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Can I power a multiwire branch circuit with generator?

Posted by AVInstaller (My Page) on
Sun, Sep 4, 11 at 23:54

I have a Briggs and Stratton Elite 6200 watt (8750 starting watt) generator that I plan to hook up to my main panel thru a transfer switch (probably a Reliance Controls 10 circuit 30amp) manual switch.
My first hurdle is that I have a 240volt well pump (with bladder tank), water softener brine tank, a radon water mitigation system with blower, a secondary above ground pump 115volt (with bladder tank) that draws water out of the bottom of Radon mitigation reservoir, and an oil boiler 1/7hp motor......Just to get a hot shower during power outage!! I have a watt meter that I will use to get real world numbers, but I feel I am already straining the generator with those items, but I checked my main panel, and I realized that my refrigerator is on a multiwire branch (double pull 20 amp w/shared neutral)....I wanted, at the very least, to have a hot shower, and keep the refrigerator alive during power outages.
I realize I would use a double pull 20amp (240volt) circuit to power my well pump thru the transfer switch, but is there a way to power the multiwire branch circuit to power up my refrigerator with the transfer switch as well without the shared neutral causing a huge fire hazard?
Anyone else have to power up their entire well water treatment center with their generator?
Thanks in advance for any/all advice.


Follow-Up Postings:

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RE: Can I power a multiwire branch circuit with generator?

"...is there a way to power the multiwire branch circuit to power up my refrigerator with the transfer switch as well without the shared neutral causing a huge fire hazard?" Yes, there is.


"Anyone else have to power up their entire well water treatment center with their generator?" I don't know if "have to" applies, but many people do so.

Your intuition regarding the generator is well founded, 5,000 watts is a realistic load for it, notwithstanding attractive decals and optimistic marketing staff.


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RE: Can I power a multiwire branch circuit with generator?

And even if you believe the marketing, the presence of motors on the circuit is going to make it infeasible to load it all the way up to capacity.


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RE: Can I power a multiwire branch circuit with generator?

You want a manual system. Is there any reason that you don't want to power the whole panel? Just turn off the circuits that you don't want straining the generator. If you are lucky, you can just get an interlock kit for your main panel and maybe move some circuits around to different legs if needed.


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RE: Can I power a multiwire branch circuit with generator?

"You want a manual system. Is there any reason that you don't want to power the whole panel? Just turn off the circuits that you don't want straining the generator."

Yes, I thought that many people frowned upon the the interlock type back fed system.


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RE: Can I power a multiwire branch circuit with generator?

"...I thought that many people frowned upon the the interlock type back fed system..."

I am one of those people, although I am aware that such "interlocks" are widely accepted by various inspectors, utilities and so on.

The primary reason that I don't care for breaker "interlocks" is that the only thing that is truly interlocked is the breaker handle position, NOT the actual status of the breaker contacts. In other words, the position of the breaker handle does not always correspond with the state of its contacts.

I prefer a simple knife switch, where I can SEE where the blades are, or a real transfer switch where the contacts (not just a handle) are mechanically interlocked.

WHY? Because I have seen what happens when someone backfeeds a generator. I have also seen what happpens when the generator is on the receiving end of said backfeed.

That said, I readily admit that "interlocked" breakers are far less likely to produce a dangerous situation than the alternative of a backyard electrical genius with a suicide cord.


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RE: Can I power a multiwire branch circuit with generator?

Hi,

I am on a well, have a water system and have an oil fired boiler. I think that you can probably leave the fridge off to take a shower. So that leaves well pump, booster pump, and oil blower. The oil blower I think is quite small relative to your well and booster water pumps.

I would check your well pump, since that looks like the most heavy load, especially starting amps. If it looks like your gen set can handle that start load, then I think you are in with a fighting chance.

My well pump is two phase 240v like yours and is on a double breaker. My gen set is 20kw, propane. I do notice the lights wink when the well pump kicks in. But that is all I see.

Look at the well pump specification. Note you do not have to pull it up. Usually the specs, at least electrical wise as on the control electronics in your basement. Sometimes with the pump power in hp, and max amp draw at start. If that info is not available look at the depth of your well and then google pumps which will pump that head of water (head = wather pump height) to get an idea of starting current needs.

Hope this helps.

Best, Mike.


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RE: Can I power a multiwire branch circuit with generator?

"My well pump is two phase 240v"

how do you get that from single phase 120/240 V power?


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RE: Can I power a multiwire branch circuit with generator?

Wayne, I think that you are letting fear of the invisible get in the way of logic.

If you do not trust circuit breakers, how do you work any circuits? Do you have to cut power the whole electrical service before working on it?


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RE: Can I power a multiwire branch circuit with generator?

"If you do not trust circuit breakers, how do you work any circuits?"

Do you not test the circuit after turning off the breaker to make sure it is dead?


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RE: Can I power a multiwire branch circuit with generator?

"If you do not trust circuit breakers, how do you work any circuits? Do you have to cut power the whole electrical service before working on it?".

You are comparing apples and oranges. Anyone who has worked in the electrical field for very long has seen a bad circuit breaker. It is not that I don't trust breakers, but rather that I'm concerned about the false sense of security brought on by proclaiming a dual breaker set up as "interlocked". To me an interlock must be failsafe- molded case breakers with a linkage between the handles are far from that.

To answer your question, I treat any circuit as energized, unless I have verified it as dead. I also take steps to make sure it remains dead until I decide otherwise.


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RE: Can I power a multiwire branch circuit with generator?

"Look at the well pump specification. Note you do not have to pull it up. Usually the specs, at least electrical wise as on the control electronics in your basement. Sometimes with the pump power in hp, and max amp draw at start."

Well, I do know that I have a 3-wire 60hz single phase, 230 volt 1/2hp submersible motor for my well. It runs at 4.8amps, but this motor has a service factor of 1.6, so I guess you would call it 5.9 service factor max amps.

Secondary pump is 1/2hp and runs on 115volts at 8.6amps, which also has a service factor of 1.6, giving it a 10.8amp max load

Radon mitigation motor is also 1/2hp and runs on 115volts at max load of 4.8amps

Oil boiler motor is 1/7hp running on 115volts at 1.7amps

And the refrigerator that is on the multiwire branch circuit (double pull 20amp breaker)

I know how to hook the well pump up to the transfer switch. As for the multiwire branch circuit with frige, I will hook each circuit on a separate leg of the transfer switch (using the other double pull 20a from transfer switch). Do you think this generator would be strong enough to power the listed loads?
Thanks again for any/all recommendations.


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RE: Can I power a multiwire branch circuit with generator?

"Do you not test the circuit after turning off the breaker to make sure it is dead?"

Yes, but how do I know it is not going to become live spontaneously. It is not a knife switch where I can see the contacts.


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RE: Can I power a multiwire branch circuit with generator?

Hi,

As I see it you would like to go for a Petrol powererd generator set. This is a good way to go because of cost, and it is the most popular, also because of cost.

I think you do not have to run the fridge while you take a shower. 30 mins down time on a Fridge should not matter too much. You just keep the fridge off while you take a shower. Like wise I think you can keep the Radon motor off for 30 mins as well.

So lets us concentrate on taking a shower. You need (a) well (b) booster pump and (c) hot water heater. I make it that running normally (not starting) you have about 2.3 Kw for these assuming normal run and a service factor of 1.0. Motors starting will draw alot more current, maybe 3 to six times normal current draw. OK do not panic. This sounds alot at it is. BUT note your gen set does not have to maintain this for very long, less than a second and not at the rated voltage of 115 or 230.

So when the motors start you have this current surge demand. In normal operation I think each motor will proabablly start at different times. May be a fraction of a second apart. This is sufficient for the gen set to supply sufficient current. If the gen set cannot supply this then the gen sets overload breakers will trip. You cannot control when the pumps kick in because they kick in when your water bladder pressure drops. So as you take your shower the pumps will come on and off. Each time there will be a 3x current surge (approx). You may notice your lights flicker as the pumps kick in, and there is a momentary undervoltage.

The Honda EU is 6500, mrsp approx $US 4,500 will supply 6.5 kw. So approx 3x what your normal running demand is when taking a shower. So I think you should be OK. This is one of those things that is easiestly solved by "try it". But if you get it wrong it is sort of expensive.

A friend of mine did buy one, that Honda inverter. Note you can only get these from Honda dealers. That dealer was so confident that he said that if you cannot take a shower, you can return the gen set for a full refund. Note you are then out the cost of the transfer switch, wiring and installation costs.

Note the honda is also an inverter set with "Fuel Saver". Inverter means it has power electronics which allows the gen set to spin faster or slower while giving you 60 hz. Hence it can operate at max efficiency and give you a nice clean power sine wave. Cheaper sets always spin at 3,600 rpm to give you 60 hz. The fuel saver feature throttles back the engine when under low load, and speeds it up for high loads saving fuel, reduces noise and general wear and tear. BUT if slows the gensets response to peak loads. I.e. it has to throttle up to put out max power. This small delay may be too long for the motor starting, try it with it on first. If the genset trips, the try simply turn off the fuel saver feature when you take a shower so that the genset can service the pump motors peak on-off depands.

So in a nutshell I would (a) try it and I think you will be O.K. and (b) back yourself by asking for a retrun clause from your Honda Dealer.

Warmest regards, Mike.


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Ohhps RE: Can I power a multiwire branch circuit with generator?

Ohhps. I noticed you have your gen set already. At 6.2 KW I think you should be OK.

Best, Mike.


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RE: Can I power a multiwire branch circuit with generator?

"Yes, but how do I know it is not going to become live spontaneously. It is not a knife switch where I can see the contacts."

A breaker is not going to move from the off to on position spontaneously.

Ever notice the force to turn one on?
You are compressing the spring that provides the energy that opens the contacts.

About the only failure that might occur is a set of contacts failing to open if they have become welded shut.

This would only affect turning the breaker off, not on (it would not turn off and would be on all the time).

While not imposable, it is a very rare occurrence given all the testing a breaker design receives and the QA used during manufacture.

The Stab-lock breakers had QA failures, not a design failure.
The early models worked fine and passed all tests.


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reRE: Can I power a multiwire branch circuit with generator?

"It runs at 4.8amps, but this motor has a service factor of 1.6, so I guess you would call it 5.9 service factor max amps."

The serive factor does not tell you what the strting current is.

The is either a kVA code letter or a locked rotor current.

When power is first applied to an induction motor it is not moving, and pulls the locked rotor current until it starts moving.
The current will continue to fall to the running current as the motor comes up to speed.

Depending on the motor design the starting surge can be 1-2 times the running current, and sometimes even higher.

Some motors are designed to start with a significant load on them, and the starting current can get very large (5-6 times the running current).


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