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#26664 - 03/26/03 08:43 PM Wire size for 300 feet @ 160 Amps
Anonymous
Unregistered


I am having a generator installed on my 6 poultry houses. Specs are: single phase, 240 volt. There will be a set of feeders per 2 houses (3 feeders for 6 houses). The longest run is 300 feet for one of the feeders. Most of the load is motors that run more than 3 hours. The man selling and installing the generator says that 250 MCM will carry 160 Amps for 300 feet without too much voltage drop. An electrician friend of mine says it would take a 400 MCM to have a 3% VD. Could you please advise because this will be a $27,000 job and I want it to work for many years to come.
Also tell me what might be the problems I might face with too much VD.
Thanks in advance
Ricky

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#26665 - 03/26/03 10:04 PM Re: Wire size for 300 feet @ 160 Amps
Pat_McBride Offline
Super Handyman

Registered: 07/12/01
Posts: 2014
Loc: Abbotsford, BC, Canada
O.K., first of all, are the generators supplying prime power or are they backup power (emergency power); in other words, are you using the generators as a source of power or as emergency power in case your local power company loses it and you?

I would think that, in any case, the generators can be considered as a primary source of power, either as primary power generators or as emergency power supply. Here in Canada, a generator used as a primary source of supply can have a 5% voltage drop; the 3% voltage drop comes in for feeders, that is, from a main panel to a sub-panel. Where the difference lies is in the Code used, whether NEC or Canadian Electrical Code. I would check with your local electrical inspector, because the interpretation of the code rules is up to him; you'll have to have it inspected anyway, why not get the word from the inspector directly.

The figure that I'm going to use is based on Rule 8-102 and Table D-3, Canadian Electrical Code, "Distance to Center of Distribution for a 1% Voltage Drop in Voltage on Nominal 120 Volt, 2-Conductor Copper Circuits". I've checked with my Code book (CEC 2002 edition), the formula is as follows:

Distance (in meters) x Voltage drop (i.e., 3 or 5) x 240/120;

The figure for 3/0 cable, 160 Amps, is 15.7 meters; the figure for 4/0 cable, 160 Amps is 19.8 meters.

3% Voltage Drop

So, 15.7 x 3 (Voltage Drop) x 240/120 = 94.2 meters x 3.281 meters per foot = 309 ft. -- 3/0 wire at 160 Amps

19.8 x 3 x 240/120 = 118.8 meters x 3.281 = 389 ft 4/0 wire at 160 Amps

5% Voltage Drop

15.7 x 5 x 240/120 = 157 meters x 3.281 = 515.7 ft. for 3/0 cable at 160 amps. Definite overkill.

So, I suggest that you check with the electrical inspector, get him to rule what the voltage drop has to be, and you might be able to go with a smaller size of cable. Give him this message, if you want, I'll sign it officially in an attempt to help you out.


Patrick B. McBride, AScT
Applied Science Technologist, Registered in B.C. Canada
Class A Accredited Representative, B.C. Electrical Safety Act
_________________________
Being able to watch horses and cows is God's gift to Man. Peace. Serenity. Calm

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#26666 - 03/26/03 10:06 PM Re: Wire size for 300 feet @ 160 Amps
Pat_McBride Offline
Super Handyman

Registered: 07/12/01
Posts: 2014
Loc: Abbotsford, BC, Canada
Sorry to be so formal in my other reply, Ricky, but if you give it to an inspector, he'll want to know who I am.

Hope it works for you, you might be able to drop one or two wire sizes and save money. Remember, I'm using Canadian rules so it may not apply in the US.

Good luck,

Pat
_________________________
Being able to watch horses and cows is God's gift to Man. Peace. Serenity. Calm

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#26667 - 03/27/03 08:03 AM Re: Wire size for 300 feet @ 160 Amps
Anonymous
Unregistered


Pat,
Thanks so much for the very informative reply. Formal was exactly what I wanted. I know you are busy like everyone else and I thank you for your time.
The wire in question will be aluminum and originates at an Auto Transfer Switch and carries both emergency and normal power from power company.
Does this make a difference on what I should expect for an acceptable voltage drop?
Are you familiar with the ElectricCalc Pro 5060 calculator by Calculated Industries? My electrician friend has one and showed me when you enter 75 degree aluminum wire, 240 volts, 160 Amps, 300 feet, 3% VD, it shows to use 300 MCM. Also do you derate this wire? If I punch the 125% entry it shows 400 MCM. Does this sound right.

Thanks again,
Ricky

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#26668 - 03/27/03 02:39 PM Re: Wire size for 300 feet @ 160 Amps
Pat_McBride Offline
Super Handyman

Registered: 07/12/01
Posts: 2014
Loc: Abbotsford, BC, Canada
Absolutely! As you may have gathered from my e-mail, and the title of the reference that I used, the table was based on copper. My code book tables show that you can use 3/0 copper for 165 Amps, whereas 165 Amps in Aluminum (actually 170 Amps) requires 250 MCM. And that's not allowing for a voltage drop. By the way, do you have to use 3% voltage drop? If it's going to be a main service, or similiar to one, why not try and get the inspector to allow a 5% drop, reducing the wire size. See my previous posting.

For the benefit of other interested observers, this is one of the reasons why the electrical industry abandoned using copper wire for small gauge work. You require a larger size of conductor for the same current, which reduces the number of conductors that you can pull into a conduit. In addition, the aluminum tends to deform under heat; this means that connections can come loose and cause short circuits, open circuits, and create fire hazards. In addition to all this, when you combine copper and aluminum in installations, you are required to coat the junction with an antioxidizing compound, another expense and typically messy.

Tell your friend he's right on the money. As far as derating is concerned, it depends on the application. Did you guys decide whether or not it's going to be a main service or a sub-service? I would suggest derating, and using 400 MCM, just to be on the safe side. That's if you have to go with a 3% Voltage Drop. If you want to upgrade at a later date, having the larger wire installed will be a saving in labour costs and will definitely remove the voltage drop factor; look at it that way.

Are you going to install all 3 conductors (and ground if applicable) in the same conduit? Don't forget, with those high currents, you may want to consider magnetically isolating the conduit from the main panel to reduce electrical currents induced by a 160 Amp magnetic field. Another thing that you might want to consider is the effect of these currents on animals. Our Code outlines certain methods that have to be used to reduce the effect of trickle voltages on livestock and fowl. We generally don't start considering this until the load current is over 200 Amps, but 160 Amps is pretty close and doesn't allow for upgrade.
_________________________
Being able to watch horses and cows is God's gift to Man. Peace. Serenity. Calm

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#26669 - 03/27/03 09:35 PM Re: Wire size for 300 feet @ 160 Amps
Anonymous
Unregistered


Pat,
There are no such thing as inspectors around here and I guess the only time something might be checked would be after a fire and lawsuit.
The 5% voltage drop would be no problem legally. I am fine with a 5% VD. The only thing I worry about is sometimes as far as these farms are from the power company you may have a low voltage to start with. Plus some branch circuits in these 400' houses are 200' or more counting turns.
For the question whether these are mains or subs I am not certain of the terminology. Here is the ladder as best I can explain. Power company, meter loop, 600 amp main disconnect, transfer switch, 3-200 Amp disconnects (one per pair of houses), the wire in question, a junction box to change the wire in question to 2 smaller wires going to each house, and lastly the breaker panel in each house. Which are mains and which are subs. If everything down stream of the 600 disco is a sub then 4 wires will have to be run to everything down stream of the 600. Is this correct?
A side note. There are hndreds if not thousands of poultry and swine farms in this country. I have been to many of them and there is not one that I have seen that has the neutral and equipment grd. seperated. I think the panels in the houses are sub-panels because the meter is on a pole outside with a 200 amp box under it with 2 100 amp breakers for feeders to the houses. There is also usually a double pole breaker at the pole for a well. If I am right what advise could you give when replacing the panel in the house considering these farmers won't let you run new 4 wire to the houses? You can't follow code with 3 wires and code won't allow you to seperate the two buses and ground the equipment ground to a ground rod because it is not an acceptable source to trip breakers. I know the answer is probably to not do the job but what is the safest of the two-- leave the buses connected or seperate and ground the grounding bus to a rod?
In response to the magnetic field. We plan on using PVC conduit and yes all wires in one conduit. Would that be considered magnetic isolation?

Thanks so much again,
Ricky

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#26670 - 03/27/03 10:05 PM Re: Wire size for 300 feet @ 160 Amps
Pat_McBride Offline
Super Handyman

Registered: 07/12/01
Posts: 2014
Loc: Abbotsford, BC, Canada
O.K., first of all I'm going to presume that you're in the US. Second, please check with your electrician buddy and make sure that all you're doing, and my advice, is O.K. with whatever Code applies.

Also, reading in your last e-mail 'A side note', are you doing this for yourself or for somebody else.

If you're doing it for somebody else, and getting paid for it, then I'm not allowed to give you advice for professional and ethical reasons. I can only practice in British Columbia, you have to engage me as a consultant, and then we have to do it all by e-mail because it doesn't belong on these forums unless the Administrator approves it for informational purposes. Even if you're not getting paid for it, because of the size of the job involved, you need an engineering professional such as myself to check your calculations and provide proper informtion.

By doing it for someone else, and everybody on the forum should note this, there are a lot of legal considerations that have to be taken into account. This particular case involves a high amperage distribution system that would be a possible source for a catastrophic event in the event of a failure. From what has been described, and in light of the absence of inspections, a number of legal issues would have to be resolved, particularly the issue of assigning blame in the event of death, injury, or damage to property. Even though there are disclaimers on this forum, the fact that I am an engineering professional will leave me liable since I am supposed to be aware of the legal and ethical aspects of my profession.

So, I'm not turning you down on your request for information, PROVIDED that you're doing the work for yourself and are prepared to accept full liability. Sorry that I have to be like this, but I wasn't aware of the scope of your project; the last e-mail has clarified it for me.

I don't see a problem with you and your electrician friend doing the work, I don't mind providing the information, I just want you to understand all of the issues around this particular post.

Pat McBride, AScT







_________________________
Being able to watch horses and cows is God's gift to Man. Peace. Serenity. Calm

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