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Author Topic: Need opinions on wire size for 110 volt to inverter  (Read 3646 times)
belfert
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« on: September 16, 2009, 08:59:30 AM »

If I get a new inverter it will require 30 feet of wire to get to my 110 volt electrical panels.

The input and output circuits will be 30 amp.  Is there any reason I need wire larger than 10 AWG for this?  The guy I will buy the new inverter from is recommending 8AWG due to the length, but that seems overkill and doubles the expense for the wire.  I can understand going larger if it was DC and not AC.
« Last Edit: September 16, 2009, 09:01:41 AM by belfert » Logged

Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #1 on: September 16, 2009, 09:07:30 AM »

Hi Brian,

I ran 6 AWG from my inverter to the panel.   50A service/3000w inverter and about 9 ft. in length.

I would use the 8 AWG for your 30a service. Length= amps usage..

Good Luck
Nick-
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« Reply #2 on: September 16, 2009, 09:13:14 AM »

A better Idea...

Try and get the distance between your inverter and panel closer... You may have to rethink your layout.

Usually, most of your electrical equipment is located centrally in one or two bays and an interior panel not too far away from that.

Then your branch runs after the breakers can run the long distance runs.

Nick-
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« Reply #3 on: September 16, 2009, 09:21:35 AM »

I agree with Nick, move it closer if you can.  However, the distance from the inverter to the batteries is much more critical than the distance to your electrical panel.  Unless you can move the batteries as well, it's not a good idea.

8 gauge is the way to go, both in and out.  Consider that if your inverter is 30 feet away from the panel that is 60 feet when in pass through mode.

There are lots of tables on the net that will confirm that, or, you can just trust those who have the experience.
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« Reply #4 on: September 16, 2009, 10:28:56 AM »

Brian,

#10 is allowable.  However, as previously noted, bigger is better.  If the issue is raceway space, you could do #8 on the input side and #10 on the output side, since there will almost always be more current carried on the input lines if it is a charger-equipped model.

Also, remember that you can go one trade size smaller for the grounds.  So #8 hot and neutral with a #10 ground, or #10 hot and neutral with #12 ground.

If it were my installation, I would go with four #8 for the current-carrying wires, and two #10 for the grounds.  Anything you run from this circuit will thank you for the lower voltage drop, especially important if that includes a motor, such as an air conditioner.

JMO and FWIW.

-Sean
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« Reply #5 on: September 16, 2009, 10:45:33 AM »

My currently broken inverter is mounted next to the electrical panels, but it is also some 25 feet from the batteries.  Yes, everything really should be together, but that doesn't always work out.  The battery cables are 4/0.  My batteries are all the way in the back of the bus.  I guess nothing says I couldn't move the current inverter if I fixed it.

If I get a new inverter it will be mounted next to the batteries in the back of the bus, but it means the AC cables will be longer.

I suppose I could move the electrical panels, but man would that be a lot of work to finish in a week by the time I ordered wire.  I also run into the issue of having to run the 50 amp service an extra 10 or 15 feet whcih might mean going to 4 AWG cable.  I suppose the cost of wire wouldn't be much different between the two options, but the amount of labor would be huge.  I also have the delimma of where to run all the cable and wire to the rear with minimal cabinetry and woodwork to hide it all.

It sounds like I really need to go with 8 AWG for the 110 volt for the new inverter unless I move the electrical panels all together.
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« Reply #6 on: September 16, 2009, 11:24:12 AM »

One of the issues was extra cost (double) for the 8 AWG cable and the second issue was do I really need 8 AWG.  They run 10 AWG for 30 amp circuits pretty far in houses all the time.

I have room for the larger cable.  I am planning to use flat marine cable with three 8 AWG conductors for this.
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« Reply #7 on: September 16, 2009, 11:32:14 AM »

I would probably rethink putting the invertor right next to the batteries. The out gassing of charging batteries is not very healthy for electronics. I am installing a 3500 heart. Manual says as close as possible, but op to 20' is acceptable. I am going to use welder wire for the connection between the two. Higher amp. carring cap. due to the braid, more flexible, and usually better insulated than battery cable. The connectors on the ends are also critical. Soldered ends make a better connection than the clamp on ones. Just my $.015 cents worth!! Cheesy Cheesy
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« Reply #8 on: September 16, 2009, 11:59:25 AM »

I have room for the larger cable.  I am planning to use flat marine cable with three 8 AWG conductors for this.


At the risk of opening the "boat cable" can of worms here, I strongly recommend you stick to NEC-approved and rated wiring materials for the 120-volt side.  That would not include marine cable.

... The connectors on the ends are also critical. Soldered ends make a better connection than the clamp on ones. Just my $.015 cents worth!! Cheesy Cheesy


With all due respect, soldered ends on battery cables are extremely dangerous.  Best practices call for mechanically secured ends.

The reasons are simple:  Ohmic heating of the cable (possibly exacerbated by other heat sources such as the engine, inverter, charger, etc.) can easily and quickly approach the melting temperature of the solder.  As the solder softens the conductors can move inside the fitting; in the case where there is any mechanical tension on the cable at all (not uncommon in battery installations), this can even go as far as the conductors pulling partially or fully out of the fitting.  That, in turn, can cause higher resistance in the connection, leading to even more ohmic heating in a vicious cycle.  A fire, electrical short circuit, or battery explosion could result.

The other reason is that the solder impregnates well into the stranding of the conductors, making the end of the cable unacceptably stiff and brittle.  In a moving vehicle, vibration and other flexing induced in the cable can actually cause the conductors or even the entire joint to fracture and break, again possibly leading to the above conditions.

Crimp your cables; do not solder.

-Sean
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« Reply #9 on: September 16, 2009, 12:05:17 PM »

Brian,

What Eric and Len said.

Getting the inverter closest to the bats is the real issue and the wire costs between 4-0 stranded and #8 should prove that out at a glance.

House wire size is specified with temp rise as the issue....not voltage drop.  I have been told.  So saying that a given size "will do the job or meet the spec" is missing the mark.  Me thinks.  I have always been one to upgrade if possible.  I note that my house voltage goes down dramatically as the current draw approaches 30 amps.  Sooo, to avoid the normal deterioration that we associate with age, go bigger.

What I have done in the past is to NOT remove the original wire but rather run an additional wire of equal size in parallel.  By doubling the wire size you increase the wire "gauge" by three points.  So if you run the second wire the effective gauge would be #7 and that should tickle the pants off everyone.  And, it will definitely be cheaper and the added benefit is that you will be carrying a "hot spare".  Electronics tech joke, that.  That wire doesn't have to be in the same chase.  Also, make sure that when you take Sean's advice you make the NEUTRAL the same size as the hot lead and the unshielded GROUND is the one that can be downsized.  Probably knew that and that is not to say he wasn't clear....sorta like up sizing the wire....overkill on my part. Huh Grin Grin Grin

# 12?  #14?  total capacity for each config?

Good luck with your project.

HTH,

John

« Last Edit: September 16, 2009, 12:23:20 PM by JohnEd » Logged

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« Reply #10 on: September 16, 2009, 12:11:05 PM »

#10 is minumum for 30 amp.  Run #8 in case you add some more appliances. 
Listen to Sean.  He knows what he is talking about.

David
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belfert
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« Reply #11 on: September 16, 2009, 12:11:16 PM »

I have room for the larger cable.  I am planning to use flat marine cable with three 8 AWG conductors for this.

At the risk of opening the "boat cable" can of worms here, I strongly recommend you stick to NEC-approved and rated wiring materials for the 120-volt side.  That would not include marine cable.

So, I should use solid copper cable that will break with the flexing and potentially cause a fire when the conductor is only partially broken and it heats up?  Yes, the RVIA allows solid copper (romex), but is that really the best option for an RV?  Remember the RVIA  is a group of manufacturers who want to spend the least amount when building a product.

The biggest complaint I have seen over the years on the bus boards is lack of UL listing for marine wire and cable.  ALL of the marine wire and cable I have used is UL listed.
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« Reply #12 on: September 16, 2009, 12:15:57 PM »

Getting the inverter closest to the bats is the real issue and the wire costs between 4-0 stranded and #8 should prove that out at a glance.

I already have the 4/0 cable installed and paid for.  I got the 4/0 cable for scrap price or less from a guy I know.  It is welding cable. 

I don't have any 10 AWG or 8 AWG cable.  The 4/0 cable needs to stay in place as I have DC distribution next to the current inverter.  I plan to relocate the inverter, but not the DC panel for now.  I will need to get a small amount of additional 4/0 cable to connect the inverter to the batteries at the new location.
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« Reply #13 on: September 16, 2009, 12:22:13 PM »

Sean makes very good points.  In practice, the soldered joint does not become more resistive with age and is the preferred method and is "old school" as well..  It does have a down side , as Sean mentioned, in that if not done properly, the stranded cable is stiffened.  But, the crimp/clamp joint will also have the stiffened point at the connector junction point.  Assembly technique is important.  You can't simply fry the connector and then apply a glob of solder(my method Grin Embarrassed).  Do that and the solder will get wicked into the wire an inch.  Heat it, apply the solder and make sure it filled the connector all the way around.  Applying the solder will lower the temp and slightly prevent excessive molten time and heat transfer to the cable.

The spec for all joints is "mechanically secure and electrically complete"...old school.  I would not use one of those clamping type terminals....too much corrosion over time.  Get the copper crimp type and crimp it before you solder it. You can enjoy the benefit of the soldered joint and not worry if the bat terminal connection deteriorates in the future and generates heat.  Make sure you put a wrap of vulcanizing tape on the seam/ joint of wire and connector and then wrap it with electrical tape for a vapor proof joint and NEVER any corrosion.

That is at least 3.5 cents,

John
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« Reply #14 on: September 16, 2009, 12:30:41 PM »

Brian,

I agree with you about the stranded but I don't have a tech/spec reason.  Just what I know is the reason for installing stranded in any location.  I think stranded is superior for any environment that has vibration.

About that UL listing.  Lots of stuff has UL listing but it is not "approved" for a certain application.  I think the point is that stranded Marine isn't approved for RV applications.  I have not a clue as to why but I bet Sean does.

Having all the 4-0 you need gives you great flex. in siting.  Good for you.

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
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