Need some advice

Status
Not open for further replies.
B

BobbyBJr

Member
Location
Upstate, SC
Hello,
I am a hvac service tech who has recently passed our state electrical contractor's exam and I need some help. I have a customer who has a straight cooling unit and they want to do away with the electric duct heaters in the ductwork in the building and have us put electric heat strips in the unit on the roof. I have calculated that they need 56kw of heat for the area. I then took the 56kw and calculated the load to be 155 amps. I got that by dividing the watts by 208 x 1.732. It is three phase. I have a 1/0 wire feeding up through the roof into the bottom of a 100 amp disconnect that is fused at 70 amps for the cooling unit. When I look at table 310.16, I see that 1/0 is good for 150 amps before any derating. The problem is, the boss and the customer both want me to use the 1/0 wire to feed the heater and I have refused because I don't think it is large enough. I think that since it runs on the roof for even a short distance, some derating should be considered. If I just use 120 degrees on the roof, the value is 0.75 of 150, but it was mentioned that the heat won't be on when it is 120 on the roof. Actually, the heat will only be running when it is below 70 and when you use the 1.05 mutiplier, the result is 157.5. Am I close on this? I know that passing the state test don't make me an electrician, but I am trying to do the right thing. I don't think the wire is large enough and want to run a 3/0. Any advice or suggestions will be appreciated.

thanks,
Bobby
 
Last edited:
Any way to back the load off to 150 amps ?Keep in mind it's likely bit colder on that roof than 70 if your needing heat.Also as you should know that body heat helps reduce your needs.Also the lights add heat.
 
Bobby,

I have a 1/0 wire feeding up through the roof into the bottom of a 100 amp disconnect that is fused at 70 amps for the cooling unit.
Click to expand...

I'm guessing you will be changing or adding a 200A disconnect?

The correct wire size for 155A is 2/0. I would not be concerned with any temperature adjustment unless there is more than ten feet of conductor on the roof.

You mentioned there is a 70A A/C load existing? If so, to use the heat you will be running the blower as well, yes?
Don't forget to add the blower amps to the 155A heating!
 
BobbyBJr said:
Hello,
I have a customer who has a straight cooling unit and they want to do away with the electric duct heaters in the ductwork in the building and have us put electric heat strips in the unit on the roof. I have calculated that they need 56kw of heat for the area. I then took the 56kw and calculated the load to be 155 amps. I got that by dividing the watts by 208 x 1.732. It is three phase. I have a 1/0 wire feeding up through the roof into the bottom of a 100 amp disconnect that is fused at 70 amps for the cooling unit. When I look at table 310.16, I see that 1/0 is good for 150 amps before any derating. The problem is, the boss and the customer both want me to use the 1/0 wire to feed the heater and I have refused because I don't think it is large enough. I think that since it runs on the roof for even a short distance, some derating should be considered. If I just use 120 degrees on the roof, the value is 0.75 of 150, but it was mentioned that the heat won't be on when it is 120 on the roof. Actually, the heat will only be running when it is below 70 and when you use the 1.05 multiplier, the result is 157.5. Am I close on this?
Bobby
Click to expand...

Electric Heat is considered a continuous load. As such you must use the 1.25 multiplier. Load then is 155.6 amps x 1.25 =195 amps. The conductor must be
rated for at least 195 amps which requires 3/0. If you need to derate for temp
you can use the 90C rating of the conductor but the result must not be less than the 195 amps. What is the advantage of changing the duct heater location?
 
Hi Bob,

In South Carolina I would not expect a heating unit to run for 3 hours or more without cycling off on the thermostst. But if it did, I would agree with your post.

Up north here, different story!
 
websparky said:
Hi Bob,

In South Carolina I would not expect a heating unit to run for 3 hours or more without cycling off on the thermostst. But if it did, I would agree with your post.

Up north here, different story!
Click to expand...

Might want to check out 424.3(B) then. It doesn't seem to have any exceptions for where you might live, or how long you guess the heat will run.
 
Thanks guys. I could not find the code reference to strips being a continuous load when I was looking for it. Once I knew that the x1.25 applied to size the wire, it was a easy decision. We have given the customer the option of a smaller heater and hoping for a mild winter, or a larger branch circuit up to the unit. Thanks again for the help.

Bobby
 
bob said:
What is the advantage of changing the duct heater location?
Click to expand...

None that I can see or think of. You just have to know this customer. I am sure we have all had one that looked better going than they did coming. This one is quickly becoming one of those.

Bobby
 
Status
Not open for further replies.
Top