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Old 08-19-2008, 06:15 PM   #1
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First post... Hello all :D

I recently had a heat pump system installed, and I'm now planning on doing some rewiring. I want to feed the entire HVAC system from a sub panel, and install a power meter inline so I can monitor total electrical usage for the system. I am planning on going through the permit and inspection process (for the first time), so I need to make sure my duckies are in a row.

The question I have right now is, what size cable do I need to feed the panel with? The feed from the main panel is under 20 feet long, and the feed breaker will be 80 amps (HP breaker is 20, furnace breaker is 60).

Another question I have is, with the sub panel that close to the main, is there any reason to ground it separately? Is a simple three conductor cable right for the job?

More generally, is there a simple method to determine cable sizes? It seems simple enough when the inspector looks at his little chart and comes up with a number, but I've never found a chart that seems to agree with anything I've been told


More info... The cable to the panel will be run at least partly through steel indoor conduit.


Last edited by gp_wa; 08-19-2008 at 06:26 PM.
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Old 08-19-2008, 06:24 PM   #2
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For actual loads you would do a load calculation. These numbers are rarely known for sub-panel so we typically go as large as possible.
You DO NOT add up the breaker numbers to get the load. Your 20 plus 60 does not equal 80.
If all you will ever have in this panel is this HVAC stuff then you know what your load is.

Grounding is a very complicated and confusing issue for many folks.
All I will say is you DO have to run a ground with the feeder to the sub-panel. You must not install the bond screw/strap in the sub-panel and the grounds and neutrals must be kept isolated. More than likely you will need to buy and install an add-on ground bar.

If I were you I would run #2al or #4cu to the sub-panel using a 100A breaker in the main. If your AHJs will not allow the 100A breaker use a 90A. For what you are doing there is really no reason to go less or smaller.

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Old 08-19-2008, 06:31 PM   #3
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Quote:
Originally Posted by Speedy Petey View Post
If all you will ever have in this panel is this HVAC stuff then you know what your load is.
I may add more, but no immediate plans. The condensate pump will also be attached to this panel, but that's a minuscule load.

Quote:
Originally Posted by Speedy Petey View Post
If I were you I would run #2al or #4cu to the sub-panel using a 100A breaker in the main. If your AHJs will not allow the 100A breaker use a 90A. For what you are doing there is really no reason to go less or smaller.
100 AMP? That's not what I expected... Why would I need a bigger breaker to the feed than the total of all the breakers in the sub? That's moderately confusing.

The actual maximum load is closer to 60 amps. The 60 and 20 amp breakers are the recommended sizes for the HVAC equipment. Minimum ampacity for the furnace is about 50, and 15 for the HP.
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Old 08-19-2008, 06:35 PM   #4
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Quote:
Originally Posted by Speedy Petey View Post
If I were you I would run #2al or #4cu to the sub-panel using a 100A breaker in the main. If your AHJs will not allow the 100A breaker use a 90A. For what you are doing there is really no reason to go less or smaller.
Does your AHJ let you get away with this?

I'd probably feed the sub with #2 AL SER cable and put it on an 80 amp breaker as long as the connected load is less than 75 amps.
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Old 08-19-2008, 06:52 PM   #5
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Does your AHJ let you get away with this?

I'd probably feed the sub with #2 AL SER cable and put it on an 80 amp breaker as long as the connected load is less than 75 amps.
A 90 is legal... why use an 80?
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Old 08-19-2008, 07:15 PM   #6
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Quote:
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Does your AHJ let you get away with this?
"Get away with" WHAT?
What I propose is perfectly legal (in my area) and safe.
If your AHJ is opposed to a 100 a 90 is fine.

And no, I don't feel like getting into this discussion again.
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Last edited by Speedy Petey; 08-19-2008 at 07:19 PM. Reason: typo
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Old 08-19-2008, 07:18 PM   #7
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Originally Posted by gp_wa View Post
100 AMP? That's not what I expected... Why would I need a bigger breaker to the feed than the total of all the breakers in the sub? That's moderately confusing.
Why not go 100 or 90? #2al or #4cu can handle it.

#6CU is only good to 70A so you would be at full capacity with just the immediate loads you propose.
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Old 08-19-2008, 07:18 PM   #8
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Quote:
Originally Posted by Speedy Petey View Post
"Get away with" WHAT?
What I propose is perfectly legal (in my area) and safe.
If your AHJ opposes to a 100 a 90 is fine.

And no, I don't feel like getting into this discussion again.

I believe its a safe installation also, I've done that same install for years.
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Old 08-19-2008, 07:52 PM   #9
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Is it correct that you cannot put a breaker on a circuit that is larger than the circuit cabling can handle, regardless of actual running load?
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Old 08-19-2008, 08:02 PM   #10
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Is it correct that you cannot put a breaker on a circuit that is larger than the circuit cabling can handle, regardless of actual running load?
Depends, there are situations where it is legal.
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Old 08-19-2008, 08:13 PM   #11
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Damnit, this is supposed to be simple
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Old 08-19-2008, 08:13 PM   #12
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Depends, there are situations where it is legal.
just curious, in which circumstances would it be legal to do so?
-thanks
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Old 08-19-2008, 08:15 PM   #13
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Quote:
Originally Posted by I_think_I_conduit View Post
just curious, in which circumstances would it be legal to do so?
-thanks
Air Conditioning is a big one, many times I've run 14 AWG backed with a 30 amp breaker.
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Old 08-19-2008, 08:16 PM   #14
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Air Conditioning is a big one, many times I've run 14 AWG backed with a 30 amp breaker.
What makes that application special in this regard?
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Old 08-19-2008, 08:18 PM   #15
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What makes that application special in this regard?

The overcurrent protection is built into the motor, all i'm providing with a breaker is short circuit and ground fault protection.


Here is a picture to get an idea what i'm talking about....

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Last edited by chris75; 08-19-2008 at 08:21 PM.
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