I'm about to wire for a 7500 KW Generac generator which is something my nephew has already done. In his case the system was wired by an electrician using a double pole 50amp breaker at the panel and 100amp wire. The system worked flawlessly. Then years later, in an attempt to upgrade the system, he called upon a second electrician that rewired using double pole 30amp breakers and 30amp wire sighting that the Generac was only providing 30amps and that the way the system was wired was overkill. Made sense at the time but when the first emergency took place, and the generator put to use, it surged and blew its breaker, while the fridge and fish tank pump made unnerving sounds, while lights would flicker and wall switches failed to work? He has since rewired to original and things are back to working fine.

My electrician is on the same page as my nephews second electrician, and who am I to argue with a professional, but I fear the same results if I wire using the 30amp logic.

Can anyone tell me why the 50amp breaker/100amp wire works while the 30amp solution fails??? Are the watts unavailable to the house due to circuit size?

Kind Regards, Bewildered

  • 1
    Do you mean 7500W? 7500KW is like one of those "diesel gensets in a 20' box" size... Mar 4 '18 at 20:33
  • 7500KW: that's not a generator, it's a power station! Anyway: The most probable issue if the cord is long (longer than 10m) is that it may have a consistent voltage drop so bigger wire is required to carry the 'same' power.
    – DDS
    Mar 4 '18 at 22:32
  • @ThreePhaseEel 7500kw is more like "a gas turbine in a 20' container" sort of power. I don't believe a 7.5MW diesel would fit in a 40’ let alone a 20'. Mar 5 '18 at 0:18
  • @Harper -- yeah, you're right -- a diesel that size would be more like 1MW or so :P Mar 5 '18 at 4:41

There's nothing wrong with overdesign. Tearing out 100A-rated cable simply because it's "too big" is just wrong. I'm not saying you should oversize every wire, but if you are in an situation where the existing wire is larger than needed, it works in your favor.

However there is something wrong with overfusing. Fuses and breakers protect wiring and equipment. A breaker which is too large will not protect the equipment. That's not a worry if the generator has its own breaker onboard.

If the generator has its own fuse or breaker protection that is correct to protect the generator, then the input breaker in the panel is allowed to be larger than the generator. The original setup was fine.

  • Motors love to pull ridiculous amounts of current as they start up. Overkill is the safe way to go. Mar 5 '18 at 3:16
  • I agree with using larger wire than the gen set requires for the simple reason most gen sets have a basic derate of 1.39 from there continuous rating. Similar to inverse time breakers they can deliver 1.4 to 3x the rated value at a in rush condition, using the minimum wire size can have heating problems because most generators are pushed to the max. Fuses at the max are the limit also because just as inverse time breakers fuses will also pass about 3x there rated value for a period of time so bigger wire is always a good idea on generators.
    – Ed Beal
    Mar 5 '18 at 3:23
  • Is DERATE much like the SAFETY FACTOR on a bridge for example, where the weight rating is set below what a bridge can safely handle? In other words, the spec is set to 30amps but the generator may produce 40amps at times when it ramps up to satisfy demand? If this is what happens it would explain why the system acted the way it did with the 30amp solution. PS: Your insights are very helpful. Mar 5 '18 at 12:15
  • @DavidCastellucci Kinda, but with different motivations. The safety factor on a bridge is quite a lot about corrosion, metal fatigue, and other deterioration. It is also about static or dynamic loads that are beyond design basis, because people don't read "Weight Limit" signs. .....However generators do not have unlimited ability to answer surge loads, and they will bog if overloaded. This can happen if someone is careless about which loads they ask the gen to support. Mar 5 '18 at 15:06
  • David this is the same as your bridge example. Most generators have a running and peak value listed for example I have 1 generator that is 8500 watts but it's peak is 12000 or 13500 I don't remember the exact peak value but I purchased it because of the peak value and its efficiency rating, I have a large well pump and my 6kw gas arc welder /generator has trouble starting the pump because its peak value is only +8kw it will start the pump but it can take up to 3 minutes for generator to stabilize. This is hard on the generator, the motor being started and everything else that is being powered.
    – Ed Beal
    Mar 5 '18 at 15:21

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