Divergence of opinion on service

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Old 04-07-15, 11:49 AM
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Divergence of opinion on service

Hello,

We have an existing 200amp main panel inside an existing structure of 4510 sq ft which has power. Have all the wire run to the outside of the house and the rough-in electrical inspection complete on another 200amp panel in an addition of 4200 sq ft., and a 100amp panel in an outbuilding of 520 sq ft.

In our county in Florida (Volusia) found out I can get everything powered if I have a new outdoor panel installed connecting the two additional and the existing powered panel work done by a licensed electrician.

I just met with the second electrician 30 minutes ago(experienced since 1987). The first electrician came yesterday and works for a large company here locally.

One insists the new outdoor panel and meter must be 400amp, and the other said a new outdoor panel and meter could be 200 amp to serve all the old and new requirements.

While the 400amp panel kind of makes sense, I met one of the local power company's electricians who indicated although we're adding to the demand, the transformer placed on our property and the wire they're already using to the existing panel won't change......To me that means the wire they ran from the pad-mounted transformer can feed everything, which is the same size wire as was mandated by code for the new 200amp panel.


400amp or 200amp for the new panel?

Any insights would be most welcome.

flbuild, central florida
 
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Old 04-07-15, 12:09 PM
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Remember electrical company wire size standards (applying the NESC) are different from NEC mandated wire sizes. NEC mandates a certain service rating (100A, 150A, 200A) based on square-footage and actual loads expected in the house. It's more of a worse case calculation and wire sizes are much more conservative (also taking into account voltage drop as well as wire heating from bundling conductors in a conduit or cable assembly, burying conductors, and passing cables through insulated walls).

Utilities usually assume you won't be using all 200 or 400 amps and apply more of an average case determination using estimated, not worse-case demand. Many times a transformer may be shared by 4 or more houses, and you will blow the transformer fuse if you actually draw a full 200 amps. Additionally aerial utility drops are in open air and benefit from additional cooling so wire sizes can be smaller.

tl;dr just because the utility won't change wire size doesn't mean your panel or service entrance cable (NEC jurisdiction) don't need to be upsized.
 
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Old 04-07-15, 12:48 PM
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You will want to do a load analysis using the rules in the NEC. For starters, allow 3 watts per square foot to cover the small stuff like lighting, TV, cake mixer, computer, etc. You will have something like 9200 sf when you are done building which comes out to about 115 amps worth of 120/240 volt service.

I suspect that when you add up the amperage for air conditioning, large appliances, etc. you will be well over 200 amps.

You may well need a 300 amp service to meet code. I don't know whether they make 300 amp panels so 400 amps would be the next size. At any rate, a 400 amp panel would cost trivially more than a 300 amp panel.

Or you can have a (physically) small 300 or 400 amp rated panel outside with just double breaker sets for the existing 200 amp panel and a 150 or 200 amp panel in the new part of the house and the panel in the outbuilding respectively. The large panels would be (re)configured as subpanels.

When you order your 400 amp (or 300 amp) service from the power company, they will know that the whole house overcurrent protection will be for 400 (or 300) amps and they will install or upgrade pole feeders, etc. as they see fit.
 

Last edited by AllanJ; 04-07-15 at 01:28 PM.
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Old 04-07-15, 08:20 PM
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No one can sit at their computer and size the service without knowing load information. The two most important things are whether there is electric space heating or electric water heating.

I'll go out on a limb and say you need the 400 amp service, based on typical loads I remember from years back.

I met one of the local power company's electricians who indicated although we're adding to the demand, the transformer placed on our property and the wire they're already using to the existing panel won't change
I seriously doubt this guy was an electrician, probably was a lineman who looked at his work order to give you the answer. Have you provided the utility company with the calculated load yet? They need the load information in order to calculate their feeder and transformer size.
 
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Old 04-08-15, 02:06 PM
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I would be very surprised if a 10,000 sq. ft. building (other than an airplane hanger) calculated out at 200A or smaller service. You will probably need a "residential" 400A service which is designed as a 320A continuous. This is often installed as a 400A meter base which feeds two 200A "main panels". Even still this is a guess -- you need to do the demand load calculation to know for sure.
 
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Old 04-08-15, 04:08 PM
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I would be very surprised if a 10,000 sq. ft. building (other than an airplane hanger) calculated out at 200A or smaller service.
Depends entirely upon what use is made of the space. We don't know what is going to happen in this building, it could be just a storage garage for a bunch of old automobiles. With minimal lighting and no electric heat or cooling it could be a load of less than 30 amperes.
 
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