I read with interest the article on subpanel installation(FHB154), but it left me with a question for you “sparkys”. My new home has a remote located entrance panel with it’s own driven ground. The planned subpanel in the house has it’s own ground via rebar cast in the footing. Does this mean I will only need to pull three conductors, vs four as in the article. Also, what size cable for 150 amps. The run is about 150 feet in 2″ conduit, buried. Thanks for any help.
Harvey.
Replies
Harvey, I haven't seen you since your rabbit days.
You didn't say if this sub-panel is in a different building but I'll assume it is since 150' would make a pretty big building. In any case you need the four conductors or wires or cables. Take your pick. 2 hots, a neutral and a ground. Everything has to be tied together.
As for the wire size, I would guess 1/0 but then I haven't accounted for voltage drop and conduit fill.
~Peter View ImageGo S-3s
Harvey,
Thanks for the compliment on the article..at least I think it's a compliment. It's too bad we couldn't cover subpanels in outbuildings, but you can only fit so much in six pages.
So, assuming that your sub is in a separate building, here are your options:
1. If there are no metallic pathways (pipes, tel or CATV cable, etc) between the buildings, you can treat the subpanel as a sort of separate "service". Run three wires (two hots and a neutral) from the main panel or service, and tie the ground and neutral buses together at the sub. Run a grounding electrode conductor (#4 copper) from the Ufer ground (the rebar in the concrete) to the ground/neutral bar on the sub.
I'd recommend against this approach. Sooner or later, you (or someone) will want to run a phone wire to the other building..
Or,
2. Run fours wires (two hots, neutral, and grounding conductor) from main (supply) panel to subpanel. Keep ground and neutral terminal bars in the sub separate (i.e., a floating neutral bar, electrically isolated from the panel). Run a #4 copper from the Ufer to the grounding bar. You always need a grounding electrode system for a subpanel at a separate building--it's the drain for high voltage spikes like lightning or High Voltage power line crosses...
As for the conductor size, if you really want to feed 150 amps, you're looking at AWG 1/0 copper. That'll get you just under 3% voltage drop at 150A. To stay under the recommended 2% drop in the feeder, go with AWG 2/0. Three 2/0 copper w/THWN insulation with a #4 THWN ground require a 1 1/2" Sched 40 conduit, so your 2" will give you a little easier go of it. Not that it'll be easy, mind you...
But remember, it's the actual load that is used in voltage drop calcs, not the panel rating...I based the above on an actual 150A demand. A 100 amp demand requires AWG 1 copper to achieve a voltage drop of just under 2%. You could use AWG 3 if you wanted to accept a feeder Vdrop of 3%. It depends on how long the branch cicuits fed from the sub will be...since ideally you want the total Vdrop to be 5% or less.
If you have a chance to ever do this again, put steel sweeps at the 90s in the conduit. If the steel elbow has at least 18" of cover, it doesn't have to be grounded (bonded). With long pulls, the wires tend to cut through the PVC elbows. Not fun.
Good luck with the project.
Cliff Popejoy
One more thing -- When you do this pull, have a helper at the feed end pushing the wires into the first 90. Amazing how much this can ease the friction problem in that first turn.
-- J.S.
Cap, thanks for that tidbit: Never too old to learn something useful - had not been aware that PVC conduits had experienced cut-thru on long pulls.
As a 'young punk', helped with a 100% full (even in a 1 ft staight shot you could not fit another wire) pull in late 60's on the old USA supersonic transport - all instrumentation wires. 40 foot long 5 inch Al conduit with 4 ea 75 deg to 90 deg bends in different geometric planes. 20,000 pound winch attached to special pull section of stripped wires (mostly TSP, Teflon insulated) soldered together and then brazed to the pull cable. 12 of us on the push end, 3 gallons of the yellow puckey lubricant, went thru in about 1/2 hour.
That is one of those things that you know, but never think about is the slightly different application.
I have a "manufactured" pvc saw that is nothing but a section of fine braided wire and a couple of rings for handles. I have used to cut 4" pvc sewer and drain pipe and it EASY. Just don't stop in the middle of a cut or it will fuse back over the wire.
And while not quiet as easy I have seen people do it with twine.