Wire Sizing


Old 12-02-05, 12:49 PM
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Wire Sizing

I am designing a pump panel service. I am going to have a 400 amp service. I would like to know how much I need to de-rate my wire if I intend to install two parallel runs of the wire for service. My intensions are to install 2 x 250 MCM THHN CU wire.

Do any of you have experience with Type W cord? I have several spec sheets on it and they have conflicting amperage ratings. Is this common?
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Old 12-03-05, 03:29 PM
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IMO, this job is something a professional electrician should be taking on.
Old 12-03-05, 05:47 PM
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It will be an electrician doing the installation. It is my job to design, order materials, and cost the project. This is why ask.

I am most interested on input on the type w cord.
Old 12-03-05, 05:55 PM
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Originally Posted by rshackleford
IIt is my job to design, order materials, and cost the project.
If you have to ask this on a DIY board I have to wonder.
Shouldn't you know about this stuff before you undertake such a project????
How are you going to price out a job you are not sure of?

Where does Type W cord come into play? You are using THHN, right?

For derating you need to know the exact load or calculated load, voltage and distance with regard to wire size.
Old 12-04-05, 12:56 AM
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The voltage is 460 volts. The load will be 250 amps and the service disconnect will be 400 amp. The distance will be small, a maximum of fifty feet.

The type w cord is used as motor lead on temporary, portable equipment. The type w cord question is an afterthought related to the service equipment I described above. Our typical cord is type soow, but when you get into larger horsepower the so cord is not large enough.

I have done a lot of these designs, but for smaller horse powers (50, 75, and 100). I am dealing here with a 200 hp setup and am looking at some different options. I know that I could run one really large thhn, however my service disconnect is designed to take parallel runs of up to 350 kcmil wire. I figure it would be easier to run 250 or 350 than it would be to run a single large run of wire. As far as the DIY board, I have been learning on here for a while now and though you people might have some advice.
Old 12-04-05, 08:18 AM
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You might want to check the boards at Mikeholt.com.

Old 12-04-05, 11:24 AM
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First let me say this - This is NOT a DIY project so people out their reviewing...never try this at home.

Ok...got that out of the way.

Now....based on what you have given us ( we will assume 3 phase since most HP motors in that size would not be 1 phase...it is important that you state that when asking large load questions like this )

If we are talking about 460 V and 250A Load for 50' you will need to run (1) 350....or you can parallel (2) 2/0 AL wires.....BUT that does not factor in the derating for the conduit in which you are going to use....

Now if you intend to use EMT.......doing the math.....blah..blah..blah....you will need a minimum 2.50in conduit....Should allow you up to 10 # 2/0 wires....You could also go down to 2in conduit but it has a max of 6 and would be tight...I always ERR on the larger size...

Now....their are other factors here....PF% of the motor, temp. factors ( conduit space factors have been done above ) and well.....this would be better if you wrote down ALL the factors involved.....

So.....not sure if that helps you much........but those are the things we have to consider when sizing for something like this.
Old 12-04-05, 11:54 AM
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You are correct this is not a DIY project. I have been watching this board and am impressed by the help people have been giving on the site. Thank you for stating that this should not be done by non-professionals. This project will be installed by a licensed electrician.

This is three phase

Thank you for your help. I cannot size the service for 250 amps because the service disconnect is 400 amps. I have to size for this amperage on the service entrance. It is this wire run that I wonder about de-rating.

Experience tells me that the limiting factor with this type of installation is the NEC allowed wire amperages. So, the info I have listed is the most relevant factors.

Thank you for all your help so far. It is much appreciated.
Old 12-04-05, 12:04 PM
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This presumes that the rating of the over-current protective device for the Service Conductors = 400, and the system is 3-phase, 4-wire. There are two raceways, each with 4 Service Conductors, each S-C conducting 200 amps.

The derating factor for 4 conductors in a raceway= .8

X x .8 = 200 X = 200/.8 = 250

250 MCM conductior W/ 75* rating = 255.

Art 22.61 Service Neutral Load--- "The Service Neutral load shall be the maximum un-balance of the load"

This calculation would require an analysis of all loads supplied by circuits with an Grounded ( Neutral) Conductor.
Old 12-04-05, 05:34 PM
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Okay then hire an EE, and stay a bit hands off from the project.
Old 12-05-05, 05:44 AM
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I've got to join the chorus of 'you probably shouldn't be doing this'.

I understand that a professional electrician will actually be doing the installation. But the _design_ of this system is as much a part of 'getting the job right' as the professional experience making proper terminations. If you are designing this installation and buying the parts, and your specification is _wrong_ and then the electrician 'installs to spec', then the installation is still _wrong_.

If for this job you are presenting yourself as doing the _engineering_ without the requisite training and licence, then you open yourself up to even greater liability than an un-licensed electrician.

One very significant problem with electrical installations and free forums such as this one is that there are many, many details in an electrical installation that _must_ be correct. If you ask a question about one detail, and don't even know about another detail, then your installation won't be correct.

A classic example is the question 'I have a lighting circuit with 14ga wire and I want to add a couple of lights. Is it okay to use 12ga wire?'...and we say, 'It is generally considered poor practise to mix 12ga and 14ga wire, but it is not illegal.' whereupon the person asking the question installs a bunch of new wiring in a box, totally ignoring box fill rules, because they didn't know that 'box fill' was even an issue and we didn't know that it hadn't been asked about.

We have no clue what things you don't even know to ask about, and the extremely basic nature of your questions suggests that you've not even read the code book.

Calculation of ampacity, including derating for number of conductors is spelled out in NEC 310.15 specifically table 310.15(B)(2)(a). Questions to ask are 'how many current carrying conductors will be in a single raceway?' 'Is the neutral a current carrying conductor in this installation?' 'Will I use one or two raceways?'

Flexible cords and cables, including the usage and ampacity of 'Type W' cords is spelled out in NEC 400. Type W cord is not suitable for wet locations. Table 400.5(B) probably answers your ampacity questions. Questions to ask ' how many current carrying conductors will be in the cable?' ' What is the temperature rating of the cable?' 'What is the temperature rating of the wire terminations?'

Motor installations are described in article 430. This is one of the most convoluted and confusing sections of the code. The rating of breakers for motors is _totally_ different than for other loads.

I'm not going to say you should never do this design work. I am going to say that I believe that you have a bunch of reading to do

Old 12-05-05, 08:14 AM
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Thank you everyone for your time.

Most of the heavy lifting has been done for me. I provide a horsepower to a UL panel manufacturer. This manufacturer provides me with a panel that is appropriate for my situation. I must provide the service, the motor lead, and mount the panel. I know what my service disconnect size is, I know what my voltage is, and I know what my motor load is.

I also appreciate that this is a Do It Yourself site. I apologize for bringing a professional question to the forum. I did not realize that this would be such an issue. I guess I can see why agricultural irrigation dealers get such a bad rap for poor installation. Thank you again for your time and input.

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