Feeder conductors

[ammklq143]

I am a new contractor and I'm bidding on a shed for a farmer. It's 6000 sq. ft. and he gave me a copy of a bid from another contractor (without pricing) to use so we bid the same materials. The other contractor has 400 ft. of 250mcm - 250mcm - 3/0 direct burial, which seems find, but then has 400 ft. of #6 green THHN Cu. Is it ok to bury the #6 THHN? Also, do I need to have an UFER ground and a ground rod?

Thank you all for sharing your wisdom with others.

 

[raider1]

Welcome to the forum.

What size is the overcurrent protective device protecting the feeder?

What I am getting at is that if the ungrounded conductors are increased for voltage drop the equipment grounding conductor must also be upsize proportional to the increase of the ungrounded conductors. (See 250.122(B)) So the #6 THHN may not be big enough.

Also the shed will need a grounding electrode system in accordance with 250.32. If you have a UFER ground then you would not need a ground rod.

Chris

 

[ammklq143]

Thanks Chris. The panel in the building will be a 200 amp panel and the conductors are 250mcm alum to allow for voltage drop. Would the #6 THHN be too small? Also, can that be direct buried?

Am I required to have a protective device at the transformer pole also?

This leads me to another thing I don't quite understand. 250.66 shows the size of the grounding electrode conductor to be 4 copper or 2 aluminum, but 250.66(A) says "shall not be required to be larger than 6 AWG copper or 4 AWG aluminum". Why does it say it needs to be 4 copper, but then says it shall not be required to be larger than 6 copper?

Thanks again

 

[petersonra]

people bury copper wire with no insulation at all. why would you not be able to bury an insulated one?

think connections to ground rods that are buried. those are underground and can be either insulated or not.

 

[Dennis Alwon]

This leads me to another thing I don't quite understand. 250.66 shows the size of the grounding electrode conductor to be 4 copper or 2 aluminum, but 250.66(A) says "shall not be required to be larger than 6 AWG copper or 4 AWG aluminum". Why does it say it needs to be 4 copper, but then says it shall not be required to be larger than 6 copper?

Thanks again

The gec based on Table 250.66 is for all electrodes not just the ground rod. We have exceptions on the ufer- no larger than 4 copper and a ground rod which only needs to be #6. If you had to run a GEC to a water pipe then you must use T. 250.66 based on the size of the service conductors or feeders in your case.

Apparently running a wire larger than #6 to a rod does not have any advantage since the rod itself is the weak link. A ufer is much better than a rod.

 

[ammklq143]

people bury copper wire with no insulation at all. why would you not be able to bury an insulated one?

think connections to ground rods that are buried. those are underground and can be either insulated or not.

I know you can bury bare copper, but is thicker stranding than some insulated stranded wire. I wondered if it would make a difference if the stranding of the #6 was finer and the THHN's insulation were to break down.

Thanks.

 

[ammklq143]

The gec based on Table 250.66 is for all electrodes not just the ground rod. We have exceptions on the ufer- no larger than 4 copper and a ground rod which only needs to be #6. If you had to run a GEC to a water pipe then you must use T. 250.66 based on the size of the service conductors or feeders in your case.

Apparently running a wire larger than #6 to a rod does not have any advantage since the rod itself is the weak link. A ufer is much better than a rod.

Thanks for the explanation. That makes sense.

 

[ActionDave]

Also the shed will need a grounding electrode system in accordance with 250.32. If you have a UFER ground then you would need a ground rod.

Chris

Did you mean to say "not need"?

 

[Dennis Alwon]

Did you mean to say "not need"?

yes, he did.

 

[cpal]

I am a new contractor and I'm bidding on a shed for a farmer. It's 6000 sq. ft. and he gave me a copy of a bid from another contractor (without pricing) to use so we bid the same materials. The other contractor has 400 ft. of 250mcm - 250mcm - 3/0 direct burial, which seems `.

It looks as if 250 kcmil Al is rated for 205 @ 75 deg C. I don't see where the cma is increased for length of run. What is the demand load on the feeders? Have you adjusted the conductors ampacity for temperatures cooler than 30 deg C?? These are DB ?????

 

[ammklq143]

It looks as if 250 kcmil Al is rated for 205 @ 75 deg C. I don't see where the cma is increased for length of run. What is the demand load on the feeders? Have you adjusted the conductors ampacity for temperatures cooler than 30 deg C?? These are DB ?????

Yes, these are DB. I was thinking that 4/0 would be the normal size conductor for this as per 310.15 and the other contractor bid the 250 for voltage drop, but 310.15 is for single phase dwellings. That would mean the #6 should be suitable for the 250mcm with a 200 amp panel, since 250mcm is the correct conductor size for this application according to 310.16, right?

 

[raider1]

Did you mean to say "not need"?

yes, he did.

Yep, I did.

Thanks for cleaning it up for me Dennis, sometimes I get typing so fast I mistype.

Chris

 

[raider1]

Yes, these are DB. I was thinking that 4/0 would be the normal size conductor for this as per 310.15 and the other contractor bid the 250 for voltage drop, but 310.15 is for single phase dwellings. That would mean the #6 should be suitable for the 250mcm with a 200 amp panel, since 250mcm is the correct conductor size for this application according to 310.16, right?

You could use a 4/0 aluminum USE on a 200 amp breaker provided that the calculated load does not exceed 180 amps. 240.4 (B) allows the use of the next size up overcurrent protective device for this situation.

Chris

 

[ammklq143]

You could use a 4/0 aluminum USE on a 200 amp breaker provided that the calculated load does not exceed 180 amps. 240.4 (B) allows the use of the next size up overcurrent protective device for this situation.

Chris

So........does that mean I'm oversizing the ungrounded conductors for voltage drop if I use 250 and I would then need to oversize the equipment grounding conductor also? If so, does the #6 GEC need to be a #4 (Nec 250.122b), since the feeder conductors for the 200 amp service are oversized for voltage drop? It says to increase in size proportionally to the ungrounded conductors circular mil area. 4/0 to 250 mcm increases the size of these conductors by 18%. Since 4/0 would require a #6 at 26240 circ. mils. I multiplied 26240 by 1.18% and got 30693 circ. mils. #4 is 41740 circ. mils. Do I need to use a #4 or can I use a #6, since it only requires 30693 circ. mils and not quite the circ. mils of a #4?

 

[Dennis Alwon]

Not in that case. Since 250kcm is appropriately sized for 200 amps then I would not upsize the egc.

 

[raider1]

Not in that case. Since 250kcm is appropriately sized for 200 amps then I would not upsize the egc.

I would agree.

One of the problems with 250.122(B) is that there is no real direction to what is the starting point of when I am upsizing.

In this case 250kcmil conductors have an ampacity of 200 amps where 4/0 has an ampacity of 180 and under certain circumstances can be used with a 200 amp overcurrent protective device. So the code is not clear which size conductors would be considered upsized.

Chris

 

[ammklq143]

I would agree.

One of the problems with 250.122(B) is that there is no real direction to what is the starting point of when I am upsizing.

In this case 250kcmil conductors have an ampacity of 200 amps where 4/0 has an ampacity of 180 and under certain circumstances can be used with a 200 amp overcurrent protective device. So the code is not clear which size conductors would be considered upsized.

Chris

I decided to call my electrical inspector and see what he said about using the #6, or if he would require a #4. He was quiet for a short time and then said, "I think you'll be alright with a 6". I guess his interpretation of the code is what counts in this case, so that's what I'll use.

Is overcurrent protection required at the transformer pole? There is a pole top disconnect that feeds out to other buildings that would be a disconnecting means for this building also. The breaker panel in the building has a main disconnect breaker too.

 

[Cow]

I'd buy a 4-conductor cable like mobile home feeder. Buying a triplexed 3-conductor and laying a seperate #6 beside it doesn't cut it. You'd have to use individual conductors like USE to be able to do that.

 

[ammklq143]

I'd buy a 4-conductor cable like mobile home feeder. Buying a triplexed 3-conductor and laying a seperate #6 beside it doesn't cut it. You'd have to use individual conductors like USE to be able to do that.

So, is it actually a code violation to use the 3 conductor cable and run the extra grounding conductor in the tile line with it?

 

[kwired]

I decided to call my electrical inspector and see what he said about using the #6, or if he would require a #4. He was quiet for a short time and then said, "I think you'll be alright with a 6". I guess his interpretation of the code is what counts in this case, so that's what I'll use.

Is overcurrent protection required at the transformer pole? There is a pole top disconnect that feeds out to other buildings that would be a disconnecting means for this building also. The breaker panel in the building has a main disconnect breaker too.

I would say that no OC device is needed as well as no EGC is needed run to the building. These are service conductors, and not a feeder. Unless the building falls under 547 then an EGC will be needed, if direct buried must be insulated or covered copper, but overcurrent protection at the pole is still not needed.