New 400A, 1P service. Decided on conductors. price consideration

[fastline]

Building a new building and having a 400A service brought over. They will bring the primary over about 300ft shy of the building and I will go underground with secondary for the entrance. I am trying to decide on two 4/0 entrances or just one. It looks like I would probably need 500mcm for a single shot.


I realize I would have 2 panels and such with two entrances but I think those panels are pretty standard, thus cheaper?

I am building a 60x96x18 and will have some light commercial and residential use in the building. I really need more like 300/100 for the separation of power but still debating.


I used to work for an electric co and now in engineering. If memory services, the entrance cannot exceed 3% Vdrop?

Also, I was considering options for a smaller neutral. My load calcs are figuring on same size neutral so I assume for reduced neutral, you have to consider hot at one resistance, then neutral as another?


I am also thinking about conduits here. Never any fun working with small conduit. I think I need to go to 4" for 500mcm? 3" for 4/0.

I have read here about "triplexed" 4/0. I think the stuff I was looking at was THHW and twisted but no common jacket. Does that sound right for conduit apps?

 

[petersonra]

Please don't take offense, but it appears to me from your post that you are not real familiar with this kind of thing. Just hire a competent electrical contractor to do it. They will know the answer to all these questions and the dozens of questions you probably don't even know to ask.

 

[kwired]

If you are going to supply 2-200 amp panels either way - you will save the cost of a 400 amp disconnect/main breaker by just feeding straight to the 2-200 amp panels - they need main breakers either way as well.

Material wise, a pair of 4/0 runs and (likely) 2 inch raceway will probably cost less then a single run of 500 and 3 or 4 inch raceway. Will have some additional labor for two runs, but 4/0 will be easier to handle then 500 so take that into consideration as well, similar labor comparison for installing 2-2 inch vs a 3 or 4 inch run.

Voltage drop? Is no definite NEC rule on this only an informational note that is basically just a suggestion. If you run separate two 4/0 conductors you will have improved voltage drop performance if you actually connect them together on both ends and make them true parallel conductors - in most cases anyway, should you have nearly constant loading in high ranges of your design capacity, then it may not make much difference.

 

[Cow]

Please don't take offense, but it appears to me from your post that you are not real familiar with this kind of thing. Just hire a competent electrical contractor to do it. They will know the answer to all these questions and the dozens of questions you probably don't even know to ask.

I agree.

 

[augie47]

These type installs have one pitfall in common and that's the "commercial" space. The residential service is simple to calculate but there are so many unknowns on the commercial side.
What type of occupancy ? The load for a pizza house or bakery or a tanning salon may be a lot different than the load for a small retail shop.' Will 3 phase power be needed ?
PVC is not expensive. My suggestion would be two runs, one for the residence and one, over-sized, for the commercial space. Then if the future load warrants, the larger conduit is in place.
Locally a lot of E/Cs will do that and install a 400 amp meter base for the commercial even if they only feed a 200 amp panel so there will be room for growth if needed

 

[jeremy.zinkofsky]

Fastline, you have a few options here. Single conductor or parallel conductors, One (for a single feed) or Two conduits (for a parallel feed).

Voltage drop should not be more than 5%, use any voltage drop calculator online or on the Mike Holt website to find the final conductor size.

Otherwise, make sure you use NEC 310.16 to size your cable, NEC Appendix C to size your conduit, and NEC 250.122 to size your ground. You can have up to six service disconnects connected in parallel just make sure that you get lugs that can hold two cables (double lugs) for each phase and that they have fuses or main breakers adequately rated for the available current (check out NEC Article 230 for info on this).

As for the neutral, it needs to be the same size as the ungrounded conductors. You can only undersize your ground according to NEC 250.122.

I would look at what is required for each scenario (single run or parallel) and compare the material cost for both. Obviously choose the cheaper one.

 

[jeremy.zinkofsky]

Additionally, to size the service for this building you really need to know all of the loads i.e. mechanical equipment, kitchen appliances, etc. This is usually done by the engineer by basically adding up all of the loads (in VA) and then finding the current by using I = S/(V*sqrt3) and then using the NEC to size the main breakers. The electrician never does this. It is the sole responsibility of the engineer to do such analysis and then dictate the basis of design to the contractor.

 

[kwired]

Fastline, you have a few options here. Single conductor or parallel conductors, One (for a single feed) or Two conduits (for a parallel feed).

Agree.

Voltage drop should not be more than 5%, use any voltage drop calculator online or on the Mike Holt website to find the final conductor size.

Per what standard? If there is a design spec, fine, if there is local codes/amendments, fine, otherwise NEC only makes a suggestion in an informational note and has no specific requirement for voltage drop.

Otherwise, make sure you use NEC 310.16 to size your cable, NEC Appendix C to size your conduit, and NEC 250.122 to size your ground. You can have up to six service disconnects connected in parallel just make sure that you get lugs that can hold two cables (double lugs) for each phase and that they have fuses or main breakers adequately rated for the available current (check out NEC Article 230 for info on this).

There should not be an equipment grounding conductor involved with service conductors, but could be supply side bonding jumpers. You wouldn't have to have double lugs, and some equipment may not have enough bending space for multiple conductors per lug, in those instances you may need splice/tap box or gutters to make such connections.

As for the neutral, it needs to be the same size as the ungrounded conductors. You can only undersize your ground according to NEC 250.122.

Again there is no EGC with service conductors, the grounded conductor only needs to be sized per the load it will see - but no smaller then that required for a grounding electrode conductor (this for service conductors, a feeder or branch circuit doesn't need a grounded conductor at all if there is no load for it to serve).

I would look at what is required for each scenario (single run or parallel) and compare the material cost for both. Obviously choose the cheaper one.

And take labor differences into consideration as well.

 

[ActionDave]

As for the neutral, it needs to be the same size as the ungrounded conductors. You can only undersize your ground according to NEC 250.122......

This is not correct.

The grounded conductor, i.e. neutral does not need to be the same size as ungrounded conductors.

250.122 has nothing at all to do with services or neutrals.

Kwired gave the reasons. 230.42, 220.61, and 220.61 are the code references.

 

[fastline]

Thanks gents! I really appreciate the references to NEC code. I will try to explain more to help. I am in the engineering field so crunching the numbers is no biggy. I own, run, and repair CNC machines and work with inductive loads every day. I have also built 3 balanced 3 phase converters. I made calcs to size the conductors and such but don't work within the code book every day like folks here. I have run my loads to determine the 400A service. Technically, I could use 600A but the rural coop is trying to limit be a bit. They technically don't want bigger than 10HP motors on the line but I think that is for inrush concerns. I plan to use VFDs for my bigger motor loads.


When I mentioned the dual 4/0 runs, I was considering installation of separate 200A panels in different parts of the building, not really paralleled conductors to a 400A panel. Actually, I am not sure you could really do that safely anyway? One thing I need to verify is my earth ground connections when running 2 panels? Do I have to bond each panel together or am I able to just drive separate rods?

Anyway, I was mostly curious about conductor costs. It sounds like neutral size will depend on my 120V loads so I might glance at that. Most of my loads will be 240V.


I hear what you guys are saying about oversizing! I agree and giving that some thought. At the very least, oversizing the conduits so I can repull the entrance conductors if needed.

3P is not financially feasible in my area or I would certainly get it! I think it is 2mi away so probably $50K to get it to me.

 

[kwired]

When I mentioned the dual 4/0 runs, I was considering installation of separate 200A panels in different parts of the building, not really paralleled conductors to a 400A panel. Actually, I am not sure you could really do that safely anyway? One thing I need to verify is my earth ground connections when running 2 panels? Do I have to bond each panel together or am I able to just drive separate rods?

Same building - you need to put both service disconnecting means at same location. If you want load centers in separate parts of building you can use one main breaker panel at service location to feed that area and put in a disconnect or breaker only as the second service disconnect in that same area and then run a feeder to the remote panel.

See 230.72 on this issue.

There is more then one way to connect grounding elecrode(s) but ultimately both service disconnecting means need connected to the grounding electrode system, or to a common point ahead of them if there is one. But since they must both be grouped together this is not really going to present much difficulty compared to what may be required if they were allowed to be in different areas of the building.