Have a 208V, 3 phase, 4 wire, 4,000A service from the transformer. The out of state engineer is requiring (11) - sets of 3" conduit with (4)-500kmil + 500kmil ground in each conduit. All copper conductors. I'm question two things, the conduit size and ground size. I think 3" is too small and I think we can use 3/0 for the ground. Any thoughts?
Service Entrance Ground Sizing
- Thread starter KMark5710
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Several questions come to my mind:
It is possible that a 3" conduit could suffice. But even if it did, pulling 5 conductors sized at 500 MCM through a 3" conduit is going to be difficult. The longer the run, the more difficult it would be. I would likely call for a 5" conduit.
I agree that 500 MCM ground is the wrong size. But I don't agree with 3/0. Presuming the N-G bond is at the main service panel, then the "ground" wire is actually a supply-side bonding jumper. The size is given in 250.102(C). I calculate it as a minimum of 700 MCM. I would like someone to check me on that, however.
- What type of conduit?
- What type of insulation?
- How far is the transformer from the main service panel?
- Where is the N-G bond (i.e., at the transformer or at the main service panel)?
It is possible that a 3" conduit could suffice. But even if it did, pulling 5 conductors sized at 500 MCM through a 3" conduit is going to be difficult. The longer the run, the more difficult it would be. I would likely call for a 5" conduit.
I agree that 500 MCM ground is the wrong size. But I don't agree with 3/0. Presuming the N-G bond is at the main service panel, then the "ground" wire is actually a supply-side bonding jumper. The size is given in 250.102(C). I calculate it as a minimum of 700 MCM. I would like someone to check me on that, however.
1. What type of conduit? schedule 80 PVC
2.What type of insulation? Unspecified
3.How far is the transformer from the main service panel? 100 feet
4.Where is the N-G bond (i.e., at the transformer or at the main service panel)? Unspecified, I would assume in the gear.
It is a utility service transformer.
2.What type of insulation? Unspecified
3.How far is the transformer from the main service panel? 100 feet
4.Where is the N-G bond (i.e., at the transformer or at the main service panel)? Unspecified, I would assume in the gear.
It is a utility service transformer.
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
I would call the utility and explain plans, and ASK them were you are going to land those extra 11-500kCMILs.
Get their response in an email and forward to engineer.
CB's math was correct....5500 x 0.125 = 687.5, or next size up = 700kCMIL.
Send and RFI to engineer and ask for verification on size and TELL him the wires won't fit in a 3" PVC-80.
I understand the formula for sizing the MBJ but. Where are you get your 5500# from?
Sent from my SM-G950U using Tapatalk
Yes 11.
You had indicated the drawings show 11 sets of (4) 500 mcm and (1) 500 MCM Ground.
Ask the power company where they are going to land those (11) 500 MCM ground wires.
They're probably going to tell you they aren't going to land them at all.
JAP>
- Location
- New Jersey
- Occupation
- Journeyman Electrician
There is no EGC required so with 4-500 kcmil's it would fit in a 3" Sch80 PVC.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I think it is likely that this is just an oversight by the engineer that had "feeder" on the brain. It should be obvious to him that the EGCs shown are not allowed and are not an option in this situation.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I agree that no EGC is required. You only install EGCs downstream of the location of the N-G bond. But was I wrong to say that an SSBJ is needed, and that it needs to be 700 MCM?
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
The issue here is that a supply side bonding jumper or an EGC would not be applicable or allowed in this case as it is a POCO transformer that will have bonded X0. That said, he might still need bonding jumpers if there are metallic turn-ups on the PVC raceways. And yes, it would be a 700 MCM if he daisy chained 1 jumper between raceways. But in this size range it is more practical to size for the ungrounded conductors in each raceway and use separate jumpers to keep the size manageable. See 250.102(C)(2).
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
Are you sure 3" is correct ?
It works for COMPACT conductors....table C.1(A) but NOT for standard stranding table C.1.
I didn't think CU compact conductors were a normal product line....typical for aluminum.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I do believe you are correct. And even if you could get compact CU you would be limited to certain insulation types as not all would fit.
You get the prize.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I should have added that a MBJ will be required within the service disconnect per 250.24(B) and sized per 250.28.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Although it might not be explicit in the code with respect to bonding jumpers instead of EGCs, the principle for parallel conductors is that the fault clearing ground must be sized according to the total area across the sets of one ungrounded conductor.
If the 11 runs are only tied at the source and have different destinations, then sizing one fault clearing conductor according to a single set conductor might be OK for a single external conductor but would certainly be fine when including one for each set.
Sent from my XT1585 using Tapatalk
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Btw, if you do need an equipment grounding conductor --not totally sure of the setup, 500 kcm is what is needed. More for your own info. Each run has to have an equipment grounding conductor sized to trip the breaker.. 250.122(F)(1)(b)
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