Voltage Drop 120 vs 240

[Power Solutions]

Hi All,
So I'm working on an estimate for a detached garage and a generator that are approx. 200 ft. away from the main panel in a home.
I start doing the voltage drop calc for the garage at 80% of125Amps (max size snap in GE breaker available) at 240 volts and then I plug in 80% of 125 amps at 120 volts and it's a big difference. So I say to myself, ok so it's based on actual load. But we have no def loads at the time of building. (Basics welder, compressor, lift, etc.)

So I'm thinking I should plan for worst case scenario and size for the 120 volt calc. 4/0 Al URD.

What do you folks do?

 

[david luchini]

Use 240V.

 

[electrofelon]

What do you folks do?

I just did a dwelling unit that was about a 300 foot run and did it with #2 AL. My rule of thumb is to use 5% VD and 60 amps for most resi stuff. Even if it does it 60 amps, it will be for a very brief period.

 

[drktmplr12]

Hi All,
So I'm working on an estimate for a detached garage and a generator that are approx. 200 ft. away from the main panel in a home.
I start doing the voltage drop calc for the garage at 80% of125Amps (max size snap in GE breaker available) at 240 volts and then I plug in 80% of 125 amps at 120 volts and it's a big difference. So I say to myself, ok so it's based on actual load. But we have no def loads at the time of building. (Basics welder, compressor, lift, etc.)

So I'm thinking I should plan for worst case scenario and size for the 120 volt calc. 4/0 Al URD.

What do you folks do?

I would assume the worst possible case based on the intended use, as you suggest. You might be penalizing yourself by assuming 80% of 125A, but it can't get any worse than that and there is nothing wrong with oversized conductors except they cost more and you have to remember to upsize the ground proportionately.

If you want to sharpen your pencil, consider which loads can be operating simultaneously. For example, what are the chances that the compressor and welder are active at the same time?

 

[ggunn]

I would assume the worst possible case based on the intended use, as you suggest. You might be penalizing yourself by assuming 80% of 125A, but it can't get any worse than that and there is nothing wrong with oversized conductors except they cost more and you have to remember to upsize the ground proportionately.

You only have to upsize the ground if you are upsizing the current carrying conductors for reasons other than required ampacity. It seems to me that in this case, ampacity is the reason for the sizing and upsizing the ground should not be necessary.

 

[K8MHZ]

If you want to sharpen your pencil, consider which loads can be operating simultaneously. For example, what are the chances that the compressor and welder are active at the same time?

If there are any air leaks, even small ones, there is a good chance the compressor may kick on while someone is welding.

 

[Strathead]

Hi All,
So I'm working on an estimate for a detached garage and a generator that are approx. 200 ft. away from the main panel in a home.
I start doing the voltage drop calc for the garage at 80% of125Amps (max size snap in GE breaker available) at 240 volts and then I plug in 80% of 125 amps at 120 volts and it's a big difference. So I say to myself, ok so it's based on actual load. But we have no def loads at the time of building. (Basics welder, compressor, lift, etc.)

So I'm thinking I should plan for worst case scenario and size for the 120 volt calc. 4/0 Al URD.

What do you folks do?

Use 240 How often do you think you are going to use all of the loads on one phase and none of the loads on the other? The code figures never, that is why you can reduce your neutral size.

 

[charlie b]

I start doing the voltage drop calc for the garage at 80% of125Amps (max size snap in GE breaker available) at 240 volts and then I plug in 80% of 125 amps at 120 volts and it's a big difference.

If you did the calc right, then the answer (in terms of volts) should be identical in both cases. But a given amount of volts dropped along the 200 foot run will be a higher percentage of a 120 volt system than the same amount of volts dropped in a 240 volt system. Your system is 240 volts, and (as others have already said) that is the value that matters.

 

[drktmplr12]

You only have to upsize the ground if you are upsizing the current carrying conductors for reasons other than required ampacity. It seems to me that in this case, ampacity is the reason for the sizing and upsizing the ground should not be necessary.

OP is sizing ungrounded conductors for voltage drop and does not have a load calculation to use as a basis, and so proposes to use 80% of 125. The garage is 200 feet away. With copper #1 the VD at 200 ft, PVC conduit, 0.8 PF, 1-phase, 240V is 2.46% so the feeder must be upsized to #2/0 to bring VD below 2%. Aluminum #2/0 same stats is 2.56%, must be upsized to #4/0.

 

[petersonra]

Hi All,
So I'm working on an estimate for a detached garage and a generator that are approx. 200 ft. away from the main panel in a home.
I start doing the voltage drop calc for the garage at 80% of125Amps (max size snap in GE breaker available) at 240 volts and then I plug in 80% of 125 amps at 120 volts and it's a big difference. So I say to myself, ok so it's based on actual load. But we have no def loads at the time of building. (Basics welder, compressor, lift, etc.)

So I'm thinking I should plan for worst case scenario and size for the 120 volt calc. 4/0 Al URD.

What do you folks do?

Why pick 80%? The actual load might be 10% or 125% if you don't know what it is. You need to start with a load calculation, at least some best estimate.

You might also have issues with adding whatever load you are to the main panel.

Are you going to size the generator at 80% too?

 

[kwired]

You only have to upsize the ground if you are upsizing the current carrying conductors for reasons other than required ampacity. It seems to me that in this case, ampacity is the reason for the sizing and upsizing the ground should not be necessary.

Maybe I read you wrong but if he puts in a 125 amp main breaker and runs 4/0 aluminum conductor because of voltage drop - this is a case where EGC must also be increased.

 

[kwired]

Many such garages you can run a 60 amp supply to them and never see main OCPD trip. But if you only run a 60 amp supply conductor you may see some significant VD when an air compressor or other larger motor load is starting, yet isn't so bad when same motor is running. That is where you may need to consider VD calculations based on starting current of such a motor, but still may be somewhat unnecessary to try to limit it to only 3% drop.

 

[don_resqcapt19]

OP is sizing ungrounded conductors for voltage drop and does not have a load calculation to use as a basis, and so proposes to use 80% of 125. The garage is 200 feet away. With copper #1 the VD at 200 ft, PVC conduit, 0.8 PF, 1-phase, 240V is 2.46% so the feeder must be upsized to #2/0 to bring VD below 2%. Aluminum #2/0 same stats is 2.56%, must be upsized to #4/0.

There is no requirement in the NEC that limits the amount of voltage drop, other than 2 or 3 specific cases. This is a design question and not a code question.

 

[drktmplr12]

There is no requirement in the NEC that limits the amount of voltage drop, other than 2 or 3 specific cases. This is a design question and not a code question.

In either case, my opinion is that OP should not disregard voltage drop simply because because code allows it, and I understand you aren't suggesting as much. The code recommends voltage drop be limited and for good reason.

Side note that International Building Code has requirements for voltage drop under energy efficiency chapter. Many states adopt IBC as a basis for building code and the inspectors can (and do, in Florida's case) enforce it. Usually it only applies to commercial and industrial facilities.