at another store and order you whatever length you want, but you would have to ask.

By the way, can SEU be used in ceilings/walls?
Yes, it's primarily used for service entrance wiring, but there are also other applications.

•For use in certain branch circuits as permitted by the National Electrical Code
Electric heat was always one of those allowed circuits. As far as I know, that use has not changed.
 
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Old 02-12-14, 06:34 PM
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This might be a handy read: Article 424: Fixed Electric Space Heating | Code Basics content from Electrical Construction & Maintenance (EC&M) Magazine

Article 424.22 Overcurrent protection (B) Resistance elements. Resistance type heating elements in electric space equipment shall be protected at not more than 60 amperes.

This is the section I saw it. (went out to the truck ) This could apply to the manufacturer of the unit as well. I would suspect your heater has some breakers/fuse built into the heater so this might not be an issue.

You will have to go to an electrical supply house to get #2/2 w/Ground NM-b. However you will need #1 copper if you are going to use NM-b as #2 is only rated 95 amps @ 60 degree.
 
  #16  
Old 02-12-14, 06:38 PM
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Yeah, I'm going to need to make some more calls tomorrow. Thanks for the advice!
 
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Old 02-12-14, 06:50 PM
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Article 424.22 Overcurrent protection (B) Resistance elements. Resistance type heating elements in electric space equipment shall be protected at not more than 60 amperes.
Thanks Tolyn. I thought I had read that somewhere, but it's not an issue that comes up often with the cost of electric heat being so high. Yes, most electric furnaces I have seen over 10 KW break the feed circuit down into smaller circuits within the unit with fuses. Interesting article.
 
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Old 02-12-14, 06:55 PM
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Food for thought: Since you might need to install a disconnect anyway, you could run aluminum from the panel to the disconnect, then run copper from the disconnect to the heater.
 
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Old 02-12-14, 10:50 PM
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You could also run Al to the disconnect like Tolyn said, then run THHN in conduit to the heater.
 
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Old 02-13-14, 01:04 AM
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Resistance heating units are actually limited to 48 amperes. When the 125% rule is factored in the maximum circuit Ampacity is 60 amperes. This applies regardless of the applied voltage.
 
  #21  
Old 02-13-14, 09:17 AM
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Okay, so lets see if I've got this right:

Resistance heaters must be protected by at most a 60A OCPD. But this requirement can be fulfilled within the unit itself if groups of elements totaling no more than 48A are each protected by a 60A breaker built into the unit. (Mine is like this...2 60A breakers built in, each protecting 42A loads)

Now if I decided to run a single feed for the 84A, I'd need a 125A breaker. [This could someday provide an additional 16A for another device... I'd need to place a subpanel and break out separate circuits for the plenum heater and the additional heater]

Because of the 125A breaker I need cables with ampacity of at least 125A based on the 60 degree column. I come up with 1/0 copper, or 3/0 aluminum.

If I were to run two separate 60A feeds [and forget about a 2nd future heater] I'd need #4 copper, or #3 aluminum.

I'd be using SEU cables, and voltage drop over 65 feet should not be a factor for any of these gauges.

These gauges seem excessive to me, especially considering that the unit diagrams call for a single feed of #2 CU [60 deg column seems to show 1/0]. Are you sure I need to use the 60 degree column? Please let me know if I'm screwing up these calculations. Thanks for all the advice so far everyone!
 
  #22  
Old 02-13-14, 05:33 PM
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Since the unit has two 60 amp breakers factory installed, if it can be wired as a two circuit unit I'd use two 60 amp circuits each installed with 6-2 NM-B cable (aka romex).
 
  #23  
Old 02-13-14, 06:01 PM
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I would not be concerned with voltage drop with an electric heater that has no motor. The #6 Joe posted would be fine. The 60 degree column must be used for any non-metallic cable (NM-b, SE)

BTW - There is a Viking Electric Supply in Plymouth that can get anything you need. They will sell to the public.
 
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