KW load

[ptonsparky]

Sizing a feeder for single phase HVAC. The backup strips are rated 20KW rated at 230 or 208. The KW will be less @ 208. Am I allowed to use the real KW of about 16.4?

 

[LarryFine]

I would say absolutely, yes. It should theoretically be 75%, or 15kw.

 

[infinity]

I agree with Larry, use the applied voltage to calculate the load.

 

[winnie]

Disagree somewhat. (Note: I don't have much experience in this area. When I say 'possible' below I mean physically possible.)

The strips are likely simple resistance devices with lower kW at lower voltage. But if the nameplate says they are 20kW at both voltages then IMHO you need to dig deeper.

Likely it is an error on the nameplate, but it is possible that these heaters were designed to compensate for the voltage change, and really are 20kW at 208V.

Possibly at installation time the strips get connected differently for different nominal voltage in order to maintain the 20kW rating.

Jon

 

[ptonsparky]

Disagree somewhat. (Note: I don't have much experience in this area. When I say 'possible' below I mean physically possible.)

The strips are likely simple resistance devices with lower kW at lower voltage. But if the nameplate says they are 20kW at both voltages then IMHO you need to dig deeper.

Likely it is an error on the nameplate, but it is possible that these heaters were designed to compensate for the voltage change, and really are 20kW at 208V.

Possibly at installation time the strips get connected differently for different nominal voltage in order to maintain the 20kW rating.

Jon

I looked at them yesterday. Only change was for the transformer to 208. I hope HVAC did that because we didn’t. Current draw is in the expected 208 range for the one 10 KW bank that came on. The 2nd bank never came on, even when Emergency Heat was selected. HVAC tech was notified.

 

[kwired]

I looked at them yesterday. Only change was for the transformer to 208. I hope HVAC did that because we didn’t. Current draw is in the expected 208 range for the one 10 KW bank that came on. The 2nd bank never came on, even when Emergency Heat was selected. HVAC tech was notified.

Next question is does HVAC guys realize they not getting 20 KW out of this unit? Many around here don't realize that is a problem when voltage is 208. Most the time they get away with it, but when weather is cold as it has been past week it maybe matters a little more.

 

[ptonsparky]

Next question is does HVAC guys realize they not getting 20 KW out of this unit? Many around here don't realize that is a problem when voltage is 208. Most the time they get away with it, but when weather is cold as it has been past week it maybe matters a little more.

I doubt if they do but the two units have been keeping up with only 1st KW of each unit working. Plus the HPs at whatever point they run @ 0 and below.

Owner of building supplied the units.

 

[kwired]

I doubt if they do but the two units have been keeping up with only 1st KW of each unit working. Plus the HPs at whatever point they run @ 0 and below.

Owner of building supplied the units.

It is when weather is like we had recently that people call that their electric furnaces aren't keeping up, that is when you find there is only one element still operating and it just can't keep up. Sequencers and high limits are probably most common failure, though elements do fail as well. Actually joule heating at a connection is probably the most common failure, but takes out sequencer or high limit with it.

 

[ptonsparky]

In this case the HVAC thought there needed to be jumper installed to bring on both banks. These are fairly new so I will let them figure it out.

 

[kwired]

In this case the HVAC thought there needed to be jumper installed to bring on both banks. These are fairly new so I will let them figure it out.

If you only have single stage signal for this function you need to tie the two input leads to that one lead coming from the thermostat. Many are like that anymore. A ~25 kW unit might even have a third stage input lead, tie all three together if controls only have one stage for this.

 

[ptonsparky]

If you only have single stage signal for this function you need to tie the two input leads to that one lead coming from the thermostat. Many are like that anymore. A ~25 kW unit might even have a third stage input lead, tie all three together if controls only have one stage for this.

Owner supplied stuff. I didn't see a 2nd stage on the stat and 'not my job' this time. As much as I hate that term, it isn't, this time..
I've always thought 'if you're not part of the solution, you're part of the problem.'

 

[kwired]

Owner supplied stuff. I didn't see a 2nd stage on the stat and 'not my job' this time. As much as I hate that term, it isn't, this time..
I've always thought 'if you're not part of the solution, you're part of the problem.'

But knowing which wires to connect is part of why we are worth every penny charged,

I have an HVAC friend that can never seem to figure out the controls and seems to find a way to burn out control transformers frequently, then calls me to figure out what he is doing wrong.