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Hello everyone. First time poster here.
I am planning an electrical project to add a 240v/120amp sub-panel to a shop and I'm looking for some guidance for what gauges of cable I will need. I have been trying to understand the formulas and tables in the NEC but i hasn't clicked for me yet.

A little about the project, we are planning to install a pool and need to run a 240v circuit for the pump, chlorinator, lights, and eventually maybe a heater. If we got everything, the pool equipment will probably consume 70+ amps. I also have a small workshop I want to upgrade. It has a 110v/15 amp line run to it with only a GFCI which I trip often. I want to add some more capacity including a 220v circuit so i can run a welder and maybe add a mini-split. It was suggested by the pool company we should overbuild a little and plan for 120 amps.

My thought then was to run a sub-panel to the shop from the main house breaker which has 200 amps and several open slots. To cut down on labor I wanted to run most of the service through the attic, about 50 feet worth, and then down the outside wall and underground with a conduit or direct bury. I will need to bury a 14 foot run between the house and shop if i can go with the shortest distance. The alternative is a 100 foot direct bury from the meter on the outside of the house.

Can anyone advise what type and gauge cable i would need? Does it all have to be the same kind or can I splice direct bury with whatever would typically be used inside of a house? Should i use aluminum or copper? I have discussed this plan with the pool company's electrician who has yet to give me an estimate for the work. Since this would be over an above what was originally quoted for the pool I am looking for ways i can offset some of the costs.

I appreciate any constructive feedback. Thanks.
 

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Red Seal Electrician
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NEC isn't my Code, but in general... aluminum, direct-burial cable save cost.
 

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Maintenance / Locksmith
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What kind of load do you presently have on the house? I trust you are not an all electric house? What kind of pool equipment will utilize 70 amps..........must be some kind of massive expansion or an electric pool heater?


If you sub panel off 120 amps then you may be leaving the house short?


What kind of panel and model number do you have as panel may not support more than 100amp breaker.



We need a lot more info to make recommendations. You probably should have your electrician making all the recommendations for you and doing the work.
 

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For 120A you will want 1/0 AWG aluminum, with a ground wire #4AL or larger, or #6Cu or larger. You can use copper if your last name is Kennecott, but then you create a dissimilar metal problem since the subpanel lugs are aluminum.

You won't find a 120A breaker so you can round up to 125A.

How do you feel about laying conduit the whole way between panels? If you do, you can use 1-1/2" conduit. If you use EMT in the house and Rigid outside, you can delete the ground wire because the pipe is the ground. Rigid is very expensive but you're not going far at all, and I really like the part where you can trench it with a garden trowel. (it only needs 6" of cover above it).

You could also use conduit outside (Rigid or PVC) and then cable inside the house, but then you'll need a (fairly large) junction box to do splices, and four $15-ish Polaris connectors for splicing.

You could also do the whole run in cable but you must then have 26" of burial depth (for 24" cover) and you have to choose the right kind of cable that's legal both underground and in an attic.
 

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... I have discussed this plan with the pool company's electrician who has yet to give me an estimate for the work. Since this would be over an above what was originally quoted for the pool I am looking for ways i can offset some of the costs...

I would highly suggest that you keep the pool company's electrician in the loop on this big project. What I mean by this is if you end up doing some of the work yourself (let's say to cut labor costs) the pool electrician may not want to utilize what you have installed or even use the materials you may have purchased. It is a matter of liability on the electrician's end.


I would suggest that you discuss your wants/needs above and beyond what is needed for the pool he is to hook up electrically. Many electricians will not pick up where a home owner started the electrical work and you may end up finding yourself in a jam spending money on materials and installing them just to find that you can't get an electrician to finish the work or if you end up in a bind and you need an electrician to finish.


This is a big mistake many home owners make. Be careful and again discuss your needs with the electrician. Also, if you do the electric for the pool this may void the warranty on the pool etc.
 
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