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Installation Issues with Symmetra LX & 100A Supply

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Symmetra LX is a redundant on-line double conversion UPS. Each Symmetra Power Module is rated at 4 KVA so when the Symmetra LX is fully populated with 5 power modules, it can draw up to16 KVA from the utility as one power module is redundant. This unit can be used either as a single phase or as a 3 phase unit, without any adjustments or changes being made to system to achieve this.

The Symmetra LX will operate at a voltage of 155 V to 276V , therefore it should draw a current of about 100A at 155V on a full load of 16KVA. Most European regulations require a 35 sq. mm cable for a 100A load but the Symmetra LX connectors will only take a maximum of 25 sq. mm.

However if we look at the power supply in most European countries, the voltage range is 230V - 240V and a voltage of under 200V would be very unusual. Taking a voltage of 240V and running the unit on full load the current would be just over the 65A, with 230V it would still be under 70A and we can drop the down to 200V before reaching 80A. The Symmetra LX should be protected by a slow blow Fuse / MCB which would need a sustained overload of over 1.2 times the rated current for about 2 hours to trip the Fuse / MCB, so even if the voltage drops below 200V the extra current shouldn't cause any problem.

In an area where a reasonably stable voltage exists there is no problem in using 25 sq. mm with an 80A fuse / MCB. Even with substantial voltage drops there is no danger of cables overheating or fuses blowing. In cases where the cable runs are very long and the regulations specify 35 sq. mm cable, the 35 sq. mm cable can be brought to the isolator adjacent to the unit and a 25 sq. mm cable can be used between isolator switch and UPS.

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