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.
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.
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?
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.
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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