480/277 sizing main breaker

[RJ_Heater]

I need help sizing my main breaker for a heater load.
I have (18) 2800 watts 277 volt heater circuits balanced over a 480 3 phase, 4 wire system.
Each has branch circuit protection sized for load (qty18, 15 amp 1 pole breakers)
How do I calculate the overall amperage or main breaker size?

 

[iwire]

There is more than one way but I would likely do it like this.

18*2,800/480/1.73=60.69 amps

Being a heating load the NEC may require we use 125% of that for conductor and breaker sizing.

60.69*1.25=75.86 amps.

I would use at least an 80 amp feeder.

 

[augie47]

I agree that I would most likely use a 80 amp breaker also, but, from a Code standpoint, and I'm shooting from the hip here, I think the 125% is for the branch circuit and your supply would be a feeder so, IMO, by Code you could use a 70 amp breaker.

 

[RJ_Heater]

Neutral buss

Neutral buss

That makes sense as each circuit is 2800 watts x 6 heaters / 277 = 60.65 amps
Do I need to size my neutral buss for 180 amps?

 

[Smart $]

I agree that I would most likely use a 80 amp breaker also, but, from a Code standpoint, and I'm shooting from the hip here, I think the 125% is for the branch circuit and your supply would be a feeder so, IMO, by Code you could use a 70 amp breaker.

Heaters are typically thermostatically controlled, but on occasion I've ran across heater applications that were not, and run continuously. OP did not elaborate to the degree of excluding this possibility... which would require 125% continuous load factoring at the feeder level.

 

[Smart $]

That makes sense as each circuit is 2800 watts x 6 heaters / 277 = 60.65 amps
Do I need to size my neutral buss for 180 amps?

No... size at the same rating as line conductors... exactly what your calculation shows... plus 25% padding for continuous loading, if that be the case.

 

[kwired]

That makes sense as each circuit is 2800 watts x 6 heaters / 277 = 60.65 amps
Do I need to size my neutral buss for 180 amps?

If the heaters are connected so that they are balanced (six per phase) AND all are running at same time, neutral current is zero. Neutral needs to be sized to maximum unbalanced load that is possible, but never smaller then 250.66 for services, or 250.122 for branch circuits, (feeders are not mentioned however). Depending on how these are controlled you may be able to go with as little as a 250.122 sized neutral or up to connected load for one phase - your mentioned 60.65 A.

 

[david luchini]

(feeders are not mentioned however)

See 215.2(A)(2)

 

[RJ_Heater]

Neutral buss

Neutral buss

The 18 circuits are all individually controller by temperature controllers with solid state relays.
Any number of circuits could be energized or deenergized at one time.

 

[kwired]

See 215.2(A)(2)

Oop's, I got that turned around, I think I meant that branch circuits don't seem to have a minimum neutral conductor requirement - other then they must have ampacity equal to load carried.

Where I run into this problem the most is with a larger motor circuit and someone decides to put a 120 volt coil on the controller. I'd have to look again but pretty sure I have determined in past that a 14 AWG would be acceptable for this neutral - but only for the branch circuit, the feeder portion still would need be sized to 250.122 minimum. The conductor definitely needs reduced before it will fit the terminal on the controller in most of these cases.

 

[iwire]

I think the 125% is for the branch circuit and your supply would be a feeder so, IMO, by Code you could use a 70 amp breaker.

The OP has not told us what kind of heaters they are or the conditions of use which is why I went with maybe on the 125%.

 

[augie47]

:thumbsup:

 

[RJ_Heater]

Neutral buss

Neutral buss

The OP has not told us what kind of heaters they are or the conditions of use which is why I went with maybe on the 125%.

Pure resistance load in this case.
Cartridge heaters.

I do this similar application with Self regulating heat trace, but in that case I use the manufactures CB chart for each breaker and size the main breaker for the sum of the branch breakers one leg. Or if the MFG requires 30 amp breakers and I have 18 ckt's I would use a (6 x 30 amp branch CB's) or min of 180 amp main.
I've always done this but it seems like over kill. Specially when you look at an 11 amp load requires a 30 amp breaker to allow for inrush.


is there a different way to figure the Main on Self reg heat trace?

 

[Smart $]

...is there a different way to figure the Main on Self reg heat trace?

That would be an engineering level determination (but AFAICT, engineer not required by NEC )... as watts/ft changes with temperature.

The easiest means of determiantion is to use software such as Thermon's CompuTrace: http://www.thermon.com/software/computrace.aspx

 

[kingpb]

I need help sizing my main breaker for a heater load.
I have (18) 2800 watts 277 volt heater circuits balanced over a 480 3 phase, 4 wire system.
Each has branch circuit protection sized for load (qty18, 15 amp 1 pole breakers)
How do I calculate the overall amperage or main breaker size?

100A main and walk away.

 

[kwired]

100A main and walk away.

Not without collecting payment first