dumb voltage drop and wire sizing ??s
Homeowner, weekend worrier, worked as an electrician’s assistant 30+ years ago, rewired own house.
Now, I’m running PVC conduit out to our old tankhouse (aka watertower) which houses my shop and is near the spa (as a northern Californian, I always feel defensive about the spa: LOML and I both have bad backs, and it’s a great help) and LOML’s pottery kilns for a subpanel. Y’all were kind enough to help me recently with the question of adding a ground rod at the subpanel in addition to the equipment ground wire coming from the house panel. Now I’ve got some new questions.
The subpanel will be 220V, so I’ll be running both hots, the neutral, and he equipment ground. Total run from house panel to subpanel is about 65 feet. Load will be a mixture of 230 V (40 amps for the kilns, probably 10 amps for the table saw) and (primarily) low to mid range 115 V (spa is likely biggest load at 15 amps). I’m hoping for a 60 amp main breaker at the tankhouse subpanel. Some years back, when our son installed drain tile behind the house, he stubbed out 1-1/4″ conduit through the drain gravel and into the crawl space, so I’m limited to this conduit size.
1. The voltage drop tables in my ancient copy of HP Richter’s book have two flavors: 230 V and 115 V. Since I won’t be drawing the full subpanel amperage unless I’m using both legs of the circuit AND running one of the 220 V loads, it seems like I could use the 230 V table. Is that correct? It lets me use No. 6 wire, while the 115 V table requires No. 4, which is close to conduit capacity (WAY easier to pull through an uncrowded conduit).
2. I remember that the equipment ground wire can be smaller than the load wires. Richter’s book recommends No. 6 for any load wires up to No. 2. Is that still current practice?
Replies
Try this one:
http://www.southwire.com/voltagedropcalculator.jsp
You base your voltage drop calculations on the figures from your load calculations.
For example ... your load calculation tells you that you need 78 amps. Well, the next size 'standard' breaker is 90 amps, so you'll need a 90 amp service. That is, you start off using wire sized for 90 amps.
Now, look at the load calc figure of 78 amps. Starting with the wire sized for 90 amps, calculate your voltage drop using a 78 amp load. There are two figures to keep in mind:
a) You generally want to keep the voltage drop to less than 5% (that's 12 volts for the nominal 240 voltage serving the panel); and,
b) You absolutely want to avoid delivering less than 108 volts on a '120' circuit, or 210 volts on a '240' circuit.
If your calculations show you're getting close, then it's best to go up in wire size.