Back feeding main panel breaker questions

[steelandcanvas]

I agree 100% that a doubled ended transfer switch is far safer.

What is wrong with an approved interlock device?

 

[steelandcanvas]

You should definitely using a 60A breaker so that you have a safety margin. Whenever I figure a load I never max out a breaker 100% that just leads to problems and doing the same job twice. Rule of thumb I try to only use 90% capacity on the breaker. If conditions ever change you're going to have problems with an undersized breaker. Transfer switch is also proper I just installed one in my bosses house for his generator. It saves liability and other potential headaches.

Breaker load: NEC says 80% load continuous load, 100% Amps non-continuous load.

 

[212sparky]

You should definitely using a 60A breaker so that you have a safety margin. Whenever I figure a load I never max out a breaker 100% that just leads to problems and doing the same job twice. Rule of thumb I try to only use 90% capacity on the breaker. If conditions ever change you're going to have problems with an undersized breaker. Transfer switch is also proper I just installed one in my bosses house for his generator. It saves liability and other potential headaches.

By code a breaker can only be loaded to 80% of it's value. #6 awg copper is good for 60 amps, and you would have no issue only going 15-20' with it. Yes wire length does effect how much a conductor can carry. Roughly you have to up size every 100' as well as take in to account temperature the conductors.

 

[Isaac-1]

You should definitely using a 60A breaker so that you have a safety margin. Whenever I figure a load I never max out a breaker 100% that just leads to problems and doing the same job twice. Rule of thumb I try to only use 90% capacity on the breaker. If conditions ever change you're going to have problems with an undersized breaker. Transfer switch is also proper I just installed one in my bosses house for his generator. It saves liability and other potential headaches.

I worded all this poorly and don't have time to fix it now, so please disregard

 

[DieselAddict]

What is wrong with an approved interlock device?

There are more potentially dangerous failure modes with an interlock setup versus a double ended transfer switch. I'm not at all condemning their use. I have a breaker interlock on my system. With that said if I was building a system from scratch it would absolutely have a transfer switch instead of an interlock setup.

FYI - Industrial setups are almost exclusively breaker interlocks. That is the only way to do a live buss transfer. There is potential for real problems there just as there is in residential breaker interlocks. They are managed with things such as reverse current relays and status switches on everything. When operated manually they require mechanical trapped key interlocks AKA "KIRK" key interlocks that offer the same function as the sliding gate interlocks we put on our home panels.

 

[steelandcanvas]

I am very familiar with Kirk interlocks. I installed quite a bit of switchgear when I was an industrial electrician. I guess I just can't understand what can go wrong with the mechanical interlock as long as the device is maintained. Obviously the main and genset breaker cannot be closed at the same time, provided the device functions as intended. Transfer switches are nice but spendy for the average homeowner. Just my 2 cents.

 

[DieselAddict]

I don't have anything against interlocks. Just pointing out they have more risk than transfer switches. That is all. No condemning them nor am I suggesting that they shouldn't be used. Only providing my thoughts from experience.

You rightly point out that properly maintaining the electrical system is a key to a system being safe and reliable. That is much more important than the type of interconnect used.

 

[datadawg]

You should never backfeed your panel linemen working on power outages could be hurt. A transfer switch the way to go.

I thought I saw an interlock on the bottom of the panel, so wouldn't that mean that the only way the generator breaker can engage (and therefore backfeed panel) is if the main panel breaker is disengaged? If main panel breaker is disengaged, the link between the utility and house is severed and OP is not really backfeeding power to the utility.

I agree a transfer switch is "better" and some modern units can give you nice features (like load shedding), but unless OP gets transfer switch that is entrance rated and can handle the entire panel load, he'll have to feed specific circuits through the transfer switch... a PITA.

 

[jimbo913]

I agree a transfer switch is "better" and some modern units can give you nice features (like load shedding), but unless OP gets transfer switch that is entrance rated and can handle the entire panel load, he'll have to feed specific circuits through the transfer switch... a PITA.

This thread has turned into a debate about interlock versus transfer switch so I figured I would mention why I went the interlock route. For starters datadawg hit the nail on the head, as I did not want to pick select circuits as with most transfer switches and I am doubtful that most electricians would have connected a 20A generator through a transfer switch to a 250A service without forcing me to chose a handful of circuits. Plus I knew the current generator would not be the final one. If an 803 does what I need and becomes the final then I might see about buying an auto start board and having an auto transfer switch installed provided that I am able to power all loads except 50A and 60A ones. I was also considering a hybrid solar system with battery backup and figured that I would have it all rewired with auto transfer at that time. I am completely comfortable with the safety of the interlock and it was the best temporary solution. If money was no object and there was an actual safety concern I would have planned to call the electrician out 2-3 times until the setup was finalized.

 

[Daybreak]

Howdy,
The interlock is fine. It also gives you the option of running larger wire and putting in a larger dual pole breaker for the size of your genset. Some states in the United States are nanny states. A proper UL approved interlock from the main panel manufacturer will be code compliant. Please also know that 6ga copper wire and a dual pole 50amp breaker can handle a lot more than 50 amps. As regarding breaker capacity, the breaker itself is overdesigned. The same thing can be said about a MEP-803a unit.(conservatively rated at 10kw) 10kw all day long too. A bargain basement generator would die if it was run at its 100% capacity. You will never find anyone to ever hook your MEP-803a with autostart. The problem being is the generator is way under rated for your house. Any autostart generator will need to cover all amps for what your service is. Don't get me wrong, a autostart can be installed, but you would then need a totally separate breakoutbox with just a maximum of 50amps in that separate panel. You have the best method now. You use you head and just watch your load. You can pick and choose what you need to run.

The topic title throws people off thinking you are actually backfeeding the panel thru say a electric dryer socket.

good luck with finding a mep-803a

 

[DieselAddict]

Here is a riddle.. When is 6ga not really 6ga..

Answer - When you bundle 4 of them tightly together and wrap them in a neoprene jacket.

Just to clarify a bit here, 6ga THHN (90 deg C) for example can handle a good bit more than 50a however 6ga cord is not rated for that temperature rise and cant handle as much current. 6ga cord is limited to 55a.

When you evaluate the circuit you have to consider the weakest link in the branch and size the overload for that link. If you put in 6ga THHN from the pannel to the inlet, install a 60a inlet, then use 6/4 cord out to the genny, you still have to size the overload for 50a because the rating on 6/4 SOOW is 55a.

So to run a 60a overload you can run 6ga THHN between the inlet and the panel, install 60a plugs but you would need to run 4ga cord between the inlet and the generator to have a true 60a circuit.

 

[datadawg]

Please also know that 6ga copper wire and a dual pole 50amp breaker can handle a lot more than 50 amps. As regarding breaker capacity, the breaker itself is overdesigned.

Umm, a 50 amp breaker will trip at 50 amps, so what does it mean it can "handle a lot more than 50 amps"? If you are drawing say, 60 amps, a 50 amp breaker will trip to protect the wire from overheating and starting an electrical fire. Whether the breaker can "handle" 1,000 amps or 100 dancing elephants doesn't really come into play, it will simply trip at its rated capacity.

 

[rhurey]

Umm, a 50 amp breaker will trip at 50 amps, so what does it mean it can "handle a lot more than 50 amps"? If you are drawing say, 60 amps, a 50 amp breaker will trip to protect the wire from overheating and starting an electrical fire. Whether the breaker can "handle" 1,000 amps or 100 dancing elephants doesn't really come into play, it will simply trip at its rated capacity.

Not exactly. A 30 amp breaker will allow > 90A through it, for short periods of time. (That's how my heat pump that's has a 96A LRA can start and run on a 30A breaker.) The trip point of a breaker is some curve that takes current and time into account.

 

[DieselAddict]

Not exactly. A 30 amp breaker will allow > 90A through it, for short periods of time. (That's how my heat pump that's has a 96A LRA can start and run on a 30A breaker.) The trip point of a breaker is some curve that takes current and time into account.

I know this is getting a little OT but it may be useful info.

It is true that a breaker will allow more current through for a short time. That time is pretty short. It takes about 1/60th of a second for the magnetic trip portion of the breaker to physically react so for that window of time you might as well not have a breaker in there. Even at 100x current rating nothing happens (assuming it doesn't explode). The breaker just isn't fast enough to react to it. This delay isn't intentional. Its strictly a matter of the physics of it. Fuses can be much faster. A fast reacting fuse isn't necessarily a good thing when starting things with high inrush current such as a motor. Thats why motor circuits use time delay fuses.

The inrush current of an accelerating single phase electric motor can be reasonably represented by a LOG curve starting with something near LRA. I say near because it will not ever reach LRA when starting since the induction of the motor helps to resist the inrush and the rotor isn't really locked. As soon as it begins to move the torque demand (amps = torque) goes down. Regardless of the exact peak current it drops significantly in the first few milliseconds to something fairly reasonable. Much shorter than the reaction time of the breaker.

So while technically correct that a 30a breaker is able to handle more current its important to acknowledge those are transient loads lasting milliseconds. That characteristic isn't useful for anything more than kick-starting a motor and doesn't really apply to real world loads.

 

[Isaac-1]

In many cases you are right, particularly about much higher inrush currents found in motor starting, however lesser overloads can take considerably longer to trip a breaker, if you look at the trip curve on a typical breaker you will see the 10% overload trip can take take up to an hour to happen, and even a 200% load will typically have to be sustained for hundreds of seconds in order to trip a common household breaker. Near instant trips do not occur until you exceed 500-1000% of the breaker rating. The reality of this in generator applications is that if the generator can sustain the load, any momentary or relatively short sustained minor load (think toaster or coffee pot running off a MEP-002a or MEP-003a) will likely not cause a breaker trip, even if you were operating a near full rated output of the breaker.

Ike

 

[DieselAddict]

Yea, breakers respond to different magnitude overloads differently. In this application short high intensity events are handled magnetically and longer low level events are handled thermally.

To meet the UL standard for long duration events a breaker must trip @135% rated current within 60 minutes. @200% current the limit is 2 min. You can see that trip curve is pretty steep. This will have more impact to the generator than to the load conductors since a 135% overload won't damage the conductors in an hour nor will a 200% overload damage the conductors in 2 minutes (assuming they are properly sized and properly installed). A 200% overload will take out the generator if sustained for more than a few seconds. But protecting the generator isn't the job of the branch circuit breakers.

The branch circuit breakers are there to protect the infrastructure. These breakers are not there to protect the load nor are they there to protect the source. The output breaker of the generator is there to protect the generator. The trip curves of the breakers in the panel are designed to not put the wire/connectors/distribution components etc at risk of failure from excess current. In context of the discussion here the overload should be sized for the wire/connectors and not for the generator. If we were talking about a generator breaker it would be a little different. The trip curves there would be more suited to the capacities of the generator.

Edit - A breaker should carry its full rated load indefinitely. People talk about the 80% rule but that is a NEC thing on the load side of the equation. Breakers should not trip at a current under their rating.