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Guage selection for additional sub-panel

1K views 12 replies 8 participants last post by  AllanJ 
#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.
 
#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.
 
#5 ·
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.
 
#6 ·
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.
 
#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?
 
#8 · (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....

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.
 
#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.)
 
#13 · (Edited)
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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