single to three phase conversion

[James M. Clarke]

I am adding some large HVAC units to a building. The facility currently has a 600 amp 240/120 single phase panel subfeeding two 225 amp single phase panels. I have to convert the entire facility to 208/120 three phase to get the total KVA I need for the new HVAC units. Obviously, the original single phase panels could be replaced with three phase panels but is there any use for the existing single phase panels with this new service. In otherwords is there a reasonable way to utililze them for the 120 VAC loads but still allow me to balance the main three phase panel board across all three phases.
Main loads that are not 120VAc are four wall mounted air conditioning units rated as follows:
Compressor Volts 208/230 RLA 13.7A LRA 75A Min Cir. 54.6A Fuse Size 60A
Heat Volts 240 Wattage 10 KW 41.67A
Thanks

 

[majpe]

three phase loads

three phase loads

yes, you can keep the single phase panels to remain. do you have three phase service coming into the building? or were you going to convert the single phase into three phase to feed HVAC components. if you have three phase available, you can easily bring in three phase and feed step down transformers for 120 VAC panels.

 

[James M. Clarke]

There is currently single phase into the building. I plan on bringing in three phase 208/120 to replace the single phase. The problem is not the 120VAc branch circuits as I could replace the single phase lighting panels with a three phase panel and simply reconnect the branch circuits. The issue is whether there is any way to reuse the two single phase subpanels after the main panel which feeds them in converted from 240/120 single phase to 208/120 three phase. Is there a easy way to balance the three phase load if the single phase panels are reused?

 

[LarryFine]

Jim, how about another idea to mull over, which just might save you some time and money:

Leave everything you can the way it is, and see if the POCO can add the third phase and deliver 240D w/hi-leg to a separate disconnect for just the new 3-phase loads.

Your new HVAC loads are already rated for 240v as well as 208v, so there's no incompatibility there, and you're allowed to have a separate service of a different type.

 

[ron]

Doesn't the existing motor loads have a 230V rating? Will that work with the 208V ph to ph service that will be brought to them? I've seen 208V services have 200V motors attached.

 

[James M. Clarke]

I thought of bringing in the three phase seperate for just the two new 20 ton HVAC units but the local POCO does not allow more than one service feed into the building even though it is within NEC guidelines. They stated the reason being for the safety of firefighters so that they always know there is only one means of disconnection.

How about this for a solution.

I replace the main panel (MDP) with a three phase panel. Most of the existing 240 VAC branch circuits to the existing HVAC units are already in the panel. I reconfigure the existing HVAC units to run on 208 instead of 230 VAC as they have that option. I leave the two subfed panels which are single phase and connect them up as subfeed panels to the new three phase panel using two phases and a neutral to feed the subpanel. I then feed the two new HVAC units (three phase) directly from the new MDP panel if their is enough slots or if not, I add a three phase subpanel for the new HVAC units.

The only thing I have to resolve is the existing HVAC units are rated 208/230 VAC for the compressor and fan but rated 240 VAC for the heat strip. I believe I could probably run the heat strips on 208 with slightly less power output but plan on checking with the manufacturer.

 

[winnie]

If the POCO will supply 'high leg delta' as Larry described above, then you can use that exact approach (3 phase MDP followed by single phase sub panels), without any of the issues of changing 240V loads to 208V. Such an installation will never have balanced three phase loading, but is not expected to be balanced.

240V resistive heaters should work safely at 208V, however they will only have 75% of rated output.

Something to keep in mind: if you have 120/240V multi wire branch circuits, and you refeed them with 120/208V 'single phase', then the neutrals will have to be counted as current carrying conductors for derating. Are there any situations where you will suddenly have too many CCCs in a conduit?

-Jon