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I need to install a sub-panel in a shed. Total run distance from main panel to sub-panel is 80 ft and includes three 90 degree turns. Burial distance will be 18". I'm thinking to install a 60-amp main breaker in the sub-panel and running #6-3 THWN in 1-1/4" PVC conduit. I hesitate using direct buried cable as it seems a bit more vulnerable (but I'm no expert). I will be driving an 8' ground rod. A worst case inventory of expected loads is about 30-amps (1500 watt heater, 300 watts in lighting, 12.5-amp table saw, other?). This gives me a bit of room to grow. Am I doing anything stupid?

  • You need 4 conductors in the feeder (hot, hot, neutral, ground). You'll also have to keep neutral isolated from ground in the shed panel. Bond the grounding conductor in the feeder, to the ground electrode at the shed. – Tester101 May 31 '15 at 12:14
  • Just a note. Your 30A expected load is at 120V. Your 60A feeder is at 240V. Amps are not amps without regard for voltage. Watts are the measurement of power draw. So 30A @ 120V equals 3600 watts. 60A @ 240V equals 14400 watts. – Speedy Petey May 31 '15 at 12:36
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Well, I would not say you're doing anything stupid. You have some very good questions.

First,

  • If you did decide to direct bury the wire the minimum depth for direct burial is 24", not 18".
  • At least three #6 ( black, red, white) and one #10 insulated ground ( green coating).
  • Anything <= 60amps just requires a #10 insulated ground with green coating.

Second, Consider voltage drop:

  • Load: 30 Amps @ 240V Single Phase.
  • Length of run: 80 feet
  • Wire Size: #6 Copper
  • Voltage: 240V
  • Voltage Drop: .81 %
  • Voltage At End of Circuit: 238.05

That is less than 3% which the NEC recommends for a feeder. Very Good !

Note: I would hesitate to install the ground rod if you have a in-ground pool in line of the transformer.

Correction: This being a feeder would require a grounding electrode!

  • Can you explain your comment about the in-ground pool?? A grounding electrode is required at a detached structure with a feeder, so omitting it is not an option. – Speedy Petey May 31 '15 at 16:22
  • Hi Speedy Petey, thanks for the great question! NEC 2014 250.32(A), Exception 1 says "A grounding electrode shall not be required where only a single branch circuit, including a multi-wire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non-current carrying metal parts of the equipment." Examples would be detached garages, sheds, and similar structures. Installing auxiliary EGC's is fine but when the fault current is in the path of the in-ground swimming pool is a big no no. – Kris May 31 '15 at 20:01
  • Thanks Kris! Fortunately no swimming pool. Would you recommend THWN? – mark May 31 '15 at 20:25
  • Mark, you're very welcome. THHN, THWN, either or should be fine. Don't forget wire soap! Makes the job go so much nicer. – Kris May 31 '15 at 20:27
  • That exception is for a single circuit or MWBC. This topic is about a feeder so the exception does not apply. A grounding electrode is required. Period. .........Also, the equipment grounding conductor run with the feeder has nothing to do with the grounding electrode or GEC. They are two different animals that serve two very different purposes. – Speedy Petey May 31 '15 at 22:23
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because of the number of ckts you want, you need 60 amps minimum to your shed see article 225.. burial depth is 18 or 24 inches depending. see table 300.5. wire size for UF, six is only good for 55A. with pipe and #6 thhn, 65A, see table 310.15B16. here in WA you need TWO count'em TWO ground rods, (NEC may or may not say one will do, since i live in WA i dont care what it says, i have to install two). separate the grounds from the neuts. ground can be bare or insulated. you are not going to a moble home. sorry, i just looked at the posting date. you're already done. bye!

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