I am a licensed electrician and work 95% residential wiring recently I was asked to wire a water heater in a commercial building , it will need 30 amps 220volts, my question is, the panel is 3phase 120volt 208 can I use a 2pole 30amp breaker to get my power

Dude, are you seriously asking this?

If the panel is 3 phase 120/208 then yes, you can get 208v single phase by using a two-pole breaker. If the unit is rated for 240v you will just get less watts from the elements.

A 240v 4500w element has a resistance of 12.8 ohms.
This same element used at 208v will give you 3380 watts and draw 16.25 amps.

Dude, are you seriously asking this?

If the panel is 3 phase 120/208 then yes, you can get 208v single phase by using a two-pole breaker. If the unit is rated for 240v you will just get less watts from the elements.

A 240v 4500w element has a resistance of 12.8 ohms.
This same element used at 208v will give you 3380 watts and draw 16.25 amps.

I don't find it unusual for a resi electrician not to know this!! I'm not going to get into the reasons. I also don't find it unusual for a commercial electrician not to know residential wiring.

I have the opinion that just being licensed you should know this. Just because someone does "95% residential" doesn't mean that's all they need to know.
What about the other 5%?? The water heater the OP is wiring falls square into that 5%.

The technical aspect of the question at hand is not strictly a residential/commercial issue.

I have the opinion that just being licensed you should know this. Just because someone does "95% residential" doesn't mean that's all they need to know.
What about the other 5%??? The water heater the OP is wiring falls square into that 5%.

The technical aspect of the question at hand is not strictly a residential/commercial issue. I agree 100%! You and I are on the same page! But don't expect others to see things the way we do! Yes, all electricians should have a firm grasp of Ohms law, but many do not. Many have forgotten the calculations and rules all together (add-in 1.732, and you can confuse the entire issue). Speaking from experience, it is uncommon that a residential electrician has to use calculations to do their job. If it wasn't for me being online helping others, and being consistent in my studies, then I would be on the same boat--lacking basic electrical math. We are in an embryonic trade, but many stay behind, because they lack motivation to stay consistent in their studies, and lack having to do (Note: We should make doing calculations mandatory )! To be on top, you must be consistent---surrounding yourself with people who want more than a 9-5 job! It has been my experience, that most residential electricians aren't electrician at all ( I'm sure there are commercial industrial electricians on the same boat). They a purely installers--lacking education in calculations, and basic NEC rules! I can tell you this, it's rare to see residential electricians sitting around doing 3 phase transformer calculations. Why, because they feel it's a waste of time--it's a prerequisite with no purpose to their everyday work life. To them, it's a bunch of college coarse that they will never use! To me, it's a step towards superseding my current position!! It will take an overhaul of the mindset and trade to get people more interested in retaining electrical calculations! Some people only learn it to pass their exam<<<<<this temporary preservation is enough for them. ;)

Just my .o2 on what I know about "some" residential electricians.

The heater should have been purchased with 208V elements or changed after the fact. It's not mandatory, though.

I expected Stan to respond to the book I wrote :)

Stan is full of surprises...

Oh oh I better duck...

Boo! :d