Generator hook-up to home electrical system

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finlfox

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Electrical Designer
I have been asked if connection to a home electrical panel can be accomplished by routing both a 50 AMP and a 30 Amp cable from a single generator to the home panel. In effect a parallel connection from two supplies on a single generator to a single input to the house mains. Not withstanding all of the disconnecting means etc... I am having trouble explaining why this is a bad idea and what NEC sources can be used to validate.

My main concern is safety and overheating of the 30 amp cable
 
It’s not the overhearing of the cable but overloading of the generator. Divide gen kw by volts to get amps should be about 50 amps. Not 50 and 30
 
Welcome to the forum.

Thee 50a outlet carries the maximum output, so using a second cable will not provide any more capacity.

Plus, you can not parallel multiple cables like that.
 
NEC 310.4(B): All conductors in a parallel set must be the same type, size, and length.

But overriding that is 310.4(A) which states that only conductors sizes 1/0 AWG and larger may be paralleled.
 
NEC 310.10(G) prohibits paralleling conductors if they're smaller than 0 AWG. (American wire gauge)

When a portable generator has both a 50-amp and a 30-amp receptacle, it's to provide the flexibility of plugging in a variety of equipment without requiring an adapter, not because the generator's capable of supplying 80 amps. (unless its rated capacity is 20 kW or more, in which case parallel conductors are still prohibited)

Has anybody calculated how much power this house actually needs? 50 amps is a lot.
 
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drcampbell said:
NEC 310.10(G) prohibits paralleling conductors if they're smaller than 0 AWG. (American wire gauge)

When a portable generator has both a 50-amp and a 30-amp receptacle, it's to provide the flexibility of plugging in a variety of equipment without requiring an adapter, not because the generator's capable of supplying 80 amps. (unless its rated capacity is 20 kW or more, in which case parallel conductors are still prohibited)

Has anybody calculated how much power this house actually needs? 50 amps is a lot.
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Somebody wants their central air to work, I'll bet.
 
Jon456 said:
NEC 310.4(B): All conductors in a parallel set must be the same type, size, and length.

But overriding that is 310.4(A) which states that only conductors sizes 1/0 AWG and larger may be paralleled.
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Correction...

NEC 310.10(H)(2): All conductors in a parallel set must be the same type, size, and length.

But overriding that is NEC 310.10(H)(1) which states that only conductors sizes 1/0 AWG and larger may be paralleled.

(I was originally going off the top of my head from the older NEC section numbering.)
 
Jon456 said:
Correction...

NEC 310.10(H)(2): All conductors in a parallel set must be the same type, size, and length.

But overriding that is NEC 310.10(H)(1) which states that only conductors sizes 1/0 AWG and larger may be paralleled.

(I was originally going off the top of my head from the older NEC section numbering.)
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They also have to be connected at both ends, and connecting to a common bus through breakers doesn't count.
 
Doesn’t matter.
You’ll only get the rated output from the genny
Some 7500-8000 watt generators have a 50 amp outlet.
that doesn’t mean you’ll get 50 amps.
You will need a 12kW generator to at least get to the 50 amp mark.
However, if you had say a 12000W generator with a 50 and a 30, both of these breakers are fed from the same “bus” inside the generator. Your just wasting your time connecting both of them.

The NEC reasoning doesn’t matter here because people using a generator aren’t going to care. You have to explain the generator reasoning to them.
 
If you "need" to explain your reasoning, maybe it's time to regress to,
"I won't do it that way because it won't work, it's unsafe and it's a code violation. It's a code violation because it won't work and it's unsafe. I'll charge you double to do it right when it doesn't work and you call me back, and I'll testify about your prior knowledge at the coroner's inquest."
 
LarryFine said:
Welcome to the forum.

Thee 50a outlet carries the maximum output, so using a second cable will not provide any more capacity.

Plus, you can not parallel multiple cables like that.
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You could parallel the cables but they would both have to be at least 1/0. :) And not thru two CBs.
 
ggunn said:
They also have to be connected at both ends, and connecting to a common bus through breakers doesn't count.
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And they have to be terminated in the exact same way. I didn't bother with these details because they were beyond the scope of the OP's question and because they were moot due to the 1/0 requirement.
 
drcampbell said:
If you "need" to explain your reasoning, maybe it's time to regress to,
"I won't do it that way because it won't work, it's unsafe and it's a code violation. It's a code violation because it won't work and it's unsafe. I'll charge you double to do it right when it doesn't work and you call me back, and I'll testify about your prior knowledge at the coroner's inquest."
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True, but if it’s explained WHY it won’t work, not the code violation parts, but simply why it won’t work, then they understand.
When you tell them all you have written they will just say, “It can’t be that hard, I have a _________ that wired stuff all the time. He’ll do it..
 
Hv&Lv said:
Doesn’t matter.
You’ll only get the rated output from the genny
Some 7500-8000 watt generators have a 50 amp outlet.
that doesn’t mean you’ll get 50 amps.
You will need a 12kW generator to at least get to the 50 amp mark.
However, if you had say a 12000W generator with a 50 and a 30, both of these breakers are fed from the same “bus” inside the generator. Your just wasting your time connecting both of them.

The NEC reasoning doesn’t matter here because people using a generator aren’t going to care. You have to explain the generator reasoning to them.
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That is the most relevant comment I've seen. Thanks
 
Hv&Lv said:
True, but if it’s explained WHY it won’t work, not the code violation parts, but simply why it won’t work, then they understand.
When you tell them all you have written they will just say, “It can’t be that hard, I have a _________ that wired stuff all the time. He’ll do it..
Click to expand...
"OK then let him, but make sure your homeowner's insurance is paid up."
 
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