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I am connecting a portable generator to my electric sub-panel in my garage. My generator is a 6kw running watt unit with a 30amp 220 volt output. I will be using this connection (L14-30) to a box outside my garage. This box will be connected to a shut-off panel inside the garage. This shut-off panel will be connected to the sub-panel, via a 30 amp double pole breaker (220 volt). I would like to measure each leg for volts, amps and watts. Am I correct in that I can measure each (2)"hot" wire for volts, and amps, or do I need to include the "common" wire and add the amps from that for my total? I also want to keep an eye on the total running watts from each leg to keep from overloading the generator. I may need to remove some devices as they start, because of their starting current.
I am not using an automatic transfer switch, so I will be shutting off the Main breaker before I start the generator. I will then shut off ALL breakers, before I switch the shut-off switch to ON. I will then switch individual breakers to feed certain areas of my house, during a power outage.

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    The experts will come along soon enough with the details. But (a) you measure the hots, no need to measure the neutral and (b) you must have an interlock such that you can't have the generator and utility power running at the same time. Otherwise, you could start the generator because "power is out" and zap the linemen trying to restore power. Feb 25 at 2:27
  • What make and model are your panels and can you post photos of them, including the labeling on the inside of their doors, please? We can probably rearrange this into something useful, but we'll need to know far more about what you have for equipment, as your "checklist" approach simply won't fly. Feb 25 at 2:45
  • I appreciate your input. My panels are both Square D panels. I will get photos this weekend to show the current set up. I am planning on adding the outside L14-13 box, as well as the inside shut-off switch in the coming weeks. I will check to see what kind of interlok I can find. Feb 26 at 5:33
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No. You can't do that.

You have to have an interlock in the main panel.

Your idea is clever, and you think "Why not a checklist"? The answer is people got hurt doing it that way, often enough that they changed the Electrical Code to stop the carnage.

In a crisis people do not "rise to the occasion". They descend to the level of their training, and if they aren't well trained, the result is chaos. Code requires your system be built to endure through that. Simple as that.

Since your inlet equipment must be at the main house anyway (you can trench a separate cable to send it to the garage if you really want to), you can simply select commercially available inlets that include amp meters. You are interested in amp meters; there's no need to monitor voltage (plus, it's pretty easy to do with other devices such as a Kill-a-Watt).

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As mentioned by manassehkatz, a reliable mechanical interlock is mandatory ( users with less technical know how, misunderstandings in the operational procedures, etc.) in order to avoid injuring or killing people. The interconnection locking device must ensure that the generator can only be connected to the building's net if all connections to the grid are disconnected.

For total wattage and each leg wattage 2 Ampere meter and 2 Voltage meter in each leg are sufficient. The current through Neutral is always the difference of both leg currents - and thus does not reveal any new/additional information.

In the specification of the generator, there should be information if and how much asymmetric load is allowed.

In case of an older or defective generator, many devices could be destroyed in case of a missing spike/overvoltage protection.

All GFCI must work as before, i.e. Ground/Protection Earth and Neutral connections of the generator must be done according to the instructions. All GFCIs must be tested when the generator is running.

The exhaust must not get into the building - CO is odorless. A CO and CO2 alarm sensor is not very expensive.

Most frequent problem with generators is that they do not work just when they are needed, since they are not tested/maintained regularly.

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  • My generator is a Champion Model 40023. It is rated as 6000 Watts running, 7000 Watts Start. This generator will be sitting outside the detached garage, in a covered (open) patio with the exhaust facing away from all buildings. Feb 26 at 5:40
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Aside from needing to comply with the code issues others have stated, you should only have to calculate or measure the watts per leg. Measuring the neutral will just tell you the difference between the hot legs.

But 6000 watts / 240 volts = 25A, so even though the generator may have a L14-30 it won't be capable of 30A simultaneously on both legs. The engineering of the generator is likely not available to determine if it will sustain an imbalance exceeding 25A on one leg.

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  • Since the generator is capable of 7000 Watts Start, it will generate up to 30 Amps. Thus I am installing a 30 Amp breaker for my connection in the panel. One of my biggest concerns in the fact the Champion does not do a good job in providing me with accurate information regarding the THD. I may have to replace this Champion with a unit that addresses this issue showing me it is <6%. Feb 26 at 5:45

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