panel and wire size
panel and wire size
(OP)
I am trying to figure out the Amps rating of a 240V panel and the wire size feeding it. It is being fed from a 100A, 120/208V, 3%%C panel through two 2kva, single phase transformers in 1" conduit. The supply breaker is 90A/3P. I assume the 240V panel is single phase and three wire. It currently has a few two pole breakers serving receptacles and a water heater. How would I determine the amps of this panel and the size of the wires feeding it?? Please advise.
Thank you!
Thank you!






RE: panel and wire size
RE: panel and wire size
RE: panel and wire size
Two things -how are you connecting them two single phase transformers connected from line to neutral on the 120/208 system won't give you a 120/240volt system. It would be 120 /208 again. Better draw out the phasor diagrams.
Second- The short circuit current from two 2 kva transfromer may not be enough to trip a breaker in the new panel. If you have a fault at the end of a wire the breaker may let through enough current long enough to start a fire.
RE: panel and wire size
The autotransformer boost connection might be used to boost the 208V 3-phase voltages to 40V to feed 240V single phase receptacles (no neutral) and 240V heater loads. The two transformers may be connected open delta boosting teh 208V phase-phase votlages to 240V.
The "amps of the panel" should be on the nameplate of the panel. This will be the amps of the bus bars, not necessarily the amps for the feeder design. Are you sure it is a single phase panel? It could be a three phase 240V panel with single phase (2-pole) breakers feeding 240V loads. If that is the case, the panel cannot feed 120V loads.
If this is an exisitng installation, your best bet is to get an electrician, deenergize the panel, follow the proper safety procedures and do a visual inpsection.
One lesson to be learned is to help those that follow your work by providng adequate information on panel schedules and as-built drawings so the elelctrical ssytem can be understood.
RE: panel and wire size
Otherwise, each 2kVA is a 120V transformer that is only 16A. This is not really enough current to feed multiple circuits and not really enough for most water heaters.
So, the panel is probably capable of supplying 62A at rated transformer current. You may want to put a safety margin on that number though.
RE: panel and wire size
All tha would only work if the transformers are 104-120 volt. Other ratios will work as autotransformers.
RE: panel and wire size
I concur with BJC that I would be concerned about whether a typical breaker would trip for a remote fault. Is this a case where fuses or supplemental protection circuit breakers are appropriate? I have never dealt with such devices.
RE: panel and wire size
RE: panel and wire size
The system is still grounded via the grounded neutral of the 120/208V system. The autotransformer does not isolate the system. A new grounded, "neutral" cannot be established unless a two winding transformer is used.
The 2 KVA autotransformers' 32 V windings are rated for 2kVA/32V = 62.5A. 62.5A x 240V = 15KVA per autotransformer. The two 2 kVA transformers could feed (2 x 15 kVA) = 30 kVA of load. (This isn't quite correct. The 3-phase KVA rating for the open delta bank is different because the two transformers also supply power to the third phase. Search for other threads on open-delta connections, there have been some excellent posts on this subject.)
This arrangement matches the rating of the circuit breakers mentioned by the OP.
Could be confusing to someone just looking at it. While this is a good cost effective design, it could be made better by a couple of nameplates, "CAUTION - 240V PANEL. DO NOT CONNECT 120V LOADS." or "Supplied from 208-240V Autotransformers - Do not connect 120V loads."
RE: panel and wire size