I am installing four infrared heating units on one 30 amp 3 phase branch circuit. Each heater is 4.8kw, 480V 3 phase. The heater do not have built in over-current protection or motors. Does 424.22(A) allow me to connect the heaters to the 30 amp circuit without individual over-current protection? Can these be cord & plug connected?
Infrared Heaters on one branch circuit
- Thread starter SDENKI
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juptonstone
Member
- Location
- Lady Lake, FL
3 heaters?
3 heaters?
Correct me if I am wrong...4.8 KW is 4,800 watts... divided by 480 volts and multiplied by 1.73 for three phase equals 17.3 amps per heater (never mind the power factor)... multiply that by 3 heaters and you have 51.0 amps total draw. I would suggest that #10 wire is significantly under capacity for your run. Using the 90 degree table for thhn, you should probably be using #6 which is listed for 75 amps which at 80% would be good for 60 amps.
3 heaters?
Correct me if I am wrong...4.8 KW is 4,800 watts... divided by 480 volts and multiplied by 1.73 for three phase equals 17.3 amps per heater (never mind the power factor)... multiply that by 3 heaters and you have 51.0 amps total draw. I would suggest that #10 wire is significantly under capacity for your run. Using the 90 degree table for thhn, you should probably be using #6 which is listed for 75 amps which at 80% would be good for 60 amps.
Riograndeelectric
Senior Member
- Location
- Lakewood Colorado
according to my calculations I come up with each heater counsuming 5.7 amps
480x1.73=830.04 4800/ 830.04 = 5.7amps :-?
480x1.73=830.04 4800/ 830.04 = 5.7amps :-?
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
A 4800W three phase heater is 1600W per phase, at 277V is (rounded) 5.8A. Four such heaters will use about 23A per phase.
brother
Senior Member
4.8 X 1000= 4,800/(480 X 1.73)= 5.78 amps 5.8 X 1.25= 7.25
7.25 X 4= 29 amps
Do you know what the amps the motor pull?? or is this included in the 4.8 kwatts??
As for the cord as a disconnect and permenant wiring method read NEC 2008 400.7, 400.8.
Im surprised the heaters dont have 'overloads' so it appears you need to provide them.
Riograndeelectric
Senior Member
- Location
- Lakewood Colorado
OK 1600w per phase but if these are indeed actual 3 phase 480V then would the ampacity not be 5,7 per unit.
if they were 277V at Watts x3 x1.25% would be 23A.
I am calculating the load at 4800W divided by 830V= 5.7. am I missing something in my calculations?
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
We both calculate the current consumed by the heater by two different methods, you do it using three phase maths, and I do it using single phase math, and we both get the same answer, only I'm a lazy sod, so I've pressed less buttons on the calculator to do it! And, of course, the single phase equivalency only works on loads that are balanced over all three phases.
The real reason I'm partial to calculating single phase loads as in the fields I've worked in (IT and theatre) three phase loads are rare, its almost always shedloads of single phase loads.
And just to be clear: the single phase slack cheat calculates the current on one phase for one third of the load, as the load is actually over three phases.
On top of that, to calculate wiring ampacity and breaker size, the 80% rule applies, hence the multiplication by 1.25, which Brother uses in his calcs.
The real reason I'm partial to calculating single phase loads as in the fields I've worked in (IT and theatre) three phase loads are rare, its almost always shedloads of single phase loads.
And just to be clear: the single phase slack cheat calculates the current on one phase for one third of the load, as the load is actually over three phases.
On top of that, to calculate wiring ampacity and breaker size, the 80% rule applies, hence the multiplication by 1.25, which Brother uses in his calcs.
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