# Upgrading Service 200 amp to 320 AMP Question



## harborremodel (Sep 26, 2018)

I'm looking to add a hot tub and a few other electrical items to my home over the next year. I currently have a 200 amp panel that's full and a sub-panel for some garage circuits. To make room for the new hot tub and AC units I'm looking to upgrade to 320 amp service. I talked to the power company and they told me my supply lines and meter are already setup for up to 600amp service so all the upgrades to service happen after the meter. 

So to upgrade to a new service do i need to run a new panel feed by the existing feed lines or do i need to replace the current panel to a larger panel fed by existing lines. Example pic attached. 













Panel Samples



__ harborremodel
__ Sep 26, 2018


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## hornetd (Sep 26, 2018)

Neither of those line drawings is correct.  What you need to do is to feed the original panel with 250 MCM Aluminum Wire.  You then run a second set of conductors to the new panel that is sized in accordance with National Electrical Code Table 310.15(B)(16) (formerly Table 310.16) Allowable Ampacities of Insulated Conductors.  Since neither set of Service Conductors will be the "the feeder conductors supplying the entire load associated with a one family dwelling" no reduction in size of the conductors is permitted.  Since 4/Ø Aluminum has an ampacity of 180 Amperes it cannot supply a 200 Ampere Breaker like the one in the existing 200 Ampere panel when it is not "the feeder conductors supplying the entire load associated with a one family dwelling."  The reduction in size of a single set of conductors is permitted because of what is called the "Diversity" of the load in a dwelling unit.  The safety of the smaller size is based on all of the loads in the load calculation being unlikely to be used at the same time.  When a set of conductors supplies only part of the load of a dwelling unit the likelihood of one set of conductors being loaded to the maximum ampacity of the circuit breaker protecting it rises sharply.  That is why the ampacity of multiple sets of service conductors must be no less than the over-current device which are protecting each set. 

To maximize the flexibility for future additions to your homes wiring you may want to consider installing a second 200 Ampere panel supplied from service conductors which are also sized for the full capacity of the 200 ampere breaker.  That would be a second set of 250 MCM Aluminum conductors.  You should do a service size calculation to assure that the total size of the service is large enough to serve the calculated load even though I have trouble imagining your surpassing the ampacity of a 400 Ampere Service.  Be aware that the National Electrical Safety Code which governs the size of utility installations uses 80% of the installations maximum ampacity to size it.  The National Electric Code which governs installations inside of customer premises, on the other hand, sizes at the maximum ampacity of the installation but applies de-rating factors to some parts of the load to avoid overloading. 

--
Tom Horne


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## slownsteady (Sep 29, 2018)

Tom...I am often amazed at the level of detail in the code that you quote. So tell me (us), do you sit with the book in front of you when you reply to a post or do you quote this stuff out of memory????


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## afjes_2016 (Sep 29, 2018)

SNS I agree with you. I believe Tom being in the business for 50+ years has something to do with it.

I always enjoy reading your replies Tom. Although I am retired now due to physical problems I like reading what you post.


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## hornetd (Sep 29, 2018)

I found out within a month of starting in the electrical craft that I was never going to be the fastest installer in a company so I was worried about being retained or laid off at the first opportunity.  I found out that if I showed up every day, gave the employer the best 8 hours I could, and AVOIDED RECALLS LIKE BUBONIC PLAGUE I was a very valuable employee.  The challenge was that a fairly large number of fellow electricians were doing the same things and I wanted to be employable even when work was scarce.  That got me started on control, fire alarm, and emerging communications installations.  I stayed curious and every time Local 26 had a course I had the prerequisites for I took the course.  I burned up a lot of evenings and weekend days in those courses but I gradually learned a lot. 

There is an old cartoon character called Sgt Mike.  He is the quintessential Drill Instructor.  He is always standing in front of a group of recruits and he is always screaming.  One of my favorites is "That there is called a pit.  That there is called a rope.  The alligator is called an incentive." 

Once I had seen a couple of failed inspections I didn't ever want to be the cause of one of those expensive oopses.  So every time I heard about an inspection oops I would find out what was behind it and study the code sections being cited to support the corrective order.  I attended the code course and the code changes courses like religious observances.  One of the things I learned from being taught by active electrical inspectors was that if you actually new the code sections which applied and argued your point calmly you could actually get out of some of the jams others had to rebuild their way out of.  Example:  Every set of Service Entry Conductors (SECs) which is supplied by a Wye transformer's windings MUST have a neutral run with the Conductors.  It does not matter what load those Service Entry Conductors supply.  They can be supplying a Delta configured transformer primary winding and you still have to install a neutral and run it with the SECs.  There is simply no exception in the US National Electric Code.  This rule is hard and fast!  So when I was sent out to a job as additional staff and found that had not been done I new we had to correct it or fail inspection.  Luckily we still had the supplying utility's wireman on the site.  Since the Transformer in question was only going to supply Delta loads we were able to get them to convert the transformer to a corner grounded Delta Secondary.  That made one of the phase conductors the grounded conductor of the SECs and no fourth wire was required.  On another job we converted the derived system of the generator from Wye to Delta.  Since it was supplying a Delta Primary transformer no grounding of the SECs was required and the three conductors which had been pulled were sufficient.  Those 2 examples are fairly simple ways of correcting without rebuilding and keeping the job on budget.  Do that even a couple of times and years later the Foreman will remember you as someone who saved his but.  You will never be first on the layoff list. 

--
Tom Horne


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## hornetd (Oct 4, 2018)

slownsteady said:


> Tom...I am often amazed at the level of detail in the code that you quote. So tell me (us), do you sit with the book in front of you when you reply to a post or do you quote this stuff out of memory????


I do cut and paste the applicable code sections from my electronic copy.  That is because there is so much bad information on the internet that I feel I should provide the justification for my assertions so that people can act on the advice.  I do remember the lyrics to a lot of songs but code language not so much.  The issues involve human safety and are too important to depend on memory. 

--
Tom


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## slownsteady (Oct 4, 2018)

Thanks. Strangely I feel better. That's because i suffer from C.R.S.  (can't remember s**t)


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## David Newell (Oct 8, 2018)

hornetd said:


> ------- The issues involve human safety and are too important to depend on memory.--------------
> --
> Tom



As an Engineer, Safety,
I'm very supportive of your stance.

My
*"HOO --HAW!  YES" *
when I read your response
does not translate well to written form.


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## WyrTwister (Oct 8, 2018)

harborremodel said:


> I'm looking to add a hot tub and a few other electrical items to my home over the next year. I currently have a 200 amp panel that's full and a sub-panel for some garage circuits. To make room for the new hot tub and AC units I'm looking to upgrade to 320 amp service. I talked to the power company and they told me my supply lines and meter are already setup for up to 600amp service so all the upgrades to service happen after the meter.
> 
> So to upgrade to a new service do i need to run a new panel feed by the existing feed lines or do i need to replace the current panel to a larger panel fed by existing lines. Example pic attached.
> 
> ...


     First of all , in our area , a 320 amp meter base is the largest meter base we can get .

     Larger than that , the utility uses CT's ( current transformers ) to operate a corresponding meter instrument  .

     Different things are done slightly different in different parts of the country , as SOP ( standard operating procedures ) .

     Has any one done a load test / calculation on your existing service ?

Wyr
God bless


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