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  #1   IP: 64.38.163.205
Old May 22nd, 2003, 12:59 PM
twigpig twigpig is offline
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Default 400 amp wire and conduit size

The PUD is recomending a 400 amp service for the house I'm building. Actually it is a 600 amp service with a 400 amp disconect for the house and a 200 amp disconect for the future detached garage at the meter pedestal.

I have done the sizing calculations and come up with 129 amps for the house and attached garage. But the PUD engineer says I should have 400 amps because I want to put 240s in the attached garage for welding, compressor and table saw. He is also concerned about the 104 locked rotor amp draw on the geo thermal heat pump and the backup resistance heating that really doesn't show up in the service calculations.

So I am looking at bringing 400 amps to a box in the garage and then splitting to 200 amps. One will go to a 200 amp panel where it enters and the other to a disconect and then to a 200 amp panel in the laundry room. The inspector said I couldn't bring in two 200 amp feeders even if they are switched. I don't see what is gained by splitting them once they get into the garage since there are still two seperate 200 amp disconects. There is a 400 amp disconect at the meter pole. Maybe that is what is important.

The meter and ct can is 60 feet from the service entrande at the garage. If it were less than 15 feet I wouldn't hve to have all these disconects.

Anyway to the question. All of my wiring books only address 200 amp services or 400 amp services with dual feeders. I need to know the wire and conduit size for the feeder wires. The neutral and ground are to be bonded at the meter base so I have to run a neutral and a ground to the meter base with the 2 feeders. The inspector said I can get away with bonding at the disconects in the garage because I got my permit a couple of weeks ago but the code has already changed to bonding at the meter pedestal. I'm going to play it safe and go with the new code.

I read duckdog's post and the recomendation of 400 kcml I assume copper feeders. I think he was planning on using 3 inch conduit. I would also like to use copper feeders. 400 kcmp is going to be hard to pull no matter what size the conduit is.

Thanks in advance
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  #2   IP: 148.78.243.121
Old May 26th, 2003, 06:44 PM
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Wgoodrich Wgoodrich is offline
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YOU SAID;
The PUD is recomending a 400 amp service for the house I'm building. Actually it is a 600 amp service with a 400 amp disconect for the house and a 200 amp disconect for the future detached garage at the meter pedestal.

REPLY;
I do not understand why you want a remote pedistal. Why not install your 400 amp double or triple lug meter base on the house? That is where your largest load will be. While it is true you may have a 60 amp welder in your garage if you are the only one in that detached garage then you will only be running one machine at a time. To calculate a demand load of your detached garage find the largest machine you will operate. Increase that 25% then add the full load of any auto run machines such as air compressors and the like. This will be your minimum service size. YOu may install a 200 amp service to your garage but if you run all your load to the garage then the house you are increasing your voltage drop concerns. You most efficient design is to place you service meter on the structure that uses the most electricity over the life of your estate planning. Then place the meter on that structure and let the power company be concerned about voltage drop on the line side of where you start paying for electricity. Then the voltage drop [power you pay for but don't use is on the power company cost not yours. You may still install our 200 amp panel in your detached garage but now you are only running one long 200 amp wire instead of two sets of 200 amp service entrance conductors to the house. 200 to the garage from the house is cheaper than 400 from the garage to the house. Installing 600 amps worth of panels to a 325 amp rated meter base does not add load but only increases efficiency and room to grow. Demand load calculation is what tells you the true load being applied on a service conductor.

YOU SAID;

I have done the sizing calculations and come up with 129 amps for the house and attached garage. But the PUD engineer says I should have 400 amps because I want to put 240s in the attached garage for welding, compressor and table saw. He is also concerned about the 104 locked rotor amp draw on the geo thermal heat pump and the backup resistance heating that really doesn't show up in the service calculations.

REPLY;
Remember if you are the only one working in that garage the largest manually operated machine is all that should be your concern in design. You could have as much as 10 - 100 amp machines with only one man operating at any given time and you only need a 125 amp service even though you have 1000 amps worth of machines. If you have more than one say two people working in that garage then increase the largest machin the required 25% increase then add the full load amp rating of the second largest machine that may run at the same time and your are sized for two people working in that garage. Just don't forget to add in the full load amp rating of any auto operated machines that may turn on while you are operating that one manual machine.
General lighitng is minimal concern for the garaqe.

The locked rotor rating is not a part of the demand load calculation because it is not a concern of sizing a service. Interupting rating is the same for a 100, 200, or 400 amp residential service system.

If you calculated only a 125 amp demand load then you must have a small home and all gas appliances. That is a small demand load for a new home. The 400 amp service does not require a commercial rated 400 amp disconnect. You may run your service conductors all the way around the outside of your home or directly from the meter base across your yard and you only have to install main service rated panels or disconnects at NEAREST POINT OF ENTRANCE INTO YOUR HOME of your service conductors. The concern for locked rotor of heat pump say a 60 amp heat pump would require an overload within the hermetic heat pump motor to protect from locked rotor conditions see the following commentary.

See NEC Handbook commentary 430.4 NEC Handbook 2002;
In general, every motor must be provided with overload protective devices intended to protect the motor windings, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. Overload in equipment is defined as operation in excess of normal full-load rating, which, when it persists for a sufficient length of time, will cause damage or dangerous overheating. Overload in a motor includes a stalled rotor but does not include fault currents due to short circuits or ground faults. See 430.44 for conditions where providing automatic opening of a motor circuit due to overload may be objectionable.

In conjuntion with the overload your main service rated breaker or fuse is rated in what is called an interupting rating. This interrupting rating would be the same whether it be a 200 amp panel or a 400 amp panel. You will not gain safety by increasing service size in concern of locked rotor condition as explained in the above copied section of the NEC handbook. The hermetic motor built in overload is required to protect for locked rotor. Disconnects or controllers are required to be sized or rated to accept the limit of that overload protection locked rotor condition. You have no concern with locked rotor whether you install a 200 AMP or 400 amp service. The interupting rating of the breaker will step in as designed if the overload of the hermetic motor failed.

YOU SAID;
So I am looking at bringing 400 amps to a box in the garage and then splitting to 200 amps. One will go to a 200 amp panel where it enters and the other to a disconect and then to a 200 amp panel in the laundry room. The inspector said I couldn't bring in two 200 amp feeders even if they are switched. I don't see what is gained by splitting them once they get into the garage since there are still two seperate 200 amp disconects. There is a 400 amp disconect at the meter pole. Maybe that is what is important.

REPLY;
The disconnect at a service pole is not required unless a Utility company or local ordinance applies. The main service disconnects are required to be installed either outside or nearest point of entrance into a certain structure. If the service conductors serving the house comes through a meter base but does not enter that garage then the NEC does not require any disconnect from the meter base anywhere except where it enters that house. Teh house service and the garage service is treated as two different services unless the second building is fed from within the first building.

I would bring the 400 amp meter and mount it on the house then sub feed the garage from teh meter base as a service conductor. The NEC does not require main disconnects or overcurrent devices [aka breakers or fuses] until a service conductor enters inside the structure. Tapping from a meter base outside is not considered as entering a structure. Main breakers or disconnects are only required at "nearst point of entrance inside the structure", not outside. I suspect you were planning a main in one location and a main in a second location inside the same structure. That is why you inspector made that comment. You are required to group your main service panels inside a certain structure in one location while within that structure. You are not allowed to install main service rated panels in different locations in your home. You may install a 200 amp panel and a 200 amp disconnect grouped together at the nearest point of entrance of your service conductors into that dwelling. Then install a 200 amp sub panel from that grouped main 200 amp service disconnect. If you have a demand load of 130 amps in a home you have no need for sub panels. If you want a 400 amp service in your home install two 200 amp distribution panesl side by side as your grouped main service panels. You are allowed up to 6 main service disconnects inside your dwelling but they must be gouped. Sub panels from what you are discribing is money spent without benefit to you for you money spent.
Remember the AHJ has the last say in ruling what will pass in your jurisdiction. I am just quoting what the NEC says.

YOU SAID;
The meter and ct can is 60 feet from the service entrande at the garage. If it were less than 15 feet I wouldn't hve to have all these disconects.

Anyway to the question. All of my wiring books only address 200 amp services or 400 amp services with dual feeders. I need to know the wire and conduit size for the feeder wires. The neutral and ground are to be bonded at the meter base so I have to run a neutral and a ground to the meter base with the 2 feeders. The inspector said I can get away with bonding at the disconects in the garage because I got my permit a couple of weeks ago but the code has already changed to bonding at the meter pedestal. I'm going to play it safe and go with the new code.

YOU SAID;
I read duckdog's post and the recomendation of 400 kcml I assume copper feeders. I think he was planning on using 3 inch conduit. I would also like to use copper feeders. 400 kcmp is going to be hard to pull no matter what size the conduit is.

REPLY;
400 Kcmil is rated 400 amps for a dwelling setting per 310.15.b.6.

I would parallel if possible 2-2/0 copper conductors using double lugs or two 200 amp main service rated grouped panels to make up the 400 amps your are speaking of. A 400 amp disconnect will cost you much more money than 20 - 200 amp main distribution panels rated as service panels for a dwelling. Parallel 2/0 copper equal 400 amp rating also.

Good Luck

Wg
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