A/C CONDUCTOR SIZING

[suemarkp]

Re: A/C CONDUCTOR SIZING

Why not? The 20A breaker is protecting a feeder. I can't find anything in 430 or 440 that refers to feeder breaker sizing, only branch circuits with single and multiple motors or loads.

Doesn't 240.4 (D) still apply to feeders regardless of the type of branch circuits that it feeds?

 

[iwire]

Re: A/C CONDUCTOR SIZING

Originally posted by suemarkp:
Why not? The 20A breaker is protecting a feeder. I can't find anything in 430 or 440 that refers to feeder breaker sizing, only branch circuits with single and multiple motors or loads.

Look at the wording of 440.35, that takes 240.4(D) out of the picture.

 

[infinity]

Re: A/C CONDUCTOR SIZING

Look at the wording of 440.35, that takes 240.4(D) out of the picture.

But how does this apply to the feeder between the panel and the fused disconnect? I agree with Sue, 240.4(D) would apply to this portion of the circuit.

 

[iwire]

Re: A/C CONDUCTOR SIZING

Originally posted by infinity:

Look at the wording of 440.35, that takes 240.4(D) out of the picture.

But how does this apply to the feeder between the panel and the fused disconnect? I agree with Sue, 240.4(D) would apply to this portion of the circuit.

You may be right.

My impression was that the wording "the conductors supplying" would apply all the way back to the service point.

However it is in the part IV of Article 440.

I will think about this some more.

 

[infinity]

Re: A/C CONDUCTOR SIZING

Bob,

I'm not crazy about the concept (neither is Steve)where you add a fuse to the end of the circuit and a whole new set of rules come into play. But as I outlined in an earlier post if you read the definitions from article 100 the branch circuit is at the end between the fuse and the unit. I can't find a reference that would allow the feeder to the fused disconnect to be sized according to Article 440.


Trevor

 

[iwire]

Re: A/C CONDUCTOR SIZING

Originally posted by infinity:
But as I outlined in an earlier post if you read the definitions from article 100 the branch circuit is at the end between the fuse and the unit.

Trevor I got that part down.

Still not sure I agree with you, I may just need to find a different section.

In general the rules of articles 440 and 430 are similar.

If this disconnect was feeding a motor 240.4(D) would not apply.

 

[LarryFine]

Re: A/C CONDUCTOR SIZING

Originally posted by infinity:
Steve, I see your point but I don't see where the code supports your arguement. The inclusion of the fuses at the A/C unit changes the definition of the circuit supplying it. These are from Article 100:

Branch Circuit -The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Outlet -A point on the wiring system at which current is taken to supply utilization equipment.

Feeder -All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.


So the conductors between the fuses and the A/C unit are branch circuit conductors by definition.

The conductors feeding the fuses are feeder conductors by definition. Article 440 would apply only to the branch circuit conductors.

I'm not sure if I agree with this conclusion because:

"Branch Circuit -The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)."

"protecting the circuit" looks like the job of the circuit breaker in the panel, whereas the fuses are protecting the load.

Thoughts?

 

[milwaukeesteve]

Re: A/C CONDUCTOR SIZING

WOW, I completely forgot about this post. I just read it now and saw all kinds of stuff that needed commenting on.
I apologize for bringing this one back 3 weeks later.

First of all, as Larry just asked. The circuit conductors from the branch circuit and/or beyond the fuses, will never see more than either the A/C unit can draw or the OVERCURRENT protection will allow. Whether that OCP is at the start of the circuit or at the almost end of the circuit. Current is the same in a series/loop. So either location still protects the conductors (Unless something is tapped off that feeder, but that is a different subject).
Since current is the same in a series, the Breaker at the panel will see the same current AND the in rush current that the fuses see.
If a HACR breaker is installed at the panel with a NON fused disconnect, then we size the breaker at 125% (440.52)
If a standard breaker is installed at the panel, and a FUSED (time delay) disconnect is used, then the breaker at the panel is considered the SHORT CIRCUIT and GROUND FAULT protection, and would be sized at not more than 175%, but if that doesn't allow for startup, then 225%.

I can explain that reasoning...
440.22 states that we need SCGF protection. SCGF can be accomplished by "a protective device".
440.52 states that time delay fuse(s) "shall be permitted" to serve as SCGF protection. It doesn't say shall or has to or is limited to. That is if there is no other SCGF protection in the circuit.

Therefore, conductor sizing, throughout the whole circuit, is based not on breaker size, but on the minimum circuit ampacity.
Breaker sizing and fuse sizing (any kind of breaker) is based on the motor full load current.


Bob, your quote:

As an example, think of a HVAC unit that has a labeled minimum circuit ampacity of 19.9 amps and a Maximum fuse size of 35 amps.

We could install a 60 amp fused disconnect with 35 amp fuses and the feeder breaker could be 20 amps.

IMO that would meet the minimum NEC requirements.

That said, in no way do I think that is a good design or one that would make for a happy customer.

It doesn't make sense. The feeder breaker CAN'T be 20 amps. The A/C unit would not start.
But there is nothing wrong with a 60 Amp breaker and #12's if the calculations and the rules are followed correctly (depending on breaker type and percentages), since conductor sizing and breaker sizing are not based on the same thing.

 

[infinity]

Re: A/C CONDUCTOR SIZING

It doesn't make sense. The feeder breaker CAN'T be 20 amps. The A/C unit would not start.
But there is nothing wrong with a 60 Amp breaker and #12's if the calculations and the rules are followed correctly (depending on breaker type and percentages), since conductor sizing and breaker sizing are not based on the same thing

If the minimum circuit ampacity is 19.9 amps why wouldn't the unit start? The 19.9 amps already has 125% of the largest motor figured in so the unit should start with a standard inverse time CB.

 

[benaround]

Re: A/C CONDUCTOR SIZING

Thoughts?

I agree 100% with Larry's conclusion,The b.c. starts at the c.b. and the fused disconnect protects the motor(s) or load,not the b.c.


frank

 

[milwaukeesteve]

Re: A/C CONDUCTOR SIZING

Infintiy,
At the end of your last reply, you stated for an inverse time breaker. You are correct. The problem is that since people are calling this portion of the circuit a 'feeder', then you wouldn't necessarily be using a HACR or inverse time breaker.

Remember, SCGF and/or OCP have nothing to do with minimum ckt amps.
-SCGF and OCP are based on rated load current, only.
-Disconnecting means is based on rated load current and locked rotor current
-Conductor sizing is based on rated load current for largest load AND the full load currents of other motors


You can't just look at the minimum circuit amps and base your breaker sizing. If you were to put in a 'feeder' breaker at 20A Instantaneous trip, that A/C unit would never run.

 

[infinity]

Re: A/C CONDUCTOR SIZING

You can't just look at the minimum circuit amps and base your breaker sizing. If you were to put in a 'feeder' breaker at 20A Instantaneous trip, that A/C unit would never run.

I think that you're confusing an instantaneous trip breaker with a standard circuit breaker which is an inverse time breaker. The $3 CB that you can buy is an inverse time CB not an instantaneous trip breaker. An inverse time breaker should allow the motor to start. I don't think that anyone would use an instantaneous trip breaker for an AC unit.

An AC unit with a minimum circuit ampacity of 19.9 amps should start using a standard 20 amp CB. With an instantaneous trip CB it wouldn't start because the CB would trip from the starting current which may be up to 6X the running current.


Minimum ampacity ratings for AC units are determined by the manufacturer. These ampacities have calculated the compressor motor FLA X 125% + the FLA of the evaporator fan motor. These nameplate ratings are required under Article 440 and are to be used when sizing the OCPD and the conductor size for the unit.

If the unit does not specifically mention a minimum CB or fuse size then the minimum circuit ampacity may be used to size the OCPD. If the unit cannot start than a larger OCPD may be used up to the maximum OCPD listed on the nameplate.


Many have said in other threads that they "wire for the minimum and protect at the maximum". this would be code compliant but not required.

 

[iwire]

Re: A/C CONDUCTOR SIZING

Originally posted by milwaukeesteve:
Bob, your quote:

As an example, think of a HVAC unit that has a labeled minimum circuit ampacity of 19.9 amps and a Maximum fuse size of 35 amps.

We could install a 60 amp fused disconnect with 35 amp fuses and the feeder breaker could be 20 amps.

IMO that would meet the minimum NEC requirements.

That said, in no way do I think that is a good design or one that would make for a happy customer.

It doesn't make sense. The feeder breaker CAN'T be 20 amps. The A/C unit would not start.

Morning Steve.

It doesn't make sense?

I never said it made sense, I said it would meet NEC requirements.

90.1 Purpose.

(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

I disagree with Infinity that a 20 amp breaker would always work for this 19.9 amp minimum circuit unit.

IMO a 20 amp breaker on this unit would be very likely to trip.

IMO that is why a unit with an MCA of 19.9 would likely have a maximum OCP of 30 to 40 amps.

125% is the figure used to find the conductor size and if that figure is used to size the breaker it will not provide enough 'extra' to start a motor load. That is why IMO we have Table 430.52 which allows Branch-Circuit Short-Circuit and Ground-Fault Protective Devices to be rated anywhere from 150% to 1100% of full load current depending on the application.

For the standard inverse time breakers we use 250% of full load current is allowed.

A unit with a MCA of 19.9 amps would likely have an FLA of about about 16 amps.

That would mean a breaker from 20 amps (likely to trip) to 40 amps could be used.

Personally I always use the maximum overcurrent device listed on the unit. I don't like call backs for tripping breakers.

 

[jwelectric]

Re: A/C CONDUCTOR SIZING

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted, provided that, where the protection specified is not sufficient for the starting current of the motor, the rating or setting shall be permitted to be increased but shall not exceed 225 percent of the motor rated-load current or branch-circuit selection current, whichever is greater.


440.22 simply states that the branch circuit current is to be increased to 175% for the short-circuit and ground-fault protective device, and if this won?t let the compressor start then we are not to exceed 225%.

175% of 19.9 = 34.8 so a 35 amp breaker is what is called for.

Lets not forget that when dealing with motors that the fuse or breaker is only for short-circuit and ground-fault.
A combination of breaker/fuse and running overload protection becomes the overcurrent protection.
In other words the combination of the two together is what protects the conductor from overcurrent, the breaker/fuse protects from short-circuit and ground-fault, the thermal protector that is integral with the motor protects from overcurrent.

 

[iwire]

Re: A/C CONDUCTOR SIZING

Mike that 440.22(A) does not apply, we are not talking about an Individual Motor-Compressor.

The opening post makes it clear we are talking about a combination unit, which brings us to 440.4(B).

 

[iwire]

Re: A/C CONDUCTOR SIZING

Originally posted by jwelectric:
Lets not forget that when dealing with motors that the fuse or breaker is only for short-circuit and ground-fault.

Do you think that Infinity, Milwaukeesteve or myself is not aware of that?

I think we got that part down.

 

[jwelectric]

Re: A/C CONDUCTOR SIZING

Sorry Bob got side tracked a little.

This is the way I see the original post needs to be answered.

If we take a long look at the definitions we might get another way to look at this. I agree that a feeder as defined has a overcurrent device on each end. I also see that an individual branch circuit supplies only one piece of equipment. This post fits both definitions.

Let?s go back to 440.22 and look at (B) this time. Here we are told to comply with 430.53. A quick look at 430.53 (C) we find that we can have both fuses and a breaker in this ?branch circuit?

If I am reading these sections the right way I see that although there is a breaker at the panel and fuses at the disconnect it is still a branch circuit as outlined in 430.53 (C).
Now the breaker can comply to the max. OCP

 

[infinity]

Re: A/C CONDUCTOR SIZING

I disagree with Infinity that a 20 amp breaker would always work for this 19.9 amp minimum circuit unit.

Good Morning Bob,


I didn't say that it would always work. It might but it might not. My comment was directed more towards the use of an inverse time CB. I did say that unless the unit nameplate requires a minimum size OCPD than the minimum circuit ampacity may be used to size the breaker. If the unit can't start than you could use any size breaker up to the maximum listed on the nameplate. I never use the minimum either. The last thing I want is a call back on a Sunday in July when I'm working on my tan by the pool.


Trevor

 

[milwaukeesteve]

Re: A/C CONDUCTOR SIZING

I like the way that jwelectric just stated it. That this circuit should be considered AS a circuit back to the breaker in the panel.
Since this is a circuit, and current is the same from where it is derived to the load it serves, whether there are 1,2 or 3 different forms of protection, this is still one circuit. This should also be considered a 'branch circuit' in respect to 440.

I am now confused as to who was talking to me about the standard breakers being or not being inverse time. I understand that at le depot de home, you can by standard breakers that are inverse time(that is what most breakers are today). However, what about a pushmatic, or zinsco, or FPE panel? In this case, you could run your #12's or 14's for the whole circuit. Fuse the disconnect appropriately, yet install a breaker in the panel that could be 225% of running load amps. That may look funny having the 12's on a 40A breaker, but would be right. Even if you decided to put a 'sidecar' panel next to the main for a HACR breaker, that original feeding breaker would have to be sized to handle Art 440's requirements.

 

[Jpyote]

Re: A/C CONDUCTOR SIZING

Been reading the posts and thought I would ad my two cents. If, your minimum cicuit ampacity is 19.9 a 20A Breaker would probably hold. They are HACR rated for the start up. Of course I dont't see many with the ampacity that close. More likely 20.1. At any rate the breaker in the main panel with probably be a HACR as most all are now.