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lapsis9 12-14-2006 02:24 AM

So lost - electrical requirements please help
Hi All,

We know it's DIY web site but are very lost and were hoping for some advice.
Thanks so much to anyone who can take the time to explain. We had an HVAC system with heat pump and back up heat strips put in last year. Our HVAC installer told us we needed 2 30 amp dedicated circuits for the HVAC heat pump and 1 60 amp dedicated circuit for the heat strips. Long story, but basically we need to have the dedicated circuits redone. We are having an electrician come in to redo the dedicated circuits for our HVAC system. He just quoted us for one 30 amp dedicated circuit to the heat pump. He said he didn't know what the other 30 amp dedicated circuit was for, and didn't know about the 60 amp (both were put in last year for the HVAC by another guy). He quoted us a little over a thousand (with tax & permit) for the work to redo the 30 amp circuit. We are lost. Does this sound right? What about the other 30 amp and 60 amp circuits from last year? Doesn't he need to redo those as well? IF someone could advise as to what all these dedicated circuits are needed for and also any rough estimates as to how much it would cost (we have a one story 820 sq ft house with a 4 ft crawl space underneath, heat pump is in the back of the house) it would be much, much appreciated.

acefurnacefixer 12-14-2006 06:02 AM

Your HVAC equiptment will detrmine what size circuts you need. There are max amp draw plates on every piece of HVAC equipt,ent sold that uses electricity. The Electrician should know that and be able to size your wireing correctly.

Christopher 12-14-2006 02:23 PM


In order to provide you with accurate information we need to know specific equipment requirements.

First I'll address the outdoor unit (I am assuming that the air handler with heat-strip is a separate piece of equipment).

All Heat-Pumps/AC Condensing Units have two relevant circuit ratings. Rarely are they properly adhered to.

The first rating is "MAX FUSE or HACR CIRCUIT BREAKER" This is required by law for one purpose only: fire prevention.

The second rating is "Minimum Circuit Ampacity" This is the relevant guide for sizing the circuit. The correct size circuit is equal to or slightly greater than this figure.

I'll provide my home as an example and indicate how the builder (HVAC subcontractor/Electrical subcontractor) violated the code.

My outdoor unit:
Minimum Circuit Ampacity: 22.7A

The builder installed a 30A circuit (Number 10 wire) with a 35A circuit breaker. It is a code violation for the circuit breaker to be greater that the circuit ampacity which is 30A for the number 10 wire. In reality, no fire will result from a fault in this installation because the 35A breaker meets the unit requirements and the wire will handle the 16% over-rating.

A valid no-brainer installation for my unit would be a 30A circuit (Number 10 wire) with a 30A circuit breaker. However, this would not provide adequate overload protection for the compressor.

The correct installation for my example is a 30A circuit (Number 10 wire) with a 25A circuit breaker. The circuit breaker should always be the smallest size that is equal to or greater than the "Minimum Circuit Ampacity" This is why "in-between" sized circuit breakers are made such as 25A, 35A, etc. - to provide electric motors with optimum overload protection. The unit or thermostat should have a re-cycle timer to guard against nuisance tripping on re-starts.

Being that your home is 820 sq. ft., I am hoping that you have a 2-ton heat-pump. Any larger would result in drafty short cycling. When sizing an air conditioner, less = more (comfort). A 2-ton heat-pump will typically have a "Minimum Circuit Ampacity" of less than 20A. So report on what you have.

Now onto the indoor equipment. The typical indoor air handler with heat-strip requires one circuit. 30A if equipped with a 5KW heat-strip, 60A if equipped with a 10KW heat-strip. Occasionally, the heat-strip is a separate piece of equipment requiring its own circuit. Note: if there is no heat-strip in the air handler, and thus it has its own circuit, the circuit should be 15A maximun for the air handler only. As it is, the blower motors in these units never have adequate protection.

Your circuits seem way too large for applying HVAC to 820 sq. ft. A 2-ton air handler will barely provide enough air flow for 10KW heat.

Let us know exactly what equipment you have and a clue as to why this new installation needs to be rewired. Obviously we don't know what the obstacles are your installation, that said, the quote that you received seems about double what it should be.


PATMAN 12-20-2006 09:11 AM

get at least three more estimates,ask another HVAC contractor to look at the system to find out what the first guy did. you don't say why you need to replace these circuits, but it is extremely rare to have to replace the entire dedicated circuit. $1000.00, my god we should all be electriciams!

sloooo 12-20-2006 08:09 PM

Something doesn't sound right. I can see a single 30 amp circuit for the heat pump and a single 60 amp circuit for the electric heaters. That should be all that is required. Your biggest amp draws are going to be the compressor of you heat pump (hence the 30 amp circuit), and the resistance heaters (hence the 60 amp circuit). Everything else is minimal amp draws like your blower and condenser fan. I would ask for a little for info as to why you need these multiple circuits. Make sure they put it in writting and you return here and let us know what they say and we can advise you wether or not it's correct.

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