As usual, I’m too impatient to wait for my electrician to get here and give me the answer, so I thought I would see what you think…
I am having a subpanel put in my small shop. I figure something rated for 40 amps should suffice. I base this on the highest likely simultaneous draw from a combination of sources: lights, table saw, dust collector and maybe a small radiant heater. I haven’t added up the draw on my units yet, maybe that would be a wise thing to go do.
The run from the main panel to the sub will be about 75 feet.
My two questions for the experts here are, does the panel seem to be a good size and what gauge cable should I expect for that length of run? I think a 10 gauge cable should handle it, but I may be underestimating.
I can’t seem to find my notes to calculate the current drop over that distance. I’m not sure it is even a concern, but I would like to ensure a good supply for all those expensive motors!
I have no fear of spending a little more, I just don’t want to get talked into ridiculously over doing it!
Replies
The 10 gauge wire is rated for only a 30A breaker. I would go ahead and use 6/3 w/g (55A ampacity) and a panel rated at 75 - 100A.
Yeah, you're right I should have remembered that it's only good for 30A.
Thanks for the input.
40Amp definitely not big enough.
I've got two horse motors that draw 28 - 30 amps at startup surge. Turn on the lights, radio, dustcollector, radiant heat, and then hit the motor switch and start cussin'
Thanks for the reality check. I definitely don't want to be making that 75 foot hike to the main panel too often!
I would go with a 60A panel. It is the standard size. A run of 4/3 copper UF cable would be my preference and will, IMHO, serve you well. Use a 60A breaker in the main panel and run land the cable on the lugs at the sub. Segregate the two bars. Ground gets bonded to the box while the neutral remains isolated.
Note that you can put in a bigger panel (with a bigger main breaker) than the wire that feeds it. But the breaker that protects that wire MUST be properly sized (i.e the upstream one). I point this out because sometimes the mass-produced load centers are pretty dang cheap. Like Square-D's Homeline (sp?) line. 200-amp distribution panel already populated with a mess of breakers (20-amp and 15-amp) for less than other empty panels. I've used them supplied through a 100- or 60- or 50-amp two-pole breaker and the appropriately sized wire.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
What Dave Thomas is correct and is allowed in many areas but do check with the local inspectors.
In a perfect world the downsized breaker would remain in place and properly protect the feeder. Unfortunately those with just a little electrical knowledge will, as soon as the limiting breaker starts getting overloaded and starts tripping regularly, replace that circuit breaker with a larger one. Sometimes this is done by unscrupulous builders to save a little money on the cost of the feeder. Shortly after, in one case not even before the inspector left, the breaker is replaced with a larger one. Quite a few inspectors and no small number of houses have been burned in this situation as the feeder overheats and causes problems.
To counter this some inspectors demand that the feeder be sized for the rated capacity of the subpanel. This is, IMHO, prudent. Spending a few dollars extra to buy the larger feeder will save money in the long run if, or more likely when, the new subpanel is more fully utilized. If you must use a smaller feed run it in conduit sized for the wire supporting the full panel capacity. At least them you can pull in new without having to dig.
As a minor note I add that an under utilized feeder compared to a fully loaded one runs cooler, is generally more reliable and saves a small amount of money on the power bills.
It is interesting what some people will do to save money.
I took a quick look and some bigger panels are cheaper, but unpopulated panels are not that expensive anyway.
The same goes for moving up to a bigger feeder. If I overspec the cable and put the right panel in, it will cost me less than 50 bucks Canadian!) more. That seems like very little to spend to do it right.
Now, I know some electricians don't like to pull a stiffer cable than the need to, but I'm sure I could talk him into it.
I guess some people are so cheap they are willing to risk their homes and maybe their lives to save a few bucks.
You might want an air compressor or cracker box welder one day go ahead and install a 100 amp services. It really not that much more money and it be big enough for whatever you do.
Ditto what Brownbag said -
You never know what the future holds. I'd also go for the 100 amp box.
Q: Why did God put men on Earth?A: Because a vibrator can't mow the lawn.