Single Phase vs. Three Phase Electrical

[Mark P]

Hello,

My exposure to electrical power and different systems (120/240 Volt Single Phase, 120/208 Volt Three Phase, 120/208 Volt Single Phase) and I'm having a bit of a problem grasping the concept.

If someone's supply is 120/208 Volt Three Phase can we supply a 120/240 Volt Single Phase Load Center? If not, is there such a thing as 120/208 Volt Single Phase Load Center?

Thanks for your expertise and insight clarifying this issue. I mean it... thank you very much!!!

 

[charlie b]

The short answers are No and No. An explanation, however, cannot be short. Let me just say for now that if you took two of the phase wires, the neutral wire, and the ground wire (i.e., omitted the third phase wire) from a 120/208V three phase system, and supplied that to a 120/240V single phase load center, then any load that actually needed 240 volts would only be getting 208. That is 32 volts low, or 13% low. Most loads cannot tolerate that low a supply. What you can do is to take two phase wires and the ground wire (neutral not needed) from the 120/208, and supply a single phase step up transformer, to get the required 120/240.

 

[augie47]

Mark,
You will find that a number pieces of common-place equipment is marked for
for dual voltage. Household ranges, HVAC equipment and some motors are examples of such equipment.
The affect of the voltage will be noted on this equipment as the 208 heat kw on a range or HVAC will be approx. 75% that of the 240 volt rating.

 

[Npstewart]

Not sure what the OP was asking. I thought he was asking about the rating on the load center. Most load centers as well as most disconnect switches etc say "120/240" on the sticker which means it is rated for up to 240V by UL. More times than not, the supply voltage is actually 208. Pretty sure that was his question but not sure. So if he was asking that, then the answer would be yes.

 

[augie47]

Not sure what the OP was asking. I thought he was asking about the rating on the load center. Most load centers as well as most disconnect switches etc say "120/240" on the sticker which means it is rated for up to 240V by UL. More times than not, the supply voltage is actually 208. Pretty sure that was his question but not sure. So if he was asking that, then the answer would be yes.

You may be correct.. I might have read in too much..
and I agree, "yes" the 240/120 loadcenter can be used on 208/120

 

[skeshesh]

I agree with all the response. While Charlie's answer was accurate and technical as usual, I think Augie's reponse best addresses the OPs concerns. Most modular building that I've seen (specially old one's that tend to get reused by school districts, local gov'ts and such) have 120/240V panelboard. As is often the case though, the available voltage source is 120V/208Y. As Augie's post references, careful varification is required to see if the equipment typical to such applications (wall-mount AC units are a good example) are rated for 208V. As Charlie mentioned, if they are not dual rated at 208V/230V then you will need to use a transformer. In my experience most of them (typically a Bard, Trane or Carrier unit) are dual rated. Another issue that not directly related but you may have interest in knowing about is it becomes a challenge to balance the loads if there are many modular sections used to build multiple buildings. Since a 3 phase system has 3 phase conductors but the single phase panels use only 2 phases, it becomes a challenge to ensure the phases arent extremely unbalanced. Of course this issue is easy enough to address during design, but it's difficult to make sure the contractor provides correct installation.