I have a customer who recently had a new 16 kW propane gas Generac generator installed.
When and only when the generator is running, his APC/UPS keeps toggling back and forth indication a incoming power line problem. I checked the neutrals, grounds and phase connections and all seems OK. I heard in the past that a gas aspired generator is not conducive to a UPS or computer is this the problem?

Thank You,

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The issue may be the frequency and/or voltage from the genset causing the UPS to think the power is not adequate.

The issue you mention with UPS on gensets is that a UPS is a highly capacitive load that the genset has a problem with.

Generally speaking the KVA of a genset should be at least 1.5 times the KVA rating of a UPS. Ideally there should be an equal amount of inductive load on a genset to help counteract the KVAR of a UPS.

I suspect that your UPS is not that large, so the issue is to fine tune the output of the genset. In any case if possible , add some inductive load to the genset.