I know this is a basic question, but I want to double check after reading several posts on this forum and on others. I am installing a new subpanel about 150' from another subpanel. I am pulling a ground wire from the existing panel to the new panel. I am running the wires in Sch. 40 PVC underground.

Based on another mesage I had read I was planning on running the ground (bare copper) outside the conduit (i.e., laying directly on the dirt 24+ inches below the surface -- the ground will also be bonded to a well casing and a ground rod at the new service panel). Now, I want to make sure. Should it be contained in the conduit running to the new subpanel? Also, I was planning on using bare copper as opposed to an insulated wire. Is this permissible and/or advisable?

Thanks for all the advice received on my last post about installing service to a pump house -- yes, this is the same project.

J.T.

 

Running bare ground inside the PVC conduit is fine. Inside is better than outside in case you ever need to service it or if your demand out there increases pulling larger conductors and a larger ground. In 1-1/4" sch 40 PVC you're permitted up to four #4 wires, and this gauge will get you 100 amps. I'd size it for future expansion. Actually I'd go straight to 1-1/2". Cost is negligable, and the larger size would make wire pulling easier. When you energe from the earth I would recommend transitioning to schedule 80 from 18" below up to the first 8 feet above grade. 80 is thicker, and is required in my area wherever "protection from physical damage" is required by Code. Protecting my grounding electrode conductor, for example, was a requirement when I installed a ground rod for my new service. This conductor runs down my foundation to the ground rod, and the sch 80 protects it from getting chopped by a lawnmower or student driver. My inspector told me up front that I would be need schedule 80 in order to pass. But sch 40 buried is OK since it is obviously not exposed to physical damage.

Good luck.

Juice

 

Juicehead,

Please explain how #4 AWG is good for 100 amps. I'm curious because according to Table 310-16, #4 AWG is good for 85 amps at 75 deg C.

Wiley870,

Have you considered voltage drop?

 

Thinman, as has been discussed in previous posts here I used 310-15(b)(6) for feeders to an outbuilding. At the same time I know there has been some disagreement on the usage of this table for feeders. I happen to subscribe to the interpretation previously voiced herein that feeders from a main panel to a sub-panel in a dwelling would not be required to be larger than the service entrance cable feeding the dwelling. Of course if anyone doubts the applicability of using 310-15(b)(6) they could go conservative and use 310-16 and 60 degrees C. I don't believe this is necessary in this situation though.

You got me on the volt drop though. I just figured an 8.5 volt drop with #4 at .9 power factor at 150 feet. Obviously unacceptable. A nifty little volt drop program I used came up with 2/0 at .9 pf. Kinda changes things a bit, huh?

 

Juice,

I don't think Table 310-15(b)(6) applies in this case. The reason being, Wiley870 is talking about a subpanel in a separate structure (non-dwelling unit?). I asked the IAEI people about this issue and a majority agreed that feeders, other the main power feeders, should be derived from Table 310-16.
I made a typing mistake in my first post. I should have typed 60 deg C instead of 75 deg C. Based on Article 110-14(1). Should the calc for voltage drop start at the utility transformer to the service panel and on to the new subpanel?
Enjoy the Labor Day week-end!

 

This may end up being a can of worms type of thread once the other boys get out of work and sink their teeth into this post! Mainly the 310-16 vs. 310-15(b)(6) issue. I just can't see how current carrying capacity on the load side or the main panel should be greater than the upstream wire's capability.

But you are kidding about the drop being calculated from the utility's transformer, aren't you?

Thanks for the holiday wishes, have yourself a pleasant weekend as well, thinman.

Juice

 

I had someone else do the calculations to come up with the wire sizes. I'm not saying they're right, I just went with the advice I got from the local electrical supply store. #2 for the hot; #4 for the neutral and #6 for ground.

I already bought the wire and started the install, but if you see a problem with the wire size, please let me know. As a side note, I'll never use anywhere near 100 amps with my current plans -- I'm just trying to allow for expansion in the future if I come up with some other need.

Thanks,
J.T.

 

An interpertation of the NEC is needed.This pertains to running the Equiptment Grounding Concuctor(EGC) in the trench.I prefer a bare EGC in the trench because it can connect to Ground rods driven in the trench,the EGC is in contact with the earth,and it eliminates a conductor in the feeder raceway.The pertinent Art. is 250-134,EQUIPTMENT(in place) (b)WITH CIRCUIT CONDUCTORS.By an EGC contained within the same raceway or OTHERWISE run with the circuit conductors." ???? Does OTHERWISE (my italics) permit the bare EGC in the trench? I'm inclined to say yes.At each end the EGC will terminate at the same point with the feeer conductors and the EGC parallels the feeder conductors end-to-end.Driving the Ground rod in the trench sets the rods deeper into the earth and the EGC connections are to exposed to injury.The EGC and Ground rods could serve as an excellent Supplemental Electrode connection for the service.-----Art.240-3 (b) allows the next higher standard CB rating for 85 amps. which would be a 100 amp.CB.(unless there's a 90).But if the #4 is needed to conduct 60 amps. within voltage-drop limits a 60amp CB would be in order. Cheers to All, SM

 

switchman,
90 amps is a standard OCPD size. See 240-6. Does 300-3(b) over rule 250-134(b) and require the EGC to be in the conduit? I think that it does.
Don(resqcapt19)

 

Thinman, this subject of 210-15-B-6 has be digested to the point of beyond recognition. I found disparity in all the forums replies. There is an obvious separation concerning this subject in the electrical industry. My vote is yes you can use it as long as this is not a farm house under agricultural rules. I came up with this interpretation by the following part of a sentence found in 310-15 centering on the fact this feeder attached to this accesory building's service should not be required to be larger than the conductors carrying its load on the line side of the main service rated panel. It would be kind of humorous in my opinion to require a #4 going to the main panel then the main panel carrying a load required to be a #2;

and the feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors.

The NEC does handle the demand load of farm dwellings differently than normal residential premises. This is why I centered out agricultural settings. Often times grain bins etc. may come from a dwelling. This would make this dwelling in a different lights as per the demand load requirements of farm homes see 220-41.

Wiley, if you want to use the large 2 awg feeder I see no problem with this idea.


Juice and switchman read the following complete rule that should change your mind thus requiring the EGC to be in that sheath or conduit;


300-3. Conductors
(b) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord, unless otherwise permitted in accordance with (1) through (4).

The only part of 1 through 4 that would cause a question to this requirement would be 250-134-B which switchman referred to. I do not believe that an accessory structure would fit in the discription of "equipment". Considering my thought concerning equipment not fitting the definition of a building I believe 300-3 would take control in this circumstance. The purpose of requiring the EGC to be in the conduit with the other conductors of the same circuit whether being a feeder or a branch, is to ensure a low-impedance ground-fault path between a ground fault and the electrical source.

Switchman, check 240-6, you made a nice try in using 240-3-B but 240-6 states that the next higher normal size overcurrent device would be a 90 amp breaker. Close, so close I checked up on your suggestion of using 240-3-B. Wish it would have worked. It would have resolved a long discussion and difference of opinion on the subject of table 310-15-B-6.

The bad news is there were several applications to change the wording in 310-15 to help clarify thus avoiding confusion in the electrical field as to what it actually says, but the ROP ruled that they saw no confusion in what the wording in 310-15 actually said. I enjoyed that one!

Wg

 

Wg,

Gotta set the record straight: I previously replied that I would prefer the EGC to be run inside the conduit. But, among the list you cited "...within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord...", was included the word "trench", which seems to permit this practice. Nevertheless I feel, and I believe many would agree with me, that after I went to the good time and trouble to dig a trench, run my C&W and then backfill and restore the landscaping, should anything go wrong with my EGC (gophers, worms, earthquake or other act of God or gremlins), or should I desire an upgrade for more amps at the other end someday, I would be loathe to go through digging up the original EGC when I could simply pull it out through the raceway. So for me, I would not direct-bury my EGC unless it was required.

Also, good to hear you re-iterate the supporting opinion of feeders in a dwelling not being required to be larger than the SE cable. To require this would indeed be humorous.

Have a nice day.

Juice

 

Juice the way that I read that rule if the conductors serving that feeder is thwn and is required to be in a conduit for protection then all conductors associated to that feeder must be in that conduit. If the conductors serving that feeder is single conductor USE and is allowed to be direct buried in a trench then all conductors associated to that feeder must be in that same trench. There is an exception if installing parllel wires serving as the same feeder allowing them to be in separate conduits but requiring those separate conduits to be installed close together as possible the entire length of that parallel feeder.

My interpretation

Wg

 

Wg, I follow your interpretation for using USE and following it closely with your EGC. Seems the most logical way to go, and looks to some future excavation, where the parties digging up an set of feeders for what ever reason will unquestionable find the EGC. For conduit, makes no sense to me to run the EGC outside of it. And if it's not kosher according to Code, it's just as well.

Juice