Generator: back feeding alternatives

Charles,

Three are a number of ways to do this, but many variables too. First is you may need to check with your local code enforcement to see what you can get away with.

If I remember correctly the NEC required a single main disconnect for this generator to isolate it from the line and prevent feedback. Normally this includes a transfer switch that locks out the generator input from the line in. Very expensive at 800 amps.

Or if you are allowed, move the circuits that you know you want to run into a single chain of panels, install a small transfer switch to cover that panel only. Some areas will allow you to do this, some may not. 7.5 KW will run a great deal of lighting load, but locked rotor KVA will choke out a small generator in motor loads.

You will need to balance the load and since you would need 5 100 amp, 2 pole three wire 230vac 20 circuit panels, you should be able to balance the load and cut in a couple of panels at a time, depending on the load.

Hope this helps...

Here is what has to happen.

You will have to open the main (800 amp!!)disconnect between the meter and the breaker box feeds. You will have to install another (100 amp) disconnect between the generator and the breaker box feeds, on the system side (as compared to the 'grid side') of the main breaker.

This will provide positive isolation of your 'service' from the utility grid supply. It will also give you the ability to isolate your generator from everything. (you also may be able to remove the meter to isolate your system from the grid)

You will have to manually open/close the individual circuit breakers to match you generating capacity and desires.

When the grid power supply goes, 1) open the main breaker, 2) open all 100 individual circuit breakers, 3) close the generator feed breaker, 4) close the individual circuit breakers as desired.

On restoration of power supply, you must 1) open (disconnect) your generator main breaker (I also recommend first opening the individual circuit breakers to reduce arcing potential), then 2) re-close the main breaker , reconnecting your system to 'the grid.' then 3) re-close all the individual circuit breakers.

Simple, low cost, fool proof? meet NEC not if your generator is automatic start and/or its breaker is automatic. Ideally there would be mechanical interlock requiring main line breaker open before generator breaker could close.

The underground utility line comes into a 3x3' panel outside with the meter coming out the side. The four 200A feeder cables exit this large panel and run into the four house breaker panels. There is no accessible disconnect between the house breaker panels and the large panel outside. I suppose the disconnect is inside this large panel but it is locked with a tamper tag.

I suppose four 200A transfer switches would do the trick safely and probably meet NEC code, right?

The tamper tag panel, just like a power meter belongs to the power company. It is called their service entrance. A professional engineer or competent electrician knows how to work cooperatively with the power company to get things done. That is standard procedure for them. Usually everything get installed and tested "cold", then in a cooperative move everything gets connected up properly.

If you taking ALL the advices to heart, you are on your way. Contact a competent electrician, discuss it, get his assessment to, an offer and go from there.

Safest way to do this is with a changeover switch However this would have to be rated at 800 amps. I doubt your electrical utility will allow any other method. You could if you want to take it on your own shoulders and I never suggested this Lock off the existing 800 amp main switch and feed to the main Bus Bar from a secondary switch which would also need to be lockable connected to the 7.5kW generator. This would cause a bit of a bang if you forgot to turn the live feed off before changing from 1 to the other.

I had business to be frequently in a large power room in New York city, where the smallest backup generator was 250KVA supporting among other things my equipment. In no case was it permitted the backup generator to "see" city power if someone throws a switch out of sequence. It was enforced by mechanical interlocks and big switches which were thrown in two steps: first open one side bus connection, the second step closes the other side bus.

All this calls for a professional engineer, permit and inspection. Inspectors, fire department and your insurance are absolutely intolerant on "homemade" solutions. Just for starter they shut you down right away until everything is corrected and that may take weeks or longer. Fines, etc....

On the other hand, transfer switches are generally preapproved types, and a competent electrician can install it, and if permit and inspection is needed, he can arrange that too. What i dont know is how much power is permitted at your location to be handled that way, but you can easily find it out.

I run in to this situation all the time. People try to get away with buying a smaller generator and only run partial circuits. By the time you setup the isolated circuits, you could have bought a larger generator and backed everything up. You have less total equipment, (transfer switches, breaker panels, etc) and more comfort from all your equipment running. Even though you have 800 amps of panel, how many amps are you actually using? What is the amp size of your main meter?

To remedy your situation, you will need (4) 200 amp transfer switches. The utility power from each 200 breaker panel must run through each transfer switch before it goes to the breaker panel. The generator power supply will also run though each transfer switch. Most automatic switches will switch themselves when the power goes out. But you don't want to switch all of them at the same time. So you will have to run the switching control power through a timing relay to select which one you want to transfer at what time.

I personally would not touch this situation unless your only going to run lighting, and other small loads. A 7.5KW generator will not support a/c units and heat pumps.

Without complete understanding this is not a job for DIY and may even be illegal to do so. As pointed out the method of isolation of the two supplies is fundamental, remembering someone else could operate the system in an emergency (fireman etc.)

Also the separation of circuits and staggered load switching is expensive, as stated a bigger gen.set is often the simplest, cheapest & safest solution. You do not mention if you are single or 3 phase, whatever you have a small backup gen.set.

Your gen.set - 7.5kW this is probably the max. rating (on overload; 1 hour in 12) not continuous, this would be approx. 6.8kW, so at 230V less than 30A (single phase). Without 'softstart' you are looking at the ability to start one 2-3kW electric motor at most, even that would have to be tested (Amps demand when starting an electric motor can vary from 3 to 5 times continuous current rating; look for 'locked rotor' on plate)

You need on-site help with this one and I feel it would not be responsible to encourage you to do it by text alone, we cannot switch off the supply with our keyboards! Of course asses the requirement but please seek local support / advice, this is fatal equipment.

When adding an alternative supply one also has to consider the Neutral / Earth circuits. The gen.set will have its N tied to E and is unlikely to have a two pole [L+N] output contactor.

This means considering a two pole isolator / change over switch for your gen.set. You want the N+E tied locally for gen.set operation, but when you go back to the mains you will have introduced an earth loop, this may also upset RCD protection.

An electrician will solve these issues and could end up using a two pole 35/40 Amp change over (properly positioned) and not 4x200A on the input; justifying his time.

You can make this as simple as you want:

a. The cheapest way is to purchase a circuit breaker of the proper size (you didn't specify the voltage or how many phases are in your circuit) and place it in series with your four 200 amp bkrs, then connect your generator on the load side of the new breaker. Now you must be careful to keep the generator breaker OFF until you need it. When the power goes off you must open the new main breaker and whatever circuits you want to disable, then start your generator and close the generator breaker. Now, you are isolated from your normal power source and you can do whatever you want without worrying about your generator backfeeding (when the power returns you will have to open the generator breaker FIRST, and then close the new main). A simple light bulb connected to the supply side of the new breaker can indicate the return of normal power. If you are not electrically inclined or if someone else may operate your system, this may not be a good option because if you forget the sequence you could destroy the generator.

b. If you want automatic transfer (more expensive) just connect your load to the single (or more) pole, of the double throw automatic switch and connect the sensing elements. Your generator will start automatically and the switch will throw isolating the generator and the load. If you select closed transition when the power comes back you will not have a power outage and your generator will shut itself down automatically.

c. There's a manual version of the transfer switch where you must throw the switch yourself. With any of these options, you have total isolation from normal power and the generator cannot backfeed. Options b and c are completely safe and anyone can use them if a qualified electrian installs them.

Hope this helps.

This is what we are trying to avoid suggesting potentially dangerous systems. True option a) would work electrically & operated responsibly - but it would not be allowed in the UK. If someone forgot to isolate the mains via the CB it would be possible to generate back into the grid when it was dead [back-feed] and so electrocute someone mending the grid cable [forget the generator seeing the mains]. In the UK you must have interlocked or isolation & if there is any chance of your gen.set [intended or not] connecting to the grid: G59 protection.

So yes it is something that would work but you would have to keep quiet and make sure no one professional saw it! - this is not what we want to encourage on CR4 surely?

There are simpler safe ways as I mentioned using a small change over / isolation switch on dedicated circuits but as we have mentioned this is really an installation where help should be encouraged, please. [& as I said previously there are also earthing issues]

p.s. I am glad you also said 'if a qualified electrician installs them'.

JohnBob, and anyone else who read my comment:

I'm sorry, I was dead wrong about my option a. Of course you would never take a chance to heat a dead line and cause someone's serious injury or death! The transfer switch option (automatic or manual) will preclude this from ever happening if properly installed. With the manual option you can open all your circuit breakers, start your generator and selectively load the generator to its maximum capacity.

I'm glad someone caught my mistake.

hughes838

You are welcome. We ALL make mistakes. Those, who conveniently forget them are bloody liars. As long as you or one of us catches it, no harm done.

Welcome.

That's OK - thank you, we were on our soap boxes for a moment!

A manual change over in this application should have a central 'OFF' / dead position that has to be turned through when changing supply options.

In the automatic set-up the two contactors required are both mechanically & electrically interlocked to stop the two supplies meeting (fail safe)

Both options have a second level of safety, showing how seriously twin supplies is taken - & for good reason.

The scheme is FLAT OUT ILLEGAL in most places. Not because anybody being excessively pedantic, either. Just last year my sister wanted to connect a small generator similarly, on the cheap, with breakers thrown this and that way. I have shown her the right way, not even expensive with an approved transfer switch.

Building inspectors, fire department and insurance companies do not tolerate it, because it is not foolproof. When your small generator meets (by mistake) a returning commercial power, it blows up, shorts and may very well start a fire. You want to be a contributing factor to a home or factory accident? I DO NOT THINK SO.

So let the guy do it with a local licensed electrician, and be protected from his own ignorance.

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God knows I am known to bend a rule, or two, maybe three, but not about safety.