Connecting multiple sub panels

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Old 10-19-14, 08:49 AM
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Question Connecting multiple sub panels

I have 2 out buildings I am chaining together from my main (so, main, to panel 1, and panel 1 to panel 2)

Load requirements are as such:
Panel 1:
20 amp 240v
20 amp 120v
15 amp 120v

Panel 2:
20 amp 240v
20 amp 120v
20 amp 120v

Questions are:
  1. Size of wire to use on each leg (main to p1 to p2)?
  2. What size main/feed breaker is needed at each sub panel?
  3. Is it best to use 2 black for hot wires, or black and red?
  4. Should I use larger than 60 amp sub panels despite not needing many slots?
  5. Does panel 1 need to handle 120amp, or is there some way to wire this to pass through to panel 2 without load in p1?
  6. How to wire each panel?

My main box has space remaining for a large breaker to feed all this, and I plan to run THHN underground in pvc conduit, 18 to 24" deep.

My understanding is that I need 60 amps at each panel, and I will need un bonded neutrals at each panel, and each having their own ground with 2 rods ea. As far as feeding panel 2 from panel one, I believe I am to feed panel 2 from a 60 amp breaker in panel 1 (or is there a better way, via feed through?).

I'd love some community support to fill in the gaps here. I've been pouring over NEC docs and forums to make sure I land this right.

Also curious how my requirements might change if I only need 30amp 120v at panel/building 1, i.e. two 15amp 120v.
 
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Old 10-19-14, 09:39 AM
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What do you mean by "load requirements"? Do you mean actual full load amps of the equipment to be used or the size breakers in the panel?
 

Last edited by ray2047; 10-19-14 at 10:57 AM.
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Old 10-19-14, 10:19 AM
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I am mainly trying to determine the size of wire required and yes, the size of the main breakers to feed the panels. The breakers for the anticipated equipment at each panel are what I listed out.

My main lack of understanding at the moment is how much load on panel 1 affects panel 2. E.g. If p1 has a 60amp double breaker feeding it, and dedicates 30amp on each leg to that building, does that mean p2 will be fed only 30amps via double breaker?
 

Last edited by Dapug; 10-19-14 at 10:41 AM.
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Old 10-19-14, 10:34 AM
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Totaling breaker handles does not reflect the loads actually on the circuits. A demand load calculation is the correct method.
 
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Old 10-19-14, 10:43 AM
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The 240v 20 amp Branch-Circuits suggests a specific type of load such as a electric-heat load or a cooling load, i.e., a AC unit.

Could you please describe the specific 240v loads , and any other specific 120 volt loads , such as a refrigerator, if any .
 
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Old 10-19-14, 11:32 AM
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Cool, here is some added detail on what I am really planning, more fun!

Building/panel 1 is a shed with:
  1. 2 lights, 200w max (120v)
  2. Table saw, air compressor, hand tools (up to 15a each, planning on 20amp circuit for these, require only 1 tool at a time, 2 is a bonus)
  3. 240v 16a mig welder ( optional, depending on budget, wire sizes, etc)

Building/panel 2 is a greenhouse with:
  1. 4000w HPS lighting (240v)
  2. 1500w water heater (120v)
  3. 1 gfi circuit for general power outlets (pumps, computers, 120v maybe 800w max)
  4. 1 circuit for general flouro lights, exhaust fan, (120v, 600w max)
 
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Old 10-19-14, 12:32 PM
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Run 60 amps to subpanel 1. From subpanel 2 run a 30 amp feed.

Assuming less than 100 amps.
60amp feed: Two #6 black, one #6 white, and one #10 green in PVC conduit. Or direct burial: 6-3 UF-b on 55 amp breaker.( If no 55 amp breaker for your panel a 60 amp breaker may be used).
30 amp feed: Two black #10, 0ne white #10, one green #10. Or direct burial UF-b 10-3 on 30 amp breaker.

On UF-b best practice is to not use conduit except for sleeves to protect it where needed such as where it enters and leaves the ground. If using individual conductors such a THWN in conduit use 1" for the 60 amp feed and " for the 30 amp feed. (Smaller may be code compliant but larger makes pulling easier.) Burial depth for conduit is 18" minimum, cable 24" minimum.

Usual practice in both cases is to use a 100 amp main breaker panel for each subpanel. (Main breaker only used as a disconnect.) Feed wires may need to be pigtailed to larger wires to meet minimum wire size of main breaker. (Back fed main lug panel could also be used but a Main breaker panel kit which includes some breakers is usually cheaper.) You will need to add a ground bar to each panel and drive an 8' ground rod at each panel. Neutral bars are isolated, ground bars bonded.
 

Last edited by ray2047; 10-19-14 at 12:48 PM.
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Old 10-19-14, 12:51 PM
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Ray, super helpful, thanks.

Your wire sizing is right where I was estimating (except I was thinking run #8 to that 2nd panel), and the details on the paneling give me something to think about (I hadn't considered 100a panels but this makes sense).
 
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Old 10-19-14, 12:58 PM
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!00 amp gives you the spaces you need. Probably a 12 space panel (or 20). You won't have that many spaces in a 60 amp panel.
I was thinking run #8 to that 2nd panel
And I was on the fence whether to suggest a 40 amp feed. No harm if you want to.
 
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