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Discussion Starter · #1 ·
Hello folks, have a question for those electrical pros out there.

I have a 2-phase 200amp main service with an 8/4 run of about 50ft to a sub-panel in my garage on a 2-pole 40amp breaker. Yes, ground/neutral are split properly.

Question is, if I wanted to extend from this panel to a pool house we are planning about 50ft from that sub-panel what is the right ga wire/breaker to use. I'm not sure if when calculating you have to factor the entire run from the main or just from the sub panel. I would assume a smaller 2-pole breaker regardless, but I'd like to verify before I buy anything.

I could also pull back the 8/4 and replace it with 6/4 on a 55 amp breaker at the sub. Then maybe I could re-use the 8/4 to the new pool house panel on a 40amp? Thankfully I put it in a 3" conduit so it would be easy enough to do if necessary.

Please advise.
 

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Discussion Starter · #3 ·
Ideally 50a, but 40a should be fine. I can’t think of anything larger than a pool pump/heater needing more than a 2pole 30a, and all the rest would be just lights and a few basic outlets.

I guess my question is in how it’s calculated. The short answer would be how much can I do without changing the existing wire/breaker and extending from that sub-panel. Obviously I can redo to get more.
 

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Discussion Starter · #7 ·
The ground from the main to the sub panel is insulated and in a code approved conduit. I'll be having the work inspected once completed. I appreciate the concern.

As for total amperage required, 30a includes everything planned, calculated by adding up the max wattage's of all the intended equipment. I would like to get 50a for some extra wiggle room and future expansion, but 40a would suffice if that does not require re-running the original sub panel feed.

Back to the original question please. How is the correct gauge calculated to an additional sub panel, from the connecting sub panel, or from the main panel, including the distance of the sub panel?
 

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Discussion Starter · #9 · (Edited)
Zach... Confused on your question. The wire guage is controlled by the initiating breaker that is protecting it.

But maybe you are asking about how to calc your expected voltage drop, and whether you have to upsize your feeder.

When you have mixed feeder sizes, I don't know how to calc/estimate final voltage drop.... Good question....
Yes, this is what I'm asking. Voltage drops based on the resistivity of the wire it passes through and the total length of run to its destination. In this case, there is 50ft of 6ga (Just double checked) to a sub panel off a 50a breaker.

To the individual that said gauge is determined by the breaker, that is not entirely accurate. You can put the wrong size gauge on a short run breaker and it will work. (you should not, but you CAN).

I'm asking if the recommended gauge rating of a wire is affected if being fed from a sub panel instead of a main panel. The question itself seems straight forward, but admittedly the caclucation to get to the answer seems perplexing.

feeding from a 50a breaker for 50ft rates 8ga wire. BUT if you ad another 50ft to that, assuming the load at the current panel is zero most of the time, does that new load get calculated to include the feeding distance. In this case, 100ft with a 50a breaker in the middle.

I'm guessing common sense says it does, but since it's AC and not DC, technically the electrons aren't actually traveling that total distance, but they are impeded by it, hence the voltage drop calculation. So I'm not sure.

So if no one can answer this, just say so and I'll stop asking. Otherwise, please offer a solution.
 

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Discussion Starter · #11 ·
Dilly Dilly.

I should be ok with 8ga for the second 50ft if I'm only going to feed that with a 40a breaker then.

Looking at the whole thing, it will be:

200A MP - 50A -> 50ft/6ga -> Sub -40A -> 50ft/8ga -> Sub.

Look good? (Everything buried in conduit with appropriate cable and shielded ground.)
 
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