Design/Build - Three Phase vs Single Phase

[ScooterB]

I am looking at a job for a 6000 sq ft warehouse and small office. 18' ceilings in the warehouse. There will be 4 - 6 50A welding recepts and am looking at using the new fluorescents instead of low bays. There will also be the obvious 120V recepts for grinders, etc. I will probably recommend a 100A panel for the future office. In your experience, what would be the best to recommend?

I am leaning toward the three phase, but I know that the cost would be a lot more. I believe that the utility can support either at this location.

Thanks in advance for your help!

 

[coulter]

What are the supply requirements for the welding machines? That may well determine the optimum service.

carl

 

[barbeer]

I believe three phase machines are more efficient. Did I hear something of the new code requiring separate neutrals for MWBC, if so there will not be much of an advantage to three phase. Besides I really like blue!

 

[charlie b]

Did I hear something of the new code requiring separate neutrals for MWBC. . .

No. It will say that if you wish to share neutrals (i.e., create a MWBC), then you must have the breaker handles of both (or all three) phases operate together.

 

[barbeer]

I knew I heard somethin about somethin!:grin:

 

[petersonra]

a lot depends on what the utility charges for providing a hookup. I don't know exactly why but 3 phase 208 is dirt cheap compared to 480 here. so much so that it has been common place for plants to bring in 208 and buy transformers to make 480. i worked at a place that had such an arrangement. i don't recall how big the transformer was, but either the primary or the secondary fuses were 800A.

same thing where I work now, but the 208/480 xfmr is much smaller.

 

[ScooterB]

I think that the welding machines he has now are all single phase, not three phase. But couldn't I just solve that with a 2 pole breaker? My inclination toward the three phase was more from a "future expansion" thought process. By having say a 300A or 400A three phase service now, his future expansion would be fairly easy (at least electrically).

 

[kbsparky]

Keep in mind that 3 phase wye connected service will only provide you with 208 Volts. Those single phase welders will operate with 25% less wattage given the same amps settings.

You might want to consider using a delta-connected "high-leg" service, which will give you the full juice on 240 Volt loads, while providing the benefits of 3-phase power.

 

[ScooterB]

Thanks kbsparky! That is definitely a consideration and one that I need to see what the utility is willing to offer. The utilities in our part of the country don't charge for the hooks/xfmrs/etc but they may not always provide exactly what yo want. Thanks again!

 

[coulter]

I think that the welding machines he has now are all single phase, not three phase. But couldn't I just solve that with a 2 pole breaker? ..

Absolutely, but why? Three phase is not inherently more efficient than single phase. Which one is more efficient is dependent on the loads. If the loads are single phase (2-pole) I can't think of any reason to to have a three phase service.

My inclination toward the three phase was more from a "future expansion" thought process. By having say a 300A or 400A three phase service now, his future expansion would be fairly easy (at least electrically).

Since your loads are all single phase (2 pole) - no easier. Unbalanced three phase is no treat to deal with.

As for expansion: Ask the customer what they are going to install. Get the nameplate data. Design the extra capacity to fit the expected loads.

Recommend to match your design to the expected loads.

carl

 

[mdshunk]

If there's any air conditioning involved bigger than about 5 to 7-1/2 ton, then you're really gonna want to try for a 3-phase service.