# 320 amp service and portable generator



## Locknutz (Sep 7, 2012)

Customer has dual 200 amp main panels with a 320 amp service. Bought a expensive 7500 watt ng generator for backup power. What’s the best way to feed this?

Right now thinking an indoor mounted 400 amp transfer switch and a 30 amp inlet on the outside. Price tag is coming out to $1,200. For his total price with generator and gas it makes sense to just get a small standby gen.


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## Kevin (Feb 14, 2017)

For a generator that small I'd do a small sub panel for the circuits he wants during an outtage. Maybe a 16 or 24 circuit panel. No way on a 7500 watt generator will you get to use everything from the 320 amp service...

Unless you meant to say 75 000 watt instead.

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## HackWork (Oct 2, 2009)

Installing a 400 amp transfer switch should cost way more than $1200. The switch alone is extremely expensive and you have to rework the entire service. 

But forget that idea altogether. You should just install an interlock on one of the 200 amp main panels. Backfeed a 30 amp breaker and run a 10-3 wire to an inlet. 

Then just switch any circuits that he wants during an outage to the panel with the interlock.


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## Southeast Power (Jan 18, 2009)

Locknutz said:


> Customer has dual 200 amp main panels with a 320 amp service. Bought a expensive 7500 watt ng generator for backup power. What’s the best way to feed this?
> 
> Right now thinking an indoor mounted 400 amp transfer switch and a 30 amp inlet on the outside. Price tag is coming out to $1,200. For his total price with generator and gas it makes sense to just get a small standby gen.


Ronk makes a very inexpensive double throw in both service rated and non-service rated:

https://www.ronkelectrical.com/meter-rite-grade-level-double-throw-switches-7416.php


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## 3DDesign (Oct 25, 2014)

I have a customer that wants to do the same thing, run a 320A/dual 200 service with a portable generator. Instead, I gave him a price on a standby 25KW natural gas generator with dual 200A, service entrance rated transfer switch. He hasn't decided yet.
The problem with a portable generator is, most are not rated for computer, electronics and circuit board use, and they're loud. The potable Honda EU7000iS is a good choice but costs almost $5,000. Another problem is gasoline storage.
For a $5,600, I can purchase both the 25KW and transfer switch.


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## Southeast Power (Jan 18, 2009)

Its my opinion that people have a little too much to drink and end up buying a generator for whatever reason.
They then sober up when they find out what they bought is wildly inappropriate for what need.
People somehow believe purchasing a generator is like buying a washing machine and think they can just call someone to install it.


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## nrp3 (Jan 24, 2009)

With the correct install and a customer who understands how manage loads, it could be ok, or a couple of selected circuit panels. Making people understand you need to run the portables regularly and use some fuel stabilizer seems to be one of the bigger challenges. I know he mentioned his was ng, but most aren't.


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## lighterup (Jun 14, 2013)

Southeast Power said:


> Ronk makes a very inexpensive double throw in both service rated and non-service rated:
> 
> https://www.ronkelectrical.com/meter-rite-grade-level-double-throw-switches-7416.php


just for the sake of assuming... I assume the OP's service is fed in 
parallel from the 320 amp meter socket enclosure to the 2 200
amp panels.

Does the Ronk product you posted , which is rated single phase @ 400 
amps , have parallel lugs in it (or ate least able to accomodate them?)


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## Southeast Power (Jan 18, 2009)

lighterup said:


> just for the sake of assuming... I assume the OP's service is fed in
> parallel from the 320 amp meter socket enclosure to the 2 200
> amp panels.
> 
> ...


Yes it will take one big one or two smaller.
I have installed at least three of these from what I can remember. They are cheap, only a few hundred bucks and a decent size. 
The major brand double throws are massive in size and at least 4x the price.
I purchase these through my electrical wholesaler as the price is about the same. Ronk, like most that sell to the public protects the electrical supply houses. 
I didn't take very many pic of the 400 but have a few of a 200 we installed.


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## svh19044 (Jul 1, 2008)

Southeast Power said:


> Ronk makes a very inexpensive double throw in both service rated and non-service rated:
> 
> https://www.ronkelectrical.com/meter-rite-grade-level-double-throw-switches-7416.php


I don't know how I've never seen this, thanks for sharing.


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## Southeast Power (Jan 18, 2009)

This is a pic of an installed 









I cant deal with this sideways pic thing but.
We came out of a 320 meter, installed a 400amp main disconnect per the engineer. I could have used the SE model.
We then fed the utility side of the Ronk Switch. Out of there, we fed each of the existing 200 amp mains.
FWIW, this one was an open delta.

One more thing.
The utility side is connected to a set of 4/0 size camlocks at the bottom of the enclosure.


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## Southeast Power (Jan 18, 2009)

This one is a 200 amp.
Fed out of a meter-main combo. We fed the Ronk out of the feedthrough lugs.
the load side feeds a 200 amp main lug interior loadcenter.
This setup gives us a few circuits ahead of the generator for whatever might be needed outdoor such as a fixture indicating if utility power was restored, battery charger, heater, etc.


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## svh19044 (Jul 1, 2008)

Damn, I just looked at the price on them. Insanely high pricing!


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## John M. (Oct 29, 2016)

Put a mechanical interlock in each breaker panel, a wire out of each panel to a J-box, and a cord from there with a plug to plug into the generator


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## Locknutz (Sep 7, 2012)

John M. said:


> Put a mechanical interlock in each breaker panel, a wire out of each panel to a J-box, and a cord from there with a plug to plug into the generator




I think this is the most cost effective approach. Splitting the 30 amp inlet feed onto two 30 amp beakers with interlocks in each panel.


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## HackWork (Oct 2, 2009)

John M. said:


> Put a mechanical interlock in each breaker panel, a wire out of each panel to a J-box, and a cord from there with a plug to plug into the generator





Locknutz said:


> I think this is the most cost effective approach. Splitting the 30 amp inlet feed onto two 30 amp beakers with interlocks in each panel.


I don’t like that idea because if there is a bastard splice anywhere into the house you could backfeed into the grid when only switching one of the panels over.

Generally doing one panel should be enough for them, especially if you switch some circuits over to it.


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## Locknutz (Sep 7, 2012)

HackWork said:


> I don’t like that idea because if there is a bastard splice anywhere into the house you could backfeed into the grid when only switching one of the panels over.
> 
> 
> 
> Generally doing one panel should be enough for them, especially if you switch some circuits over to it.




Trying to picture this in my head? Example? Been working all day maybe my brain is too fried.


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## HackWork (Oct 2, 2009)

Locknutz said:


> Trying to picture this in my head? Example? Been working all day maybe my brain is too fried.


 If you flip one interlock that will isolate that panel from the grid, but the other panel will still be connected to the grid and half of the circuits in the house. 

I have found bastards splices in many houses in which two hots from completely different circuits were spliced together in a box. Nothing happened because they were on the same phase. But if this type of splice is in their house, when flipping only one interlock they could backfeed out into the grid. Or when the power returns it could feed through and destroy the generator. 

It’s only 30A max, feeding a single 200A panel should get them what they need within the limits of the generator.


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## Kevin (Feb 14, 2017)

HackWork said:


> If you flip one interlock that will isolate that panel from the grid, but the other panel will still be connected to the grid and half of the circuits in the house.
> 
> 
> 
> ...


But with an interlock on only one panel this is still a possibility no?

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## CoolWill (Jan 5, 2019)

First, you need to get two dryer cords...


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## HackWork (Oct 2, 2009)

Kevin_Essiambre said:


> But with an interlock on only one panel this is still a possibility no?
> 
> Sent from my SM-G920W8 using Tapatalk


Yes, I didn't even think about that. :sad:


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## nrp3 (Jan 24, 2009)

With that said, possible in any scenario short of a single panel and interlock.


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## CoolWill (Jan 5, 2019)

HackWork said:


> Yes, I didn't even think about that. :sad:


I personally am not that concerned about the what-if. But it would be a relatively easy thing to check now that we know about it. With the mains off, check for continuity between the panels at the load side of each main. Well, that might still ring due to the neutrals being tied. So continuity isn't the right check. Resistance. There should a high resistance. If it is close to zero, then there is a direct tie.


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## nrp3 (Jan 24, 2009)

If you had two interlocks, one interlock and one panel not backed up, or a selected circuit panel, my guess would be if you see voltage at the main when the generator is running, then you have what Hack describes.


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## HackWork (Oct 2, 2009)

Yeah, you're right. Even a typical 6 circuit transfer switch has the ability to do what I said. 

Forget I said anything, I am so stupid :sad:


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## nrp3 (Jan 24, 2009)

No, I've never given that much thought so it may be something worth checking next time I install one.


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## splatz (May 23, 2015)

@HackWork, those Reliance PowerBack! alerts you install would help with this... did you ever have one go off on a backfeed?


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## HackWork (Oct 2, 2009)

Nope, not yet. Good idea though.


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## Kevin (Feb 14, 2017)

HackWork said:


> Yeah, you're right. Even a typical 6 circuit transfer switch has the ability to do what I said.
> 
> 
> 
> Forget I said anything, I am so stupid :sad:


You could get an ATS and feed a small sub panel. You'd have to hack some of the wiring on it so that it'll shut off the generator power if it sees any power on the utility side (as opposed to if it sees one leg it'll transfer to generator power.)...

I guess you could use 2 contactors with 120 volt coils with double pose double throw. Wire one phase to one contact and the other phase to the other contactor. Wire the generator into one contactor, then the next (contactor in series). Then if there's a hot connected to a non-generator circuit the contactor will open the generator power cutting the power... except it would cycle continuously... but you'd know it's backfeeding and you'd have to identify the wrong joint and fix it. Then it should work fine.

This is a lot of work, just sell a 400 amp transfer switch 

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## Monamac (Jan 13, 2021)

3DDesign said:


> I have a customer that wants to do the same thing, run a 320A/dual 200 service with a portable generator. Instead, I gave him a price on a standby 25KW natural gas generator with dual 200A, service entrance rated transfer switch. He hasn't decided yet.
> The problem with a portable generator is, most are not rated for computer, electronics and circuit board use, and they're loud. The potable Honda EU7000iS is a good choice but costs almost $5,000. Another problem is gasoline storage.
> For a $5,600, I can purchase both the 25KW and transfer switch.


I know this is an ancient topic but I have a similar situation. 320a, 1ph 240v service and a 16.5kw propane Generac. I’m Set up with 1 ea 200amp panel and 2 ea 150amp panels. Want to split out to all 3 loads.
can you tell me what equipment you used for switching and a simple diagram if exists.
thanks in advance for any help.


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## mjbasford (Oct 2, 2016)

Either a small 6 place outdoor breaker box, or inlet -> splice box -> 3x interlocks.

If you're worried about the back feed issue above, turn off panel b and c, with A still on, check for power at each breaker. Turn off A, on B, check again. Off with b, on with C final check.


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## Kevin (Feb 14, 2017)

Monamac said:


> I know this is an ancient topic but I have a similar situation. 320a, 1ph 240v service and a 16.5kw propane Generac. I’m Set up with 1 ea 200amp panel and 2 ea 150amp panels. Want to split out to all 3 loads.
> can you tell me what equipment you used for switching and a simple diagram if exists.
> thanks in advance for any help.


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