I know this is for code questions but I have been caught up into another problem. One of our furnaces keeps blowing out heaters and then the problem just cascades to where an entire zone and wiring is damaged. These heaters are all connected Delta and Production Engineers want to change them to connect to wye. I am the Facilities Engineer and am trying to explain what it will take to change to wye. Only have 3 500 MCM Line feeds off of buss duct through 3" Pipe. I have told them we would have to pull in the neutral and to do that all 4 feeds would have to be pulled, can't just pull another 500 MCM cable into a 3" pipe, especially with twists and turns in pipe. What I really want to explain is this will not fix the problem and I can not figure out how to get it across to them. Or am I wrong and this is the way to go. Any help or direction to somewhere else would be appreciated.
Furnace Heater Problem
- Thread starter 69boss302
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Furnace Heater Problem
is the existing supply already wye?
I'm not sure why your heater wiring is being damged. Is the insulation melting? maybe it just getting too warm and you need higher temperature insulation on the wiring. Is the overcurrent protection for this wiring sized and operating correctly?
Are you suggesting they want to rewire each element so it goes phase to neutral instead of phase to phase or are they planning to leave each element connected phase to phase?
is the existing supply already wye?
I'm not sure why your heater wiring is being damged. Is the insulation melting? maybe it just getting too warm and you need higher temperature insulation on the wiring. Is the overcurrent protection for this wiring sized and operating correctly?
Re: Furnace Heater Problem
4 Zones, 3 Zones with 4 sets of 9 heaters, and 1 zone with 5 sets of 9 heaters, 1000 watts per heater. Yes switchgear and bus way wired to support neutral. Incoming transformer Delta Wye, and we have run 277 panels for lights. Biggest thing is problem seems to be only in one zone. I have told them they really need to check out all the wiring from their control panel to the heaters, there could very easily be a chaffed wire. Unit was installed only about 6 Months ago. Yes they want to change the banks of heaters to wye internally, heaters are easily accessible. What happens is one heater may go but then the others get loaded up to compensate and they then burn up. It's kind of obvious the problem is in just that one part of the furnace (to me anyway), but they want it set up so that if a heater goes out they can keep running without burning up more heaters in a very short time span.
4 Zones, 3 Zones with 4 sets of 9 heaters, and 1 zone with 5 sets of 9 heaters, 1000 watts per heater. Yes switchgear and bus way wired to support neutral. Incoming transformer Delta Wye, and we have run 277 panels for lights. Biggest thing is problem seems to be only in one zone. I have told them they really need to check out all the wiring from their control panel to the heaters, there could very easily be a chaffed wire. Unit was installed only about 6 Months ago. Yes they want to change the banks of heaters to wye internally, heaters are easily accessible. What happens is one heater may go but then the others get loaded up to compensate and they then burn up. It's kind of obvious the problem is in just that one part of the furnace (to me anyway), but they want it set up so that if a heater goes out they can keep running without burning up more heaters in a very short time span.
Re: Furnace Heater Problem
Connect those heaters delta. Voltage will remain stable if the load changes because of a burnt out heater. They would be connected to one transformer winding. Wye connections need a balanced load when connected only phase to phase. They share two windings with the other heaters and the voltage will change as those loads change. Incidentally, why bring a neutral if nothing is connected to it?
Connect those heaters delta. Voltage will remain stable if the load changes because of a burnt out heater. They would be connected to one transformer winding. Wye connections need a balanced load when connected only phase to phase. They share two windings with the other heaters and the voltage will change as those loads change. Incidentally, why bring a neutral if nothing is connected to it?
Re: Furnace Heater Problem
That is the problem. The center tap must be connected to the neutral (center tap) of the supplying transformer. That center tap of the secondary of transformer must be connect to ground. If the transformer is installed properly, the center tap is connected to a grounding electrode which is probably your building steel. As in my earlier post, I mentioned the voltage changing with an imbalance on the transformer windings. This is not an issue when you are connected 277. You are now connected to one winding. You are not sharing windings with another load.
Don't try to pull another conductor in that conduit. The existing conductors will probably be damaged. I've been there.
Think about cable tray. It goes in easy. Offers expansion capability, Dissipates heat easier than a conduit. Has a life time usefullness.
That is the problem. The center tap must be connected to the neutral (center tap) of the supplying transformer. That center tap of the secondary of transformer must be connect to ground. If the transformer is installed properly, the center tap is connected to a grounding electrode which is probably your building steel. As in my earlier post, I mentioned the voltage changing with an imbalance on the transformer windings. This is not an issue when you are connected 277. You are now connected to one winding. You are not sharing windings with another load.
Don't try to pull another conductor in that conduit. The existing conductors will probably be damaged. I've been there.
Think about cable tray. It goes in easy. Offers expansion capability, Dissipates heat easier than a conduit. Has a life time usefullness.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Furnace Heater Problem
If the heater's are wired in delta and for 480 volts there should not be a problem when one element burn's out as you should have 6 element's with two in series between each set of phases. connected in a delta pattern (triangle) If they are wired in a star pattern ("WYE") but they don't have a neutral Then yes when you loose one of the element's then it will cause the other element to burn up as it is now receiving a higher voltage because of there not being a balancing neutral. It sound's like the heaters are not connected for a true delta connection as I said above. and are wired in a WYE but without a neutral. Two things have to happen to correct this:
1. leave them wired in a WYE and run a neutral or:
2. reconnect the elements in a delta with two element's in series between each set of phases.
Maybe we could get someone to post a diagram of this so you can see how they should be wired.
I had this problem on a kiln furnace and the same thing happened. the elements were wired for a WYE but they didn't have a neutral, rewired the element's into a delta and all was good.
If the heater's are wired in delta and for 480 volts there should not be a problem when one element burn's out as you should have 6 element's with two in series between each set of phases. connected in a delta pattern (triangle) If they are wired in a star pattern ("WYE") but they don't have a neutral Then yes when you loose one of the element's then it will cause the other element to burn up as it is now receiving a higher voltage because of there not being a balancing neutral. It sound's like the heaters are not connected for a true delta connection as I said above. and are wired in a WYE but without a neutral. Two things have to happen to correct this:
1. leave them wired in a WYE and run a neutral or:
2. reconnect the elements in a delta with two element's in series between each set of phases.
Maybe we could get someone to post a diagram of this so you can see how they should be wired.
I had this problem on a kiln furnace and the same thing happened. the elements were wired for a WYE but they didn't have a neutral, rewired the element's into a delta and all was good.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Furnace Heater Problem
I just did some figuring and the zone with 5 sets of 9 will not work with all the element's. As I said in a delta pattern you need 2 in series between each set of phases and this requires 6 elements to be a balanced load. but you could run 8 of the 5th set and leave one element out of the circuit and the load on the one phase will only be off by the 1 KW that the element would apply. otherwise the other zones with 4 set's of 9 will work.
Keep in mind that with these element's wired in a delta pattern they will only receive 240 volts each
so the wattage over all will be reduced. I don't think this would be a big problem but could be if the requirement of the BTU's are close. as each wattage loss of each element will be added together for an over all loss of the entire system. so it could be enough amount that might cause the furnace not to make temp, or keep temp when a cold load is applied.
[ October 01, 2004, 08:40 PM: Message edited by: hurk27 ]
I just did some figuring and the zone with 5 sets of 9 will not work with all the element's. As I said in a delta pattern you need 2 in series between each set of phases and this requires 6 elements to be a balanced load. but you could run 8 of the 5th set and leave one element out of the circuit and the load on the one phase will only be off by the 1 KW that the element would apply. otherwise the other zones with 4 set's of 9 will work.
Keep in mind that with these element's wired in a delta pattern they will only receive 240 volts each
so the wattage over all will be reduced. I don't think this would be a big problem but could be if the requirement of the BTU's are close. as each wattage loss of each element will be added together for an over all loss of the entire system. so it could be enough amount that might cause the furnace not to make temp, or keep temp when a cold load is applied.
[ October 01, 2004, 08:40 PM: Message edited by: hurk27 ]
- Location
- Illinois
- Occupation
- retired electrician
Re: Furnace Heater Problem
Don
They don't have to be connected to a neutral or grounded conductor to operate at 277 volts on a 480 volt system. Just connect the 277 volt heaters in a wye, with identical heaters in each leg and connect each leg to one phase of the 480 volt system. The heaters in each leg will only see 277 volts and the grounded conductor is not required.
Don
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Furnace Heater Problem
That is true Don but I think they have the WYE connection point of each parallel set of element's also connected together instead of being isolated. This is why they are blowing out elements when one burns out. this causes a higher voltage at the other elements that are paralleled with the one that went out, just like when we loose a neutral. If this neutral is kept separate for each set of three elements that's connected to all three phases then if one goes out the other two will only see 240 volts which would not burn them up. But I do see where they would work in a WYE configuration if the center of the WYE was kept isolated from other sets. I bet there is a common buss bar that all these elements connect to for the neutral/common connection and if so this is the center point that is floating when they loose an element. Draw 4 parallel sets of elements connected in a WYE connection including the center connection. now remove one element and you will see that the other three elements on the same phase still connected will have a higher voltage on them. Now isolate this center connection and if one element is lost then the other two elements will only have 240 volts on them as they are now just two elements in series with 480 volts. The other way as each element opens the voltage goes even higher until all elements open on that phase.
That is true Don but I think they have the WYE connection point of each parallel set of element's also connected together instead of being isolated. This is why they are blowing out elements when one burns out. this causes a higher voltage at the other elements that are paralleled with the one that went out, just like when we loose a neutral. If this neutral is kept separate for each set of three elements that's connected to all three phases then if one goes out the other two will only see 240 volts which would not burn them up. But I do see where they would work in a WYE configuration if the center of the WYE was kept isolated from other sets. I bet there is a common buss bar that all these elements connect to for the neutral/common connection and if so this is the center point that is floating when they loose an element. Draw 4 parallel sets of elements connected in a WYE connection including the center connection. now remove one element and you will see that the other three elements on the same phase still connected will have a higher voltage on them. Now isolate this center connection and if one element is lost then the other two elements will only have 240 volts on them as they are now just two elements in series with 480 volts. The other way as each element opens the voltage goes even higher until all elements open on that phase.
Re: Furnace Heater Problem
Thanks guys for all your comments I think hurk27 has it all described about the best as to what's happening and I will have to check on the ability to isolate at wye connections. I'll let you know what I find out. It does look like though what we really need is a neutral and what I was planning on doing was pulling in 4 new 500 MCM's. Neutral would still have to be sized with the Line leads since it would have the potential of carrying all load from other heaters if any of them blow or possibly even more than one blows. Correct?
Thanks guys for all your comments I think hurk27 has it all described about the best as to what's happening and I will have to check on the ability to isolate at wye connections. I'll let you know what I find out. It does look like though what we really need is a neutral and what I was planning on doing was pulling in 4 new 500 MCM's. Neutral would still have to be sized with the Line leads since it would have the potential of carrying all load from other heaters if any of them blow or possibly even more than one blows. Correct?
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