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# Neutral Bonding Screw?

#1
09-26-00, 07:54 PM
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I'm installing a subpanel in my garage. The panel came with a neutral bonding screw. What is the purpose of this. Also its about a 100ft run with 10-3 wire. Is there a formula to figure out my amperage drop over this run? And if amps drop to say 27 do I put a 30 amp breaker in the main panel. Thanks Tim

#2
09-26-00, 09:12 PM
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Throw the neutral bonding screw away. You only use this for main panels.

Amperage doesn't drop. Voltage does. Electrons that go in one end of the wire always come out the other end.

Voltage drop over 100 feet of 10/3 with 30 amps 240 volts will be less than 3%.

Put a 30 amp breaker in the main panel if you're using 10-3 wire.

#3
09-26-00, 09:27 PM
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Location: Los angeles
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John Nelson is right

You ran a 10/3with ground. This means you brought the grounding conductor with you with the garage relying on the dwelling for a grounding source.

YOu must not install a ground rod that would create two grounding sources to the same structure.

When you ran the grounding wire in the 1/3 [bare] wire John Nelson recognized your wiring method designating your garage panel a sub panel of the main panel in the dwelling.

The simplified formula for a voltage drop is as follows

2xKxLxI divided by circular mill of conductor

These properties you will find in Chapter 9 Table 8 of the NEC

These calculations are not for the inexperienced.

I personally feel that unless you are using a full 30 amps in that garage on a continuous manner then your voltage drop should be ok.

If you voltage did drop your amperage would rise. You size your breaker by the ampacity of your conductors that it protects. The ampacity of a 10 Ga. wire in the wiring condictions you describe is 30 amp. You seem to be ok for your design. Just connect your bares to the grounding bar that is bolted in contact with your panel box and install your whites to the neutral bar floating on plastic. You should be ok. Remember to get Permits and inspections, and install GFI protection on all readily accessible receptacles in the garage.

Good Luck

Wg

#4
09-27-00, 08:05 AM
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Thanks for the info John & WG. This is a big help. Over the summer I noticed the old tube & post wire running overhead to the garage was bare in a couple of spots. Also I regularly got shocks if I touched metal frame of power tools during use. Thanks Again Tim

#5
09-27-00, 10:26 AM
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Ok So where does the ground wire from the 10/3 go in the subpanel? There is no lug screw to connect it to the neutral bus, which is where it is connected at the main panel. Or do I just connect it to the silver torx type screw on the neutral bus? Do I need to get a ground bar kit? If so where does the ground bar go? Thanks Again Tim

#6
09-27-00, 12:51 PM
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Yes, if the box you bought doesn't have a separate grounding block, buy one. Attach it to the box itself, normally off to one side. You may see some screw holes already there for that purpose. Do not allow the ground to contact the neutral bus, nor allow the neutral bus to contact the box housing.

#7
09-27-00, 12:54 PM
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Depends on the brand name and the design of the panel you have.

If you have a full sized panel with a jumper bar below that ties the two bars together with two screws, just remove that bar it should be in the manufacturer's specs.

If you have a series of little bitty holes in the corner area of you panel then the panel is desined for an accessory grounding bar. This is especially true if the bar connecting two neutral bars together is located behind the main breaker or the buss bar.

Hope this helps

Wg

#8
09-27-00, 02:00 PM
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Tahnks again! The panel is a Suare D load center main lugs. What I don't understand is if the neutral and ground wires are together on my main panel why not on my subpanel? One more question ! Is there anything wrong with running UF wire in conduit burieed 18" deep? My reason is that I may up grade things next summer ( three way switch for out building lights or something ) and I had a roll of 10-3 UF I bought at a local Builders square going out of business. Thanks again Tim

#9
09-27-00, 02:13 PM
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First of all, be assured that this is indeed true. You must not connect the ground and neutral in the subpanel -- even though they are connected in the main panel. The reason has to do with "ground loops". Type this term into your favorite search engine and you'll get more than you ever wanted to know.

WG, what about it? I've heard you say before not to run cable in conduit (for heat reasons I assume). Is there some way Magoo can do it?

#10
09-27-00, 02:37 PM
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hello Magoo,
its best not to run romex inside of conduit, due to heat reasions. since u have uf wire what i would do is run the wire in the ditch with the conduit. this leaves the conduit fully open for future use.

now lets address the bonding issue.
some sqD panels come with ground bars in them if yours doesn't then it will have holes to except 1. this is a sub panel and u brought the ground from main to it there for u must not/can not bond the nutural and ground together according to the code. there will be a bonding screw for the nutural bar DO NOT INSTALL THIS SCREW, it bonds the nutural to the can, and the ground bar attaches directily to the can, thus bonding the nutural and ground. it would take me to long to explain why u don't bond subs, just take my word for it dont do it, if u like put ground loops in a search eng and look it up.

#11
09-27-00, 06:23 PM
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Location: Los angeles
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Hey Magoo & John Nelson

The following is the chart concerning more than three conductors in a raceway and the deration that you have to consider. Take not this is not the section pertaining to non metalic sheathed cable in a conduit. It is ok to install Romex or UF in a cable as long as it is not overfull. The second section is a copy of the section pertaining to Romex in a conduit.

Table 310-15(b)(2)(a). Adjustment Factors for More than Three Current-Carrying Conductors in a Raceway or Cable

310-15-b-2-a

Number of Current-Carrying Conductors Percent of Values in Tables 310-16 through 310-19 as Adjusted for Ambient Temperature if Necessary
4 – 6 80
7 – 9 70
10 – 20 50
21 – 30 45
31 – 40 40
41 and above 35

Romex in a conduit

CHAPTER 9

NOTES TO TABLES

9. A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

I really do not see why Magoo would want to install the UF cable in a conduit unless it is rocky or other physical damage may occur to cause the UF cable to be protected. UF cable is approved for direct burial at a minimum depth of 18" deep. I'm with Sparky dont run the cable in a conduit unless you have damage risk to the cable due to rocks etc.

Hope this helps

Wg

#12
09-27-00, 07:23 PM
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I'm running the 10-3 uf in 2" plastic conduit. As I said before I'm running the 10-3 because I bought it earlier for this project and finally I'm getting around to doing it. Once I started to dig the trench I realized previous owner had buried field stone, not the round type the flat sharp ones used in old foudations, in driveway. Thanks again for all the info everybody. Tim

[This message has been edited by Magoo (edited September 28, 2000).]

#13
09-28-00, 02:09 PM
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See previous post Last question on this ( I promise )

#14
09-28-00, 02:56 PM
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Wg, i think that with the length of the run that Magoo should consider using a #8. With the #10, i can almost guarantee that running power tools and such will dim lights in the house. A #8 would reduce voltage drop to less than 2%, and would also handle any heavy loads better than the #10. It would also allow room for expansion of the garage circuits if, for example, Magoo decided to install more lighting, receps, etc. Not to mention things like refridgeration equipment, etc. I think the #8 would be a good investment, and also save having to dig everything up at a later date. I would also stick with the conduit, but using PVC in the type of soil Magoo describes, i would opt for Sched. 80 instead of Sched. 40, and use THHN instead of cable. Just some thoughts.

#15
09-28-00, 07:47 PM
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Dave S I don't know that I disagree with what you are saying. Most people run #4 cu for 100 amp rating and to compensate for a voltage drop that may be present. If you figure 250' from transformer to house then throught the house, and add the 100' to garage he would have some voltage drop experience as he increases load. I am not saying that you would have that much load in the garage just the the differance in cost is usually small and the advantages for future is great almost unlimited. I agree if going to the trouble to run feeders out there and digging the trench the extra cost whether #8 Ga. or #4 Ga. is well worth the add investemnt. Good advise on your side.

Keep sending in your replies. Each one of us have a value in what we say. The one with the question is smarter than any one of us who replies because he has the combined knowledge and ideas of all who gave their ideas and thoughts on the subject. We all work well together. Everyone should be proud of the product provided in this forum.

Proud to be a part of such quality

Wg

#16
09-29-00, 07:27 PM
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First of all thanks everybody for all the advice!

Ok so if I go with # 8 THHN wire, what kind of box do I use to connect the THHN to romex once I get it indoors.? Remember this 2" conduit. I've never seen a pvc junction box for 2" conduit in my hardware store. I figured on leaving " breathing room " for my 10-3 UF that I was originally gonna put in it. I don't want to run conduit all the way to the main panel. Can I use one of the connectors with the screw on cover plate. I think they call it a pulling "L" or something like that. Do I use just stardard wire nuts or are there like lug conecctors to connect it together? I kept the sch 40 pvc conduit but dug the ditch wider & little deeper the filled with sand and I'm gonna sort out the rocks when filling in. Gee I promised last question a couple of posts ago Thanks Again Tim

[This message has been edited by Magoo (edited September 29, 2000).]

#17
09-29-00, 08:12 PM
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#8 THWN is good for 40 amps. You can run sch 40 pvc in a trench 18" deep without sand back fill with a rock mix exposed to the conduit. This type installation is not considered by the NEC as subject to physical damage. Sch 80 must be used from 18" below ground to the 2" LB condulet which will allow you to make the bend into the dwelling and garage both and be able to stay flat to the outside wall. A conduit will be requiered from the house panel to the garage panel no stop to protect the THWN conductors. A 2" conduit is plenty large enough to carry the three # 8 and a 10/3 uf cable. Just watch that you do not mix line and load together on a service without an overcurrent device at the line side. You don't have to worry about the line load not mixing because both the #8 and the 10/3 would be considered loads because they are on the load side of breakers or fuse. If you run three conductors from the house to the garage I would install a 100 amp service rated box with at least 8 circuits in the panel. A ground rod and grounding service conductor must be installed with a #8 grounding service conductor from the ground rod to the service rated panel. You must marry the whites and bares together on a service rated panel. If you were to run 4 conductors from the house to the garage then you could install a 100 amp rated main lug only panel with 8 circuits or more in the non service rated panel [sub panel] if with 4 wires. This sub panel design will have the grounding conductor coming from the main dwelling panel. Remember that the neutrals and grounds must be separated in a sub panel.

Good Luck

Wg

#18
10-01-00, 05:16 PM
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Thamks everyone for your input! I'll put it to good use ! Tim

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