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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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Remodel and New Build GC
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What A's do you want at the pool house.... that's the first issue, then length of run will determine if you need bigger wire to mitigate V drop.
 

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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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flipping slumlord
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I have a 240V 200amp main service with an 8/4 run of about 50ft to a sub-panel in my garage on a 2-pole 40amp breaker.
Remember when you were advised to use a heavier wire... just in case?

(now) I want to extend from this panel to a pool house
How much more power do you need?

Ideally 50a, but 40a should be fine.
Thankfully I put it in a 3" conduit...
Do a complete load calculation this time so you KNOW how much power
is needed and then upsize that by 20% or more.
---

The short answer is to pull back the #8 wires and re-feed the garage panel.
You might be able to reuse the #8 for the sub-sub feed out to the pool.
 

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There is a big difference between running power to a pool house and running power to power pool equipment.

Article 680 changes some of the practices that you might be used to. Mandating insulated grounds is one of them.

I really think you should hire an electrician for this. What would be a minor mistake inside the home can kill someone when a pool is involved.

Are you pulling permits for this work and having it inspected.
 

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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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Remodel and New Build GC
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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?
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....

EDIT: But, it is never wrong that I know of, to upsize wire guage feed... apart from the issues of conduit size and your pocket book.
 

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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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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.
Seems like you pretty much know everything, but for your distance I would account for the entire length. Do your voltage drop on the total distance.

Electrons travel at the same rate, whether you excite them with AC or DC potential, so no need to be confused between the 2.
 

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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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Very Stable Genius
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Online wire guage resistance gave me:
0.04ohms for 100' of #6
0.062ohms for 100' of #8
Using ohms law and 80% of each breakers rating gave me Vd of:
1.6V
1.984V
Combine these two:
3.584V

At 240V that's 1.5%Vd. Well within requirements of 3% for feeders etc and
5% total.
 

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Compute the voltage drop from the garage to the pool house for the pool house load (in amperes).

Compute the voltage drop from the house panel to the garage for the combined load for both the garage and the pool house.

Add the two together, should not exceed 3 percent.

There is a third number, the voltage drop for the span from the utility pole transformer to the house main panel for the combined load for the main house, the garage, and the pool house. All 3 numbers should add up to no more tha 5 (percent). This number is more difficult to compute so it is easier to just worry about the other two which should add up to no more than 3 (percent).
 
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