APC does not recommend that you daisy-chain two or more UPSs together. Each unit should be plugged directly into a properly grounded wall outlet for optimum surge protection.
All UPS systems
All serial numbers
This document describes APC by Schneider Electric's stance on daisy chaining UPS units and the reasons behind it.
Daisy chaining UPS systems is not recommend for the following reasons:
1) It is not UL tested. Therefore, should something go wrong and damages be caused to your connected equipment, APC would not honor the claim. (Underwriters Laboratories Inc. [UL] is an independent, not-for-profit product safety testing and certification organization)
2)This configuration does not provide any extra surge protection. The UPS is designed to remove any possibility of a surge reaching the output receptacles. If a surge, strong enough to damage equipment was received, the first UPS in the chain would sacrifice itself to protect its load. This would mean that power would be removed from the 2nd UPS in the chain and force it to battery.
3)Whenever connecting a 2nd UPS into a 1st UPS, the chance of Overloading the 1st UPS is greatly increased. The amount of receptacles in a UPS are restricted. This is due to the power limitations of the UPS itself. Although, the amount of receptacles has been increased, the overall Watt capacity of the 1st UPS remains the same. The capability of the 2nd UPS, will be inversely affected by that of the 1st UPS. Therefore, the overall Watt capacity of the configuration is no greater than that of the 1st UPS in line.
4) In most cases, daisy-chaining UPSs does not allow for extra run time. If you are using a UPS that outputs a step-approximated sine wave when on battery, as soon as the fist UPS goes on battery, the second UPS will also go on battery because it will see the step-approximated sine wave as distorted or bad power. Both units will discharge together, and will not provide any extra run-time to the load.
If you are looking for extra run-time, we recommend a product with the capability of adding extra, external battery packs. If you require more runtime or VA capacity than you currently have, we recommend taking advantage of our Trade-UPS program. Through the Trade-UPS program, you could trade up your existing unit for a unit with a higher VA capacity or one which has extended run capability.