Service Conductors for 600A Service

[GTTofAK]

Hello all,

First post. Im an EIT in Alaska, would have just passed the test had Covid not canceled everything. I'm a line and customer designer and I have a customer requesting service. The customer was requesting a 400A multiple meter service so I applied 83% sizing. The electrician called me yesterday telling me they wanted 600A instead. NEC 310.1(B)(7) specifies the 83% rule 100-400A. This is the first time I have had a single phase residential customer ask for over 400A and in the NEC I don't really see where to go for sizing over 400A. Do we go back to pre NEC 2014 and size based on load? Or do we size based on the size of the service?

Now off the top of my head I would say 2 runs of 4/0 TPX but I need to be able to support my decisions in my very very toxic work environment.

I also have the problem that our service requirement book hasn't been updated since 2014 and is written for NEC 2011. Up to 800A the customer needs only one 4" service riser which I don't think is large enough.

 

[infinity]

What is the calculated load and is there more than one service disconnect (sounds like there is)?

Welcome to the Forum.

 

[electrofelon]

Rob's two questions are key to providing an answer. But I would also like to clarify that 310.15(B)(7) only applies to conductors serving an individual dwelling unit. It can not be used for common "riser" conductors.

 

[GTTofAK]

Welcome to the Forum.

Honestly I dont know. Generally one residential the customer requests a service and we size the cable at 83% of the requested service. We only require electrical drawings and load profiles on commercial buildings.

My question is the code. After 400A do we size for the load or do we size for the service?

Electrofelon its 4 200A in a multimeter so 6 of one two dozen of the other. Maybe you have a good point that each meter is 200A so the sizing for 310(B)(7) applies.

 

[Dennis Alwon]

You size for the service size not the load. If you install a 600 amp service then your conductors must be sized for 600 amps.

 

[electrofelon]

You size for the service size not the load. If you install a 600 amp service then your conductors must be sized for 600 amps.

Dennis I think we should be more complete and note that for services with multiple service disconnects, we can size to the load. Im not really clear what the OP has.

its 4 200A in a multimeter so 6 of one two dozen of the other

 

[augie47]

From your original question it sounds somewaht like you are sizing the utility aerial drop (TPX ??). Is that the case or exactly what portion of the service are you sizing ?

 

[infinity]

Sounds like there no single main so you end up with a meter stack with 4 individual 200 amp meters and mains. The conductors to that meter stack would be sized according to a load calculation. The conductors to each unit fed from the 200 amp main could be sized at 83% if these are dwelling units.

 

[GTTofAK]

Sorry it was very late and I was getting the 8 meter service and the 4 meter service confused. The 4 meter service is 150A per meter. You have to understand my work. I work for a public utility and the city short on cash has spent the last 5 years cutting corners. I remember one engineer at eng-tips commenting that there is no greater scofflaws of code and licensing requirements than municipal governments.

None of the managers are EEs, the city refuses to pay the salary so most of our managers have never worked in electrical engineering for a utility before. They have been told that we aren't even under NEC requirements.

Now I’m back in the NESC code book to find the rule where it refers you to the NEC for service cable ampacity.

 

[GTTofAK]

From your original question it sounds somewaht like you are sizing the utility aerial drop (TPX ??). Is that the case or exactly what portion of the service are you sizing ?

Its an underground service.

Our service requirement book which was last revised in 2014 looks like it was written for compliance with NEC 2011. I was working in dispatch at that time. I'm newer to customer and line design engineering. Our only PE who really knew customer engineering and code quit to take a job on California so there is no longer anyone to go to for code questions on customer engineering projects. And much of our practice and procedrures seem to be based more on wishful thinking.

 

[oldsparky52]

Its an underground service.

Our service requirement book which was last revised in 2014 looks like it was written for compliance with NEC 2011. I was working in dispatch at that time. I'm newer to customer and line design engineering. Our only PE who really knew customer engineering and code quit to take a job on California so there is no longer anyone to go to for code questions on customer engineering projects. And much of our practice and procedrures seem to be based more on wishful thinking.

Is this decision one that is in your job description? If it is not, then I'd be careful about making it.

 

[GTTofAK]

Is this decision one that is in your job description? If it is not, then I'd be careful about making it.

Well Im trying to get them to give me an answer but they came back requesting me to write a breif. Our current managemnt team is an IT guy who just got his CS degree he is the division head, an ET, and an ME.

 

[infinity]

Sorry it was very late and I was getting the 8 meter service and the 4 meter service confused. The 4 meter service is 150A per meter. You have to understand my work. I work for a public utility and the city short on cash has spent the last 5 years cutting corners. I remember one engineer at eng-tips commenting that there is no greater scofflaws of code and licensing requirements than municipal governments.

None of the managers are EEs, the city refuses to pay the salary so most of our managers have never worked in electrical engineering for a utility before. They have been told that we aren't even under NEC requirements.

Now I’m back in the NESC code book to find the rule where it refers you to the NEC for service cable ampacity.

No problem well try to help you, but if the read back through his thread we still need some information. You say you have 4 meter stack, is there a single main service disconnect for the entire stack or just a service disconnect for each meter?

 

[GTTofAK]

No problem well try to help you, but if the read back through his thread we still need some information. You say you have 4 meter stack, is there a single main service disconnect for the entire stack or just a service disconnect for each meter?

I might be wrong. Is there any requirement in the NESC as to the ampacity of the utility owned service cable. For some reason I remember reading to refer to the NEC but now I cant find it anywhere.

 

[oldsparky52]

I can envision a PoCo feeding a meterbank of 4 150-amp services (assuming apartments or condos) with a 4/0 aluminum.

 

[kwired]

From your original question it sounds somewaht like you are sizing the utility aerial drop (TPX ??). Is that the case or exactly what portion of the service are you sizing ?

Guess more info was added but that was my first thoughts as well - designing POCO equipment. They are not bound by NEC and will usually have something that is closer to actual demands vs NEC calculations. On something new this has to be based on past experiences with similar loads. Many like to have large raceways or spare raceways so they can easily upgrade or add more conductors if deemed necessary down the road. But for a four unit dwelling complex, they might have better idea of what max demand would typically be and not require much for extra room or extra conduits. Can see them requiring a 3 inch conduit but only pulling 4/0 aluminum if they think that is all the load will require.

I can envision a PoCo feeding a meterbank of 4 150-amp services (assuming apartments or condos) with a 4/0 aluminum.

Especially if they don't have electric heating, otherwise they may up it to a single 350 aluminum even if load calculation is more than ampacity of that.

 

[oldsparky52]

Especially if they don't have electric heating, otherwise they may up it to a single 350 aluminum even if load calculation is more than ampacity of that.

I know of a marina that has a 1,200-amp 120/240-volt panel that has (4) sets of 350kcmil between the panel and the CT cabinet and the PoCo feeds the CT cabinet with one set of 500 aluminum.

In the summer, I bet that 500 is ... warm.

 

[kwired]

I know of a marina that has a 1,200-amp 120/240-volt panel that has (4) sets of 350kcmil between the panel and the CT cabinet and the PoCo feeds the CT cabinet with one set of 500 aluminum.

In the summer, I bet that 500 is ... warm.

Still depends on connected load. I did upgrade in a school building a few years ago because of addition of new HVAC equipment, went from single phase to three phase service. My load calculation came out only needing maybe ~ 600-700 amps. We had 2000 amp single phase existing. That existing was only supplied by a 100 kVA single phase transformer - obviously the load wasn't there but they built it to be able to have more later - just not three phase Stupid thing was the switchboard had three phase bus and most breakers over 150 amps were three pole frame but only had guts installed in two poles making this gear useless for three phase. I installed 1200 amp three phase service to have similar extra capacity as they originally had and hopefully it will allow for some future additions if they come up. POCO only set a 225 kVA transformer for this, but at least it had room for the conductors, should have seen the single phase transformer that was originally there - I believe there was six sets of 500 aluminum run (they had 1800 amp main fuses IIRC) and they somehow landed them in a 100 kVA single phase padmount and still got the cover to close.

 

[RD35]

Company I used to work for installed new lights on a previously poorly lit H.S. football field. New lights were Metal Halide...70 fixtures @1500W each......enough to have night games on TV. Upped service panel from 200 to 600A 1-phase. PoCo came out just before the first game of the season (we had been waiting for a week for them to show up) and reconnected the existing 25KVA pole transformer via the existing drop of 4/0 Aluminum to the two new parallel runs of 350 copper hanging from the weatherhead. The lineman said it would be fine...they do it all the time. Lights came on....for about 10 minutes and.....BOOM....Dark! New bigger (don't remember how big) transformer and drop were installed and powered up just in time for the kick-off!!! Whew! In the lineman's defense, I think there was some confusion and the engineering folks at the PoCo office thought they were working with 3-phase (three 25kva pots existing instead of one) when they ran their calcs.
So my reason for this story is that in my neck of the woods the PoCo definitely does NOT follow NEC. They use in-house data for sizing services and drops. Their senior engineer told me this himself.