HVAC question

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blueheels2

Senior Member
Location
Raleigh, NC
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Electrical contractor
I have an inside AHU with auxiliary heat strips. Nameplate is:

Minimum circuit ampacity:58.3 amp
MOCP: 60 amp

This is a changeout.

Old unit was fed with 6-2 nm-b. Ampacity of this is 55 but 240.4(B) says it can be protected by a 60 amp breaker. My buddy says his old electrician wired that like this all the time and passed inspection.

I say this is incorrect that the unit needs to be wired in 4-2nm-b or installed in conduit and wired with #6 thhn.

What do you guys say?
 
I see these all the time with the #6 NM on a 60A breaker. I honestly wouldn't even give it a second thought unless it was being inspected, and then I'd ask the inspector on the phone to make sure he wasn't wasting a trip to the job.
 
All of the inside AHU unit load is the electric resistance heater. It is setup either as reheat on the duct or a second stage heat, typical with heat pumps.

If you are concerned with the 55 amp cable rating not meeting the 58.3 MCA, it is already there so I don't see the case to change the cable. The question is not right or wrong but do you reuse the cable. That decision the customer makes. It's their money.

I would get the actual amp draw with a meter at full load. Electric resistance banks usually do not draw right at the nameplate, so if the MCA is 58 but it never draws more than 52 amps on a meter, that would be enough for me. If it concerned me I would discuss it with the customer and just advise them of my concern.

My guess is the actual draw on a meter is less than the rating, and the load type itself may be such that it does not run too much and not a lot at full for a long time (just a guess I would look into). If I thought that AHU electric heat bank was itself a primary heat source, I would suspect the entire design has some flaw. It would cost them a fortune to run it. They could do it, but it might be wrong.
 
__dan said:
All of the inside AHU unit load is the electric resistance heater. It is setup either as reheat on the duct or a second stage heat, typical with heat pumps.

If you are concerned with the 55 amp cable rating not meeting the 58.3 MCA, it is already there so I don't see the case to change the cable. The question is not right or wrong but do you reuse the cable. That decision the customer makes. It's their money.

I would get the actual amp draw with a meter at full load. Electric resistance banks usually do not draw right at the nameplate, so if the MCA is 58 but it never draws more than 52 amps on a meter, that would be enough for me. If it concerned me I would discuss it with the customer and just advise them of my concern.

My guess is the actual draw on a meter is less than the rating, and the load type itself may be such that it does not run too much and not a lot at full for a long time (just a guess I would look into). If I thought that AHU electric heat bank was itself a primary heat source, I would suspect the entire design has some flaw. It would cost them a fortune to run it. They could do it, but it might be wrong.
Click to expand...

It doesn't matter what you measure. The nameplate governs., It also doesn't matter what someone got away with in the past. Make it right.
 
blueheels2 said:
I say this is incorrect that the unit needs to be wired in 4-2nm-b or installed in conduit and wired with #6 thhn.

What do you guys say?
Click to expand...

The nameplate says that the old NM cable is not code complaint. Either of your methods would work.
 
I agree absolutely with the make it right. No question there.

I would bid that over the phone without looking at it meaning that there are a limited number of factors. There's always a bidder who is using the old disconnect and cable, is a factor.

If you do get the job for a changeover it probably came from a mech contractor you do some business with. The price of the job is certainly sensitive to if you are reusing the old disconnect and cable. Then upstream to the customer, the customer may have no clue to even tell the new disconnect from the old disconnect. If you do change the cable, who is paying for it. Is it in the original bid, would you need a change, or would you think it's your mistake to pay for.

There's actually some legal doctrine that applies, what was discoverable or observable at the bid time. They are not paying me for a prebid survey.
 
A 10kW heater uses (10,000 / 240) 41.667 amps, plus the blower motor, control transformer, and a relay or few. As the motor's starting current is of short duration, I wouldn't lose sleep over this. However, I would use #2 al SE cable instead of #4 cu NM, because it's cheaper.

Is this a permitted job? Is the HO the customer, or an HVAC contractor? If the latter, I would discuss this with the contractor.
 
This is one of those situations where you can make a good argument that NEC is being overly conservative. Unfortunately you have to draw the line somewhere and once you start crossing the line you open yourself up to lots of problems/second guessing/etc.

NM-B has a 90C rating. The 75C ampacity for #6 CU is 65A. But we are required to use NM-B with the 60C rating. I'd bet $100 bucks that the installation would work fine and safely until the building was torn down for other reasons. Now would you bet your license?

-Jon
 
If you discover the violation, you have a duty to mitigate damages, which to me usually means disclosure. Things will always be difficult, there has to be a basis in honesty. If there is no honesty, I would be looking to bail at the first chance.

So the necessary information from the OP is still missing. The job is presented as a change of the existing for like. Is the new cable in the bid. In this case no because it would just be installed.

If it is not in the bid *and* it is found necessary to upgrade the cable, there would usually be a letter from the BO stating 'this work necessary to meet code'.

It does not sound like you are at that stage. It sounds like you have not talked to the BO or the customer about it. You would not be allowed to touch it without the customer's permission so talking to them about it has to happen, and then the permit for it so talking to the BO also.

Somewhere in this sequence necessary quantities are missing or are present but not in the proper order. Only the BO can order the upgrade necessary. The customer can choose to do it voluntarily. I do not see where the EC can be forced to make the upgrade without permission *and pay* by the customer since he did not do the original deficient cable installation.
 
So this is for a HVAC contractor who happens to be a very good friend of mine. Throws all of his work my way. I mentioned changing it the other day which brought up the everybody around here does it. I even see it as a gray area because of 240.4(B). However, I contend that it is incorrect. If I push him own it he said he will probably just downsize the heater to make it compliant.

I just wanted toto see if other electricians felt I was being unreasonable. Do I think it’s ok. I bet it would run the life of the house and be fine. Not sure I want to put my livelihood on that though.
 
blueheels2 said:
So this is for a HVAC contractor who happens to be a very good friend of mine. Throws all of his work my way. I mentioned changing it the other day which brought up the everybody around here does it. I even see it as a gray area because of 240.4(B). However, I contend that it is incorrect. If I push him own it he said he will probably just downsize the heater to make it compliant.

I just wanted toto see if other electricians felt I was being unreasonable. Do I think it’s ok. I bet it would run the life of the house and be fine. Not sure I want to put my livelihood on that though.
Click to expand...

I bet your right, but...
an OLD electrician I worked with told me something I’ve never forgot. The NEC isn’t a ceiling to reach for, it’s a floor to build from.:thumbsup:
 
Yes it's all there. It's OK but who picks up the liability for it is an issue

Talking about it is a sensitive issue which is why the customer and BO have not heard of it. Prime contractors can be expected to ditch subs who are letter happy. And the reverse is not true. Primes themselves like to be letter happy.

Luckily it does not sound like that's happening here. I can tell you what your friend the prime will say, 'do it as bid'.

If you have to go to the BO for the permit I would say you have to disclose to the BO, but then that's good for you. The BO will pick up the liability concern on what he says, either to keep the cable or upsize it. That's the proper resting place for it. It is sensitive how you go about it, if your prime does not want it going to the BO because of the hassle of stopping the job. But will certainly not pick up the liability himself by ordering you to proceed. He will say that's what my electrician is for.

If the word of your friend is good and you can live with it, you can go with his advise which most likely will be to forget about it.

Of course when there are problems with the job, everyone remembers what was said and are quicker than the other to point the finger at someone else.

If menu items like new disconnect or new cable are not in the bid, primes and customers will still try to get something for nothing. The whole thing is a sensitive issue.

If there is a failure of the existing cable and that can be proven, I don't see how that comes back on you as you did not install that cable.

I would meter the full worst case running load and if it cannot be proven the rating is exceeded, *that* requirement passes. You can prove did not install the cable nor overload it.

If the equipment fails that one is easier. I do not warranty your equipment.

Most likely I would suspect the usual problem is that the prime or customer would use something they can twist to balk at payment or intimidate for extra work.

It does not sound like that's the case here. Sounds like everyone is OK it will work safely forever and never actually overload.

That's key for me. You can argue with a meter the cable is actually rated for the measured load so anything in Art 240 is OK for you. If they want to argue a technical violation of 110.3 B, they can pay for the new cable.

The BO is not going to push a 110.3 B violation on you for an existing cable if you can document the metered running load less than the rating. So it may be safe to test if this is true. If it's not safe, the prime will get another sub who never goes to the BO.
 
blueheels2 said:
So this is for a HVAC contractor who happens to be a very good friend of mine. Throws all of his work my way. I mentioned changing it the other day which brought up the everybody around here does it. I even see it as a gray area because of 240.4(B). However, I contend that it is incorrect. If I push him own it he said he will probably just downsize the heater to make it compliant.

I just wanted toto see if other electricians felt I was being unreasonable. Do I think it’s ok. I bet it would run the life of the house and be fine. Not sure I want to put my livelihood on that though.
Click to expand...

Nice try on his part but he's wrong. The MCA is just that, the minimum required.
 
__dan said:
. . . if your prime does not want it going to the BO because of the hassle of stopping the job. But will certainly not pick up the liability himself by ordering you to proceed. He will say that's what my electrician is for.
Click to expand...
Well, that wouldn't be fair. "Do it how I say, but it's on you for listening to me if something goes wrong." :happysad:
 
The conductors have to safety carry the MCA of 58amps regardless of ocpd.

Yes, it's likely that it draws less than that, but the MCA on the nameplate has the required 125% factor built in.
 
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