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Hi all! Thank you in advance for your experience and time! OK, I have searched alot of places and have gathered some information, but, I am somewhat confused. I just built a workshop behind my house. I have a 200 amp panel in my house , and I want to feed my sub panel from the main house panel. I am going to run pvc conduit 18" below to the workshop. I know I have to isolate the equipment and bus bars. I plan on running 3 ,
15 amp circuts and 1 20amp circut. One lights, one dedicated for my coffee roaster (draws 7amps) , one for my computer , and the 20amp for general use. What I need to be sure of is , 1. the size of the breaker I should use in my main panel. 2. The size of the wire to the sub panel. And 3. the size of the sub panel I should install. I need a panel with a main breaker in it, not just lugs. Some of the forums I have searched with similar questions to mine, the expert tells the homeowner to use 4 wire. 2 hots , a neutral and a bare ground , and other experts tell the homeowner to use 3 wire with one hot. This is what is smoking my brain! What is the difference and which wire should I use? I have alot of room in my main panel, and I draw about 100 amps. I have a small rancher house. Any help will be awesome! Thanks, Brian
I'm not necessarily going to answer your questions directly, but hopefully will provide some insight and make it possible to converse intellegently.
1. Counting the circuit breaker sizes and adding them up to arrive at the draw of the main panel is not correct. (If I'm assuming wrong, tell us)
2. Detached structure/attached structure makes a difference.
3. Wire length makes a difference.
4. Neutral and ground must be isolated in sub-panel.
5. A main breaker in the sub-panel can be higher or the same rating as the feed in the main panel when the main breaker of the sub-panel is used as a local disconnect.
6. Sub-panel rating can be more than breaker feeding it. e.g. A 200 Amp sub-panel can be fed from a 60 Amp breaker with a 200 A main breaker in the sub-panel, but you'd still have a 60 Amp sub-panel.
7. Sub-panel should anticipate additional loads.
8. The sub-panel feed breaker in the main panel must be sized primarily for the wire.
9. Voltage drop calculations need to be done and cost depends on whether copper or aluminum wire is used.
10. Loads on for more than 3 hours at a time are calculated at 125%.
11. There is always a problem where ground is generally assumed in the trade, thus 12/2 is 2 conductors of #12 with a ground. This is best described as 3 pole, 4 wire. Poles carry current.
12. The ground conductor from the main can usually be a smaller wire.
13. Running 240 to the sub-panel makes sense, even though all your circuits are 120. Four wires will need to be run. 2 hots, 1 neutral and ground. IF and only IF the structure is detached a ground rod and ground must be provided for the sub-panel.
14. So you have three 15's and one 20 amp circuit. Lets distribute that as 15+20 and 15+15, so that's a 30A circuit for one HOT and a 45 Amp circuit for the other hot, so why not use a 60 Amp sub. Take 45 Amps and size up and size the wiring accordingly and actually you would only be allowed to draw 0.8*60 or about 48A of 120 Volts CONTINUOUSLY from each HOT. Your going to be a lot less than that and you'll have some ability to add a few circuits if there are some breaker spaces left.
15. But there may not be any real reason to not run them all as 20 amp circuits and wire with #12 wire. The loads matter and the length of time they are on matter.
16. It's always a good idea to put lighting separately with redundant lights. Redundant circuits better, but at least two lighting fixtures, so that if your using a power tool in the garage and you pop a breaker, you will still be able to get around because you will have light. If a single light popped and you were using a power tool, you'd be left in the dark unless you had two fixtures and only the light burnt out.
17. Neutral carries the difference current on the two hots.
Ok, I understand for the most part. I have allready purchased 14/2 (250') and started to wire the shop. I have no problem ripping this out and making all circuts 12/2. I know that I can still use the 15amp recepticals that I bought. New light switches. Can I still use the 15amp ground fault recepticals? Next, assuming that I follow your advice to make all 4 circuts 20amp, should I still install a 60amp sub panel? I do believe it's a good idea to run all 20's. I will be working in the shop 8 hours a day.
So, what I don't understand is , if I run 240v to my sub panel, drawing 48amps from each leg, can a 60amp sub panel handle that? To me , that is potentially 100amps going to a 60amp panel. Or can a 60amp panel handle 60amps per side? I like the way you answer my question because it's making me think. It's important for me to understand the mechanics of the path.
This is what I think you are saying, based on 4 20amp circuts. Tell me if I'm wrong!!
1. Two 40amp breakers in my main. Double pole.
2. 4 #6 wires from the main to the sub. It is about 75' from main to sub.
3. Install sub panel ???amps with isolated neutral and ground bus bars.
4. Install two atleast 40amp mains in sub panel.
5. Run a #6 bare copper ground no more than 10' from the panel to a 10' ground rod pounded in a couple of inches below the surface, with a direct burial clamp.
6. Wire building with circuts.
7.Roast coffee and enjoy!
Please let me know how the amp ratings of panels are based. This is the confusing part for me right now. Also please recommend a panel for me.
Thanks for your help! Brian
1) Use a 2-pole 50 or 60. If you are using #6 wire all the way then why limit yourself to 40 amps?
2) 75' is nothing. Use the #6 and be happy. Run in conduit the whole way if you can. If not then you MUST change over to cable. This can be more of a hassle than running conduit sometimes.
3) I would use a 12 to 20 circuit MAIN BREAKER panel for your sub. There MUST be a disconnect at a remote structure. A main breaker is the cheapest and easiest way.
4) Again, use a main breaker. 100 amp is FINE. This is ONLY serving as a main shutoff. It is NOT being used as overcurrent protection. The breaker in the main panel serves that purpose.
5) Yes, but there is NO 10' restriction. Where did you get that information?
6) I would keep the 14/2 in place for the lighting. NO reason to use #12 for that. You also thank yourself later for it.
Use #12 for all the regular receptacles though. 15A receptacles and GFIs are FINE.
OK, I understand even more now! But, (there is always a but!) I am now confused about the difference between wire and cable? I have a straight shot from my main to my sub. The terrain is grass, so installing conduit is my plan. What is the ideal feeder? Wire/cable?? Thanks, Brian
The 10' foot from the panel is just something that I thought I remembered from when I wired my addition. I obviously forgot what I thought I remembered!
Distilled my suggestion, but didn't quite make you understand it.
I'm going to take a couple of your topics:
A: "Use the GFCI outets I got":
1. Why not use GFCI breakers?
2. In many cases, the first GFCI has less of a rating than the rest of the string. e.g. For a 20 amp GFCI circuit, the GFCI outllet may be OK for 15 Amps, but the rest of the circuit can be rated for 20 amps. See the literature that came with the GFCI.
3. If the GFCI is for outside, then having the rest at the panel may be desireable.
B. Use the 15 Amp outlets, well they are about a buck a piece. You can use a 20 amp breaker, #12 wire and 15 amp recepticles, but they are a buck each. Take more labor to change them to 20 A recept's later.
C. Isolated neutral in the sub-panel. Not isolated ground.
In the context it's a 240 V/120V single phase 60 Amp sub-panel.
This is supplied by a 60 amp 240 volt circuit.
This means that you have 60 amp circuit of 240 V available and a 120 amp circuit of 120 V available. Do the breakers installed in the panel have to add up to 60 amps? NO!! CAn they be greater than 60 AMPS? YES!.
Can the number of circuits exceed the number of slots available? YES!
Can the number of poles exceed the panel rating? NO!!!!!
Can a 20 amp circuit provide 20 amps for more than 3 hours. NO!
Should a 20 amp circuit circuit drawing 20 AMPs continuously trip? NO!
NEC makes reccommendations on what's required in a dwelling and will dictate certain minimum requirements. Where GFCI's go, where arcfault breakers go etc.
Your dealing with a garage/workshop. Do me a favor. Just jot down what you might like in the workshop over time. You might include things like: garage door opener, compressor, welder, table saw, mill, lathe, aoutlet for outside electricity, lighting, high pressure outside lighting, a dust collector system. That's what you need to size the service for.
A 20 amp circuit is not a 20 amp load. Continuous resistive loads require derating. Their circuits are sized at 125% their continuous load. Not all loads operate at the same time.
Wire length, thickness and length inluence the voltage drop. Generally you'd like to be with in 3%. It takes thicker aluminum wire than copper wire to have the same voltage drop.
Large motor loads add en entire new dimension. We are not talking a fridge here.
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