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Old 06-16-2017, 07:44 PM   #1
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Advice requested on small subpanel


I would like to re-purpose a 30A (240V) circuit. It currently serves a defunct heat pump that is a long way from the main panel. We would like to replace the dead heat pump with some mini-split units in key rooms at that end of the house.

In order to connect a couple of 15A or 20A AC units to that 30A circuit, I assume I will need to install a sub-panel with appropriate circuit breakers. Can I use a 15A and a 20A circuit in this panel? That adds up to 35A, but these are high efficiency units that don't usually use their full capacity. It's likely to be very rare that they would exceed 30A total. Can the sum of the potential loads exceed the 30A supply breaker and by how much?

The existing cable is 10-2 w/ground. Normally you need a separate common and ground to a sub-panel. If I use 240V mini-splits (I've checked and they are probably the ones I want, anyway.) Can I just use the existing cable and do away with the common in my sub-panel? I won't be using it. I would, of course, still use the ground!

Assuming this is OK, does it require a special sub-panel or can I just use any small sub-panel and not wire in a common leg? What exactly am I looking for in a sub-panel and am I likely to find it at Home Depot or will I need to go to an electrical supplier?

I am fortunate that this cable runs through an unfinished room. That will make installing the panel easy. I would then need to run the supply wires to the AC units underneath the house. I've done a lot of household and farm wiring, but have never run wires beneath a house. Is conduit required for this? Are there any other pertinent rules that are different from running wires inside of walls or in a ceiling?

Thank you!
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Old 06-17-2017, 03:52 AM   #2
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Re: Advice requested on small subpanel


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It's likely to be very rare that they would exceed 30A total. Can the sum of the potential loads exceed the 30A supply breaker and by how much?
Short answer: No. The feeder needs to supply 100% of the load in this case.
But the load isn't the sum of the breakers. You need to find the exact load from the nameplates or installation data of both split units.

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Can I just use the existing cable and do away with the common in my sub-panel? I won't be using it. I would, of course, still use the ground!
Yes, just mark the existing white conductor with electrical tape on both ends.

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Assuming this is OK, does it require a special sub-panel or can I just use any small sub-panel and not wire in a common leg? What exactly am I looking for in a sub-panel and am I likely to find it at Home Depot or will I need to go to an electrical supplier?
You just need any small panel that will physically fit in your desired location and hold the two double pole circuit breakers. You'll find several at any given home center or hardware store. You'll likely also need to pick up a matching ground bus kit.

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I would then need to run the supply wires to the AC units underneath the house. I've done a lot of household and farm wiring, but have never run wires beneath a house. Is conduit required for this? Are there any other pertinent rules that are different from running wires inside of walls or in a ceiling?
Conduit under the house is rarely necessary. But there are other considerations. Will this wiring under the house be in a basement or a crawlspace? If a crawlspace, how damp is it? Will you be drilling through the floor joists or running under them?
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Old 06-17-2017, 01:28 PM   #3
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Re: Advice requested on small subpanel


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Short answer: No. The feeder needs to supply 100% of the load in this case.
But the load isn't the sum of the breakers. You need to find the exact load from the nameplates or installation data of both split units.
Thank you. Yes, I do realize the actual load cannot exceed the 30A. In fact, I'm hoping I either have or can get a 100% continuous load 30A breaker.

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Conduit under the house is rarely necessary. But there are other considerations. Will this wiring under the house be in a basement or a crawlspace? If a crawlspace, how damp is it? Will you be drilling through the floor joists or running under them?
It will go through a crawlspace in southern CA where it is generally pretty dry. I would hang it below the joists as there is insulation between them.

Thank you very much for your other answers. With temps in the high 90s predicted this week, I only wish I'd begun this a month ago! :-)
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Old 06-17-2017, 02:28 PM   #4
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Re: Advice requested on small subpanel


If you are running from joist to joist in the crawl space, you will need to install running boards when running cables smaller than 6/2 or 8/3.
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Old 06-17-2017, 02:50 PM   #5
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Re: Advice requested on small subpanel


You realize that you are wiring the sub for 240 only, you can't use it for 120 volt loads. You might want to run that by your local inspector as some might have a dim view. Problem being that down the road someone may connect a 120 volt load and use the ground for the neutral. After all they connect too the same place in the other panel
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Old 06-17-2017, 03:12 PM   #6
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Re: Advice requested on small subpanel


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You realize that you are wiring the sub for 240 only, you can't use it for 120 volt loads.
Yeah, I realize this. I don't care if I can never use it for 120V because it will probably be at full capacity with the AC units on it. I plan to get it permitted and inspected. Hopefully the inspector will agree. If someone in the future adds a circuit and doesn't get it permitted and inspected, I doubt that I will be liable.
But this is one reason I was wondering if there was a special box for this purpose. Lawyers being what they are, a box with the common bus removed would probably be completely different than a box with the common bus removed and placarded for 240V only.
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Old 06-17-2017, 03:15 PM   #7
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Re: Advice requested on small subpanel


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If you are running from joist to joist in the crawl space, you will need to install running boards when running cables smaller than 6/2 or 8/3.
Can you explain running boards?
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Old 06-17-2017, 04:03 PM   #8
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Re: Advice requested on small subpanel


Here is a image of the two methods approved for running small cables in an unfinished basement or crawl space. Since you mentioned insulation in place, I did not mention the thru the joist method.
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Old 06-17-2017, 05:51 PM   #9
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Re: Advice requested on small subpanel


Oh, that makes sense. It must be to keep the cables from being pulled back and forth through the staples as people walk around on the flexible floor above.
Thanks.
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Old 06-19-2017, 03:29 AM   #10
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Re: Advice requested on small subpanel


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In fact, I'm hoping I either have or can get a 100% continuous load 30A breaker.
There's no need for that. The feeder simply needs to have the capacity for both AC loads at the same time. While calculating the load of a feeder you can apply a demand factor which will decrease the required capacity. But your loads are too few and too small to do this, so the feeder must be capable of supplying them both. That's all I meant by supplying 100% of the load.

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It must be to keep the cables from being pulled back and forth through the staples as people walk around on the flexible floor above.
It's to protect the wiring if struck and to keep people from hanging things on it. It doesn't make much sense in a crawlspace, but I suppose people do use the area for storage sometimes. Running boards were previously only required in unfinished basements.
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Old 06-19-2017, 03:32 PM   #11
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Re: Advice requested on small subpanel


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There's no need for that. The feeder simply needs to have the capacity for both AC loads at the same time. While calculating the load of a feeder you can apply a demand factor which will decrease the required capacity. But your loads are too few and too small to do this, so the feeder must be capable of supplying them both. That's all I meant by supplying 100% of the load.
My long term plan is to eventually scatter minisplits throughout most of the house. While each one will have a smaller load, I can see them all adding up to close to 30A. Since a "normal" breaker is rated for 80% of the 30 amps at a continous load, then it seems like the 30A breaker could pop occasionally on really hot days when lots of the units are running. It wouldn't be often - the whole point of the minisplits is to only heat or cool the rooms where it is needed. But it seems like putting in a 30A continuously rated breaker would eliminate that occasional hassle. Is there some reason not to use a continuously rated breaker?
And is the 10 guage wire I've got adequate for a 30A continuous load? I know it's right for a "normal" 30A breaker. But the point of a breaker is to protect the wire, so I assume a 30A breaker that is rated at 80% continuous load (24A) is protecting the "normal" 10 guage wire from a continuous load above 24A.
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Old 06-23-2017, 04:16 AM   #12
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Re: Advice requested on small subpanel


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But it seems like putting in a 30A continuously rated breaker would eliminate that occasional hassle. Is there some reason not to use a continuously rated breaker?
There no reason you couldn't use one. But air conditioners are not continuous loads.

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And is the 10 guage wire I've got adequate for a 30A continuous load? I know it's right for a "normal" 30A breaker. But the point of a breaker is to protect the wire, so I assume a 30A breaker that is rated at 80% continuous load (24A) is protecting the "normal" 10 guage wire from a continuous load above 24A.
Yes, the conductors are more than adequate. It's only the breaker which can be problematic.
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Old 06-23-2017, 11:10 AM   #13
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Re: Advice requested on small subpanel


So it seems the thing to do is use a regular 30A breaker and if it proves inadequate, switch it to a continuously rated 30A breaker.

Thanks very much for all of the advice.
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