Class math

[wyatt]

Need help
store 60' by 80' supplied by 240/120, 3 phase delta. min 2 wire 20 amp branch circuits requiered for lighting.

I got 60x80=4500x3(table 220.12)=14400
14400/2400=6 the answer is 8?
My thoughts are that on a 240 delta only 1 string has the mid tap so that all 120 loads is like 1 phase and the 1.732 should not be used in the calulation. I have put in that number and still cant come up with 8. what I'm I missing?

 

[wyatt]

Re: Class math

Ok i found it 125% for continuous load. but were in the code dose it say to do it. the exampel in the back used 125% but I don't see were it says to do it.

 

[bphgravity]

Re: Class math

See 210.19(A)(1) - any load that is expected to operate for more than 3 hours is going to be considered continuous. Store lighting easily fits this description.

 

[wyatt]

Re: Class math

Thanks that also answers a motor question that runs more than 3 hours.

 

[LarryFine]

Re: Class math

Originally posted by wyatt:
Thanks that also answers a motor question that runs more than 3 hours.

That's quite a question!

 

[haskindm]

Re: Class math

Any single motor is calculated at 125% regardless of how long it runs. Also the largest motor on a feeder is calculated at 125%. This has nothing to do with continuous loads. Do not try to oversimplify the code, it will get you in trouble every time.

 

[bphgravity]

Re: Class math

Originally posted by haskindm:
Any single motor is calculated at 125% regardless of how long it runs. Also the largest motor on a feeder is calculated at 125%. This has nothing to do with continuous loads.

This is not exaclty true. 430.22(A) clearly states "used in a continuous duty application". While I agree the 125% rule applies for service and feeder sizing, this is based on 430.24 and does not apply to all single motors by themselves.

 

[haskindm]

Re: Class math

bphgravity,
Thanks for catching that. I guess I was also guilty of oversimplifying. It is nice to put these concepts in a nice, neat, understandable box, but they never stay there!

 

[augie47]

Re: Class math

don't forget the requirement of 600.5 , if applicable