panel and wire size 5

[Reesh14]

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!

 

[jraef]

If you don't know how to do that you are not an engineer, and this site is not to be used as a substitute for proper engineering.

 

[Reesh14]

Thanks for crushing my confidence as a junior engineer. I am not familiar with 240V systems and using a transformer from 120/208V to 240V. This situation was confusing to me. I thought I would post on the site to learn how one would correctly approach this as I read up on it on my own. I am sorry if my post offended you in any way.

 

[BJC]

look in the catalog and pick out a 120/240 panel. Being as your feeding it with two the size can be small. I think they start at 40 amps.
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.

 

[rcw retired EE]

Reesh14 - Your information is not complete enough to provide a good answer. Is it an exisitng panel and circuit or is it one beign designed? If it is exisitng Is the 240 V panel three phase or single phase 120/240V. What is the size of the main breaker in the 240 V panel? How are the two trasnformers connected and what is their nameplate rating and connection diagram? Are they two winding 208V input 240V output transformers with a full load amps of 8.3A (= 2kva/240V) or are they connected as buck/boost autotransformers with a full load amps of 63A (= 2kVA /(240V-208V) ).

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.

 

[LionelHutz]

I expect rcwilson's description of the transformers being used in a "boost" connection to increase the voltage of 2 legs of the 208V to 240V is correct. That's the only description that really makes sense if the panel is feeding receptacles and a water heater. This would make it a 120/240V, single phase panel.

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.

 

[BJC]

The only way I can see to make it work is to connect the high side of the two 2 KVA transformers in series and connect them between two lines on the 120/208 system. The secondaries are connected in series and grounded at the center ( establishe a system neutral ).
All tha would only work if the transformers are 104-120 volt. Other ratios will work as autotransformers.

 

[JensenDrive]

I guess jraef had a point that it at first looked like a Q associated with how to read the NEC, but the small xfmrs adds a interesting facet to the matter that I am learning from.

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.

 

[Reesh14]

thank you everyone for your thoughts/advice on this matter. The equipment is all existing and I think it will have to be visually inspected to make sure all the ratings are correct, obtain all the complete information and to understand how the transformers are connected.

 

[rcw retired EE]

Lionel - It is possible to use the 2 kVA transformers as autotransformers in an open delta connection to provide 240V, 3-phase power. Each 2 kVA transformer's 32V boost winding "stretches" one side of the 208V phase-phase voltage triangle out to 240V. The result is three phase-phase output voltages at 240V but the phase to "neutral" voltages would be 223V, 120V and 223V. The panel could only feed 240V loads. It would be dangerous to have it feed a 120V load.

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."

 

[JensenDrive]

Gee, I get it now. I did not realize what LionelHutz and RCWilson were saying. As autoxfmrs, the fault duty is much higher than as two winding xfmrs. I think I would need to give up on per unit and work in real ohms to figure out what the available fault duty is for such an installation.