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Wireing inlet for generator

bcrittendon

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Sep 21, 2013
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I m wiring in a 30 amp inlet for a portable generator to a transfer switch. My cord for my genset is 10 ga and 40 foot. I will be running 30-35 foot of thhn through pvc from my inlet to my switch. the wires to connect to in the switch are 10 ga. Due to the lengths involved do i need to up my thhn from inlet to switch to 8 ga? (The manual on the switch suggest 10 ga as the standard).
Thanks

Brian
 
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2ManyProjects

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I m wiring in a 30 amp inlet for a portable generator to a transfer switch. My cord for my genset is 10 ga and 40 foot.

Why such a long cord? I realize that you won't want the genny smack up against the house, for both noise & safety reasons; but 10-15 feet would seem to be adequate separation, in the absence of any other specific limitations.

I will be running 30-35 foot of thhn through pvc from my inlet to my switch. the wires to connect to in the switch are 10 ga. Due to the lengths involved do i need to up my thhn from inlet to switch to 8 ga? (The manual on the switch suggest 10 ga as the standard).

Assuming this is a 120V generator, over that length run (~75 ft., total) the whole thing (including that "cord") would need to be AWG 8, if you want to hold voltage drop to no more than 3% (which you should).

If you can hold the TOTAL run length (including both the cord and the inside wiring) to no more than about 50 feet, then AWG 10 will be (barely) adequate.

 

wyliesdiesels

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Why such a long cord? I realize that you won't want the genny smack up against the house, for both noise & safety reasons; but 10-15 feet would seem to be adequate separation, in the absence of any other specific limitations.



Assuming this is a 120V generator, over that length run (~75 ft., total) the whole thing (including that "cord") would need to be AWG 8, if you want to hold voltage drop to no more than 3% (which you should).

If you can hold the TOTAL run length (including both the cord and the inside wiring) to no more than about 50 feet, then AWG 10 will be (barely) adequate.


No. Using #8 would be overkill! And I bet the generator doesnt even do 30a continous....
 
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2ManyProjects

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Assuming this is a 120V generator, over that length run (~75 ft., total) the whole thing (including that "cord") would need to be AWG 8, if you want to hold voltage drop to no more than 3% (which you should).

No. Using #8 would be overkill!

Wrong.

Try reading what was actually written, before arguing with things you're imagining.

Once more, for the arithmetic-impaired...

CM = K x I x L / E

YOUR figure for "K" is 12.9; "L" is the round-trip distance; and "E" is the permissible Voltage Drop. So...

CM = 12.9 x 30 x 150 / 3.6
CM = 58,050 / 3.6
CM = 16,125

AWG 10 is 10,400 CM -- i.e., inadequate. AWG 8 is 16,500 CM -- adequate, but just so, and certainly nowhere near "overkill".

Now that we know this is 30A @ 240V application, we can set "E" to 7.2 and still be within 3%; at which point, AWG 10 would be adequate. But that wasn't the question on the table, at the time.

And I bet the generator doesnt even do 30a continous....

Why? Do you have some reason to believe that the OP was attempting to mislead us? In point of fact, he never actually stated what the output capability of his generator is/was. What he DID say was:

I m wiring in a 30 amp inlet for a portable generator to a transfer switch.

Besides, the generator itself may well get replaced multiple times over the life of the inlet and transfer switch. And who knows what the capabilities of those as-yet-undefined replacement generators might turn out to be? The issue is, the INLET (and it's associated wiring) is targeted to support 30 Amps. Hence, that is the figure we MUST use.

And I wonder why the voltage matters? :)

See above.

At 240V, a 3% voltage drop is, obviously enough, twice as many volts as a 3% drop @ 120V would be. Hence, with a larger (absolute value) voltage drop permissible, we can use a smaller wire, yet still stay within the nominal 3% limit.

 

pattenp

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2manyprojects, yes #8 by the math provides less voltage drop but common sense tells us that. To use #8 over #10 in the OP's case for a 30A generator inlet circuit that's 30-35 ft just isn't needed. In my opinion the added benefit to cost isn't justified. With your position on wire size and VD, all 15A lighting circuits should use #12 and 20A outlet circuits should use #10.
 

wyliesdiesels

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2manyprojects, yes #8 by the math provides less voltage drop but common sense tells us that. To use #8 over #10 in the OP's case for a 30A generator inlet circuit that's 30-35 ft just isn't needed. In my opinion the added benefit to cost isn't justified. With your position on wire size and VD, all 15A lighting circuits should use #12 and 20A outlet circuits should use #10.

He's including the THHN run length(35') AND the generator flexable cord(40') for a total of 75'.....
 

pattenp

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Yeah I know, but we're talking about a 30A generator inlet circuit that's 30-35 ft long. The generator cord length isn't fixed, the cord could be 10ft or 100ft. If the cord is so long to make VD an issue then the cord should be up-sized. That's how I see it.

He's including the THHN run length(35') AND the generator flexable cord(40') for a total of 75'.....
 

ishiboo

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Wrong.

Try reading what was actually written, before arguing with things you're imagining.

Once more, for the arithmetic-impaired...

CM = K x I x L / E

YOUR figure for "K" is 12.9; "L" is the round-trip distance; and "E" is the permissible Voltage Drop. So...

CM = 12.9 x 30 x 150 / 3.6
CM = 58,050 / 3.6
CM = 16,125

AWG 10 is 10,400 CM -- i.e., inadequate. AWG 8 is 16,500 CM -- adequate, but just so, and certainly nowhere near "overkill".

Now that we know this is 30A @ 240V application, we can set "E" to 7.2 and still be within 3%; at which point, AWG 10 would be adequate. But that wasn't the question on the table, at the time.

You're simply WAY over-thinking this with math and not practice, and sizing a circuit based on voltage drop during 100% of its capacity, and including a cordset? Under the WORST case scenario of the circuit being loaded 100% all, you would see higher than ideal but still acceptable voltage drop for practically everything.
 

wyliesdiesels

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Wrong.

Try reading what was actually written, before arguing with things you're imagining.

Once more, for the arithmetic-impaired...

CM = K x I x L / E

YOUR figure for "K" is 12.9; "L" is the round-trip distance; and "E" is the permissible Voltage Drop. So...

CM = 12.9 x 30 x 150 / 3.6
CM = 58,050 / 3.6
CM = 16,125

AWG 10 is 10,400 CM -- i.e., inadequate. AWG 8 is 16,500 CM -- adequate, but just so, and certainly nowhere near "overkill".

Now that we know this is 30A @ 240V application, we can set "E" to 7.2 and still be within 3%; at which point, AWG 10 would be adequate. But that wasn't the question on the table, at the time.

Why? Do you have some reason to believe that the OP was attempting to mislead us? In point of fact, he never actually stated what the output capability of his generator is/was. What he DID say was:

Besides, the generator itself may well get replaced multiple times over the life of the inlet and transfer switch. And who knows what the capabilities of those as-yet-undefined replacement generators might turn out to be? The issue is, the INLET (and it's associated wiring) is targeted to support 30 Amps. Hence, that is the figure we MUST use.

See above.

At 240V, a 3% voltage drop is, obviously enough, twice as many volts as a 3% drop @ 120V would be. Hence, with a larger (absolute value) voltage drop permissible, we can use a smaller wire, yet still stay within the nominal 3% limit.


The main issue with your #s is that u ASSuMEd the generator is 120v. It turned out it was 240v which changed the math...

Even IF the generator was 120v, the voltage drop would be about 9v AT FULL LOAD 30a. If the generator held the voltage at 120v that would mean devices would be seeing about 110v which us acceptable for most devices.

Also, the reason I brought up wether the geny is 30a continous is because many generators HAVE a 30a recepticle because their capacity is just over 20a so a 20a outlet is too small! If this is indeed the case, then using 30a for amperage in our VD math is also overkill!

#10 is fine...end of story!

Yeah I know, but we're talking about a 30A generator inlet circuit that's 30-35 ft long. The generator cord length isn't fixed, the cord could be 10ft or 100ft. If the cord is so long to make VD an issue then the cord should be up-sized. That's how I see it.

Exactly! VD calcs are for FIXED wiring! So u dont include the length of a flexable cord because the cord length can change and an electrician's job is to compensate only for whats fixed inside the wall and cant be changed. The OP can add an even longer cord and mess up the VD calcs...its not our job to calculate what length cord might be plugged into an outlet, only the wire FEEDING that outlet!

You're simply WAY over-thinking this with math and not practice, and sizing a circuit based on voltage drop during 100% of its capacity, and including a cordset? Under the WORST case scenario of the circuit being loaded 100% all, you would see higher than ideal but still acceptable voltage drop for practically everything.

May be just the reason why I smell an engineer....
 
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2ManyProjects

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2manyprojects, yes #8 by the math provides less voltage drop but common sense tells us that. To use #8 over #10 in the OP's case for a 30A generator inlet circuit that's 30-35 ft just isn't needed.

If that was all there was to it, I would agree. But the OP explicitly specified a 40-ft. AWG 10 generator cord, which greatly exacerbates the problem. The load doesn't care WHERE some/most/all the voltage drop was imposed; it only knows it's not seeing as much voltage it wants. I questioned the need for such a long cord, but never got an answer on that. By now, it's a moot point, because it turns out that this is apparently a 240V application; hence, AWG 10 will suffice. But even so, if the OP can use a significantly shorter cord to connect the genny, then all the better. Notwithstanding any commonly used "rules of thumb", less voltage drop is ALWAYS better than more voltage drop.

In my opinion the added benefit to cost isn't justified.

Maybe. Maybe not. It's not your money, so not really your decision. There seems to be a chronic mindset around these parts that everything MUST be done in the cheapest possible way; and that quest to pinch every penny is sometimes taken to downright silly extremes. But some folks will prefer to spend slightly more, and do a "nicer than bare minimum" job, even if the return on that investment is sometimes more in the realm of "peace of mind" or "future-proofing" than "urgent immediate need". So I see nothing wrong with pointing out that there are alternatives, and why they might be of value. (Note, this applies in all sorts of contexts. Do you always buy THE cheapest car on the lot? Or THE cheapest shirt on the rack? Or THE cheapest meal on the menu?)

With your position on wire size and VD, all 15A lighting circuits should use #12 and 20A outlet circuits should use #10.

That is a gross overstatement, at best; and as such, it does NOT accurately represent my position.

 

2ManyProjects

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The main issue with your #s is that u ASSuMEd the generator is 120v.

(Still arguing with posts you vowed not to read, are we?)

Since the voltage was not (initially) specified, that was the only prudent thing to do. I did, of course, qualify my remarks to denote that assumption, and thereby establish their context. And as such, those remarks were both perfectly appropriate and 100% accurate. Yet, you saw fit to take issue with them anyway. Had I assumed 240V, and based on that assumption made a "Wylie Special Shoot-From-The-Hip One-Size Fits-All" pronouncement that AWG 10 was just fine and dandy regardless of any other considerations or possible complications, he could very easily have wound up with a real problem on his hands had it turned out that it was 120V.

So which would be the more responsible and potentially helpful (in the sense of avoiding potential problems) approach?

Even IF the generator was 120v, the voltage drop would be about 9v AT FULL LOAD 30a.

Which would be nearly three times the preferred limit. Yet you are apparently OK with that, as a design criterium?

If the generator held the voltage at 120v that would mean devices would be seeing about 110v which us acceptable for most devices.

"If", "acceptable", and "most".

Do you REALLY think that installations should be designed to be dependent upon the vagarities of "if", (barely) "acceptable", and "most"?!? At least at the design stage, the criteria should be "even if not", "ideal" (or as close to it as feasible), and "all foreseeable".

You also seem to think that if XYZ meets code, even if just barely, that's the end of the discussion; and that anything beyond that is by definition a total waste. What you apparently fail to understand is that code is ONLY the bare minimum spec needed to stay legal. It does NOT represent even a "medium quality" approach, let alone "best practice" or "ideal".

Also, the reason I brought up wether the geny is 30a continous is because many generators HAVE a 30a recepticle because their capacity is just over 20a so a 20a outlet is too small! If this is indeed the case, then using 30a for amperage in our VD math is also overkill!

Red Herring.

As pointed out previously, the generator itself was never mentioned by the OP, and is not the issue. Beyond that, any attempts to rationalize a potentially sub-par installation based on a guess that the generator MIGHT not fully tax the circuit are just silly.

#10 is fine...end of story!

Still wielding that overly wide brush, I see.

Exactly! VD calcs are for FIXED wiring! So u dont include the length of a flexable cord

You do when it is explicitly specified as part of the given problem.

...because the cord length can change and an electrician's job is to compensate only for whats fixed inside the wall and cant be changed. The OP can add an even longer cord and mess up the VD calcs...its not our job to calculate what length cord might be plugged into an outlet, only the wire FEEDING that outlet!

Your tunnel-vision is showing.

Who said anything about "your job"? The OP asked if, given that long cord, AWG 10 would be adequate for the interior wiring. As as pointed out numerous times by now, it very well might not have been, given the (most likely) scenario of it being a 120V circuit. So how exactly was it wrong to point that out?

May be just the reason why I smell an engineer....

Thank Ghod you aren't one!

 

pattenp

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I'm not saying it's wrong to up-size the circuit wire beyond standard practice, or what you're advocating is wrong. I just don't agree whole heartily with your position. And yes it's not my money and or decision and I don't know how you equated like it was out of what I said. The NEC standards are not based on marginal safety factors or what's the cheapest safe way to do wiring.

If that was all there was to it, I would agree. But the OP explicitly specified a 40-ft. AWG 10 generator cord, which greatly exacerbates the problem. The load doesn't care WHERE some/most/all the voltage drop was imposed; it only knows it's not seeing as much voltage it wants. I questioned the need for such a long cord, but never got an answer on that. By now, it's a moot point, because it turns out that this is apparently a 240V application; hence, AWG 10 will suffice. But even so, if the OP can use a significantly shorter cord to connect the genny, then all the better. Notwithstanding any commonly used "rules of thumb", less voltage drop is ALWAYS better than more voltage drop.



Maybe. Maybe not. It's not your money, so not really your decision. There seems to be a chronic mindset around these parts that everything MUST be done in the cheapest possible way; and that quest to pinch every penny is sometimes taken to downright silly extremes. But some folks will prefer to spend slightly more, and do a "nicer than bare minimum" job, even if the return on that investment is sometimes more in the realm of "peace of mind" or "future-proofing" than "urgent immediate need". So I see nothing wrong with pointing out that there are alternatives, and why they might be of value. (Note, this applies in all sorts of contexts. Do you always buy THE cheapest car on the lot? Or THE cheapest shirt on the rack? Or THE cheapest meal on the menu?)



That is a gross overstatement, at best; and as such, it does NOT accurately represent my position.

 
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