3 phase power, fuses, & balancing the amps

[max_well]

I've got an electrical question concerning 3 phase power. I have an industrial space with a 3 phase service, each line of which is connected through 70 amp fuses above our meter located in the electrical utility room for the whole building . The panel that contains the main breaker is locked (with local electric company lock), so I can't see what the actual service capacity is. The wire that feeds the breaker box in my room is #4 THW, which is rated to carry 85 amps. The run to the main is ~120 feet from my box. I have been planning a 12KW light show, cooled using 2 big (2.5 ton)mini splits, that draw max current of 14.2 amps each at 240v. By my calculations, I can best split and balance the load from lights and a/c to give 65 amps on legs 1 and 3, and 30 amps on leg 2 (all at 240 v), and then have the balance of the available power single pole on leg 2 to run fans, pumps, and other 120v devices (up to 40 amps at 120v, to stay below the 70 amp fuse limit ).
My question is: Does this sound like it is cutting things too close to the fuse ratings? I know that fuses can safely run above their rating for short amounts of time without issue (such that they won't pop when the a/c starts up, which requires lots of extra current ), but is it safe to run near their capacity? I will not have regular access to the utility room where the fuses are located. Would it be safe to change them out for 80 amp fuses considering the wire is rated to carry 85amps on each leg? or do I need to down size everything? any thoughts are welcome and appreciated!
cheers
Max

 

[hiboy]

You better be careful with that 120/240v. You might be dealing with a stinger leg identified with orange
markings.That means you dont want to run any 120v appliances from it. Use it for your 240v only. And yes i'ld bump the fuses up to the main breaker rating.

 

[floodo1]

Yes, firstly make sure that you don't have a "high leg delta" setup, which will be identified with 1 orange wire (which is the high leg). At least it SHOULD be identifiable this way, whereas a standard setup most typically uses Red, Blue, and Black for each of the 3 phases.

Make sure to use a meter to check the voltage from each phase to neutral / ground and to check the voltage for each phase. If you have the standard 3phase setup you're going to get 120v from each hot wire to neutral/ground and you're going to get 208v between phases.

NOTE: there is no 240v available in the typical 3phase 120/208 setup. All you can do is get 120 from hot to neutral or 208 from hot to hot.

Almost all magnetic ballasts have transformers which can be used for 120, 208, 240, and maybe even 277v. Almost all electronic ballasts can be run from 90-277v. So for magnetics make sure to adjust to 208v, and for electronics do nothing :)


In any case you're going to have to see if your AC can run on 208v because you have no 240v. Can't give you much advice here.


ALSO: when you tap 120 circuits (from phase to neutral).....if you balance the 120v load across all 3 phases the neutral wire ends up carrying no current, but if you unbalance the load then the neutral wire ends up carrying the 'unbalanced current'. that is, suppose you build your system up with only TWO 120v circuits. each pulls 20 amps. In this situaiton the neutral wire is going to be carrying 40 amps!!!!! However if you added a third 120v circuit and split your devices between the 3 circuits so that they pull 15, 15, and 10 amps.....well then your neutral wire is only going to be carrying 5 amps.
So, as you can see, it's very easy to overload your neutral wire if you don't use multiples of 3 120v circuits.....ie run 3,6,9,12 circuits of 120v and be careful about trying to balance the load so that the neutral wire isn't overloaded.


Be careful man, it sounds like you don't know whats up with 3 phase 120/208 and it's a slightly different beast than the more usual 120/240 single phase that houses have. The nice part is that since you can use each phase indiviually you end up with lots of current handling capability, but you lose 240v!

 

[Dr.stickerdick]

You do not know what you have until you measure it in your pannel. Phase to phase to phase to ground to neutral. The pannel that supplies your space is to be available to you, legally and by code. There are several three phase voltage set-ups, you need to know what you have and maybe find a licensed eletrician that knows three phase. stay alive and,,,,,,,,, Peace

 

[floodo1]

yeah, it may turn out that you have a setup with more than 208v....potentially upwards of 400 volts!

be careful :)