Site lighting-480v to 208v

[jflynn]

We have a customer that had a design done to illuminate there parking lot.The design is based on 480v being present @ an existing building,turns out the existing buildings service is 208.The site heads are 400w M.H. lamps,with the most shown on any given circuit is 12 heads.

The 480v design was actually based on 1000w heads,on 40amp brk;s,fed with#8;s.

My question is,- knowing now that the heads are 400w,can we use the 480v design with 208v,or would it be a better job to increase the wire size to #6;s and install 30amp brk;s?
The control for these is shown as 30amp- 4p- lighting contactors,Any help would be appreciated,thanks

 

[ducorse]

most 400w MH fixtures are multitap that can use 120, 208 ,240, 277. you will have to re size your circuit accordingly for the higher amprage and voltage drop.

 

[LarryFine]

J, what matters is that your supply voltage is 208. If the load current of each 208v, 400w fixture is equal to, or less than, the load current that each 480v, 1000w was, the wire and breaker sizes, should work fine.

1000w/480v = 2.08a

400w/208v = 1.92a

While the actual load current will be higher than these numbers, calculated using the lamp wattage, the relative numbers should have a similar ratio. According to these numbers, you should be good to go.

Added: You really should use the actual rated current specs of the fixtures in your calculations. Just do the same math with the new numbers that was done in the original design.

By the way, system voltage has no bearing on voltage drop. That's a function of load current and circuit impedance. 2a will cause the same voltage drop over the same circuit, regardless of circuit voltage.

However, the same voltage drop on a lower voltage circuit does translate into a higher voltage-drop percentage.

 

[jflynn]

J, what matters is that your supply voltage is 208. If the load current of each 208v, 400w fixture is equal to, or less than, the load current that each 480v, 1000w was, the wire and breaker sizes, should work fine.

1000w/480v = 2.08a

400w/208v = 1.92a

While the actual load current will be higher than these numbers, calculated using the lamp wattage, the relative numbers should have a similar ratio. According to these numbers, you should be good to go.

Added: You really should use the actual rated current specs of the fixtures in your calculations. Just do the same math with the new numbers that was done in the original design.

By the way, system voltage has no bearing on voltage drop. That's a function of load current and circuit impedance. 2a will cause the same voltage drop over the same circuit, regardless of circuit voltage.

However, the same voltage drop on a lower voltage circuit does translate into a higher voltage-drop percentage.

Larry great info,this makes sense to me,thank you.....