Hi all,
I have a single-phase (US, 240V, 6000W) on/off tank immersion heater that I need to decide on circuit protection for. I have been looking at overloads, 6000W = 25A. 3-Phase overloads are readily available, but as I understand it, due to the phase loss protection of the overload, I'd need to wire the return phase with the L2/T2 and L3/T3 connections in series. I've been looking at Siemens 3RV2 motor starter protectors for this, the datasheets specify a much lower rating when used with single phase for the same hp; I'm guessing this is because of the 1.732 factor between 3-phase and 1-phase power calculations. The model I'm looking at is rated to 7.5hp for single phase, meaning 23.28A, with adjustable thermal overload from 28-32A and is rated 15hp for 3-phase. I'm thinking trip class 10 is fine. These also have built-in instantaneous magnetic trip. So my questions:
1) Should I use any additional protection devices other than this? I was thinking of using a magnetic breaker ahead of this, but it seems unnecessary.
2) Should I worry about the fact that the single phase rating is a few amps below the heater current? The next step up for current range adjustment is significantly higher than the heater current.
3) What should the overload setting be set to, 1.25 x 25 = 32A approx? I see the 1.25 factor a lot, I think this is from NFPA79, is it applicable to resistance loads as well? Or should I set it lower?
4) I'm assuming that since this has a magnetic trip also, it should be installed upstream from the contactor, correct?
5) They have these in trip class 10 and 20, should I prefer one over the other? (my gut says go with 10)
Thanks!
I have a single-phase (US, 240V, 6000W) on/off tank immersion heater that I need to decide on circuit protection for. I have been looking at overloads, 6000W = 25A. 3-Phase overloads are readily available, but as I understand it, due to the phase loss protection of the overload, I'd need to wire the return phase with the L2/T2 and L3/T3 connections in series. I've been looking at Siemens 3RV2 motor starter protectors for this, the datasheets specify a much lower rating when used with single phase for the same hp; I'm guessing this is because of the 1.732 factor between 3-phase and 1-phase power calculations. The model I'm looking at is rated to 7.5hp for single phase, meaning 23.28A, with adjustable thermal overload from 28-32A and is rated 15hp for 3-phase. I'm thinking trip class 10 is fine. These also have built-in instantaneous magnetic trip. So my questions:
1) Should I use any additional protection devices other than this? I was thinking of using a magnetic breaker ahead of this, but it seems unnecessary.
2) Should I worry about the fact that the single phase rating is a few amps below the heater current? The next step up for current range adjustment is significantly higher than the heater current.
3) What should the overload setting be set to, 1.25 x 25 = 32A approx? I see the 1.25 factor a lot, I think this is from NFPA79, is it applicable to resistance loads as well? Or should I set it lower?
4) I'm assuming that since this has a magnetic trip also, it should be installed upstream from the contactor, correct?
5) They have these in trip class 10 and 20, should I prefer one over the other? (my gut says go with 10)
Thanks!