40 amp garage sub-panel

[CadiBrewer]

I have a sub-panel in my garage fed from a 40 amp breaker in my main 200 amp house panel. The sub-panel is wired with 8 gauge wires.

I have built a 30 amp control panel based off of Kal's design. I've installed a 30 amp gfci breaker in my sub-panel. The other circuits in the panel are one 15 amp breaker for the garage lights (two flourescent shop lights) and two 20 amp circuits for the outlets.

When I'm brewing, I'll have the panel going (as I understand it from Kal's design, the max it will be pulling is 23 amps), the lights on, and connected to the outlets will be my two keezers, which are both 7.5cf chest freezers. I don't anticipate running any power tools or any other loads.

I'm going to have the work I did to install the 30 amp breaker and the corresponding outlet inspected by my city. Assuming that my wiring job is up to code, does the rest sound okay? Realistically, am I going to have a problem with the 40 amp breaker in the main panel tripping during a brew day?

 

[Furore]

Sorry if this gets long winded. Part of that's going to depend on how many amps your keezers are pulling. I would basically assume that the fluro lights aren't pulling much amperage at all but you could check the ballast on them to confirm that. Without operating info on the keezers and lights I couldn't say for sure, but you might be getting close.

Unfortunately for extended use scenarios (>1hr), it isn't really as simple as just subtracting appliance amperage draw from the breaker rating. This is because your wires are going to heat up over time during constant use, and if you push it, you can melt wires especially when you're operating around the capacity of the breaker.

An important thing you will want to consider is that for any extended run time of electronics (say over an hour, so brew day would certainly qualify), you want to underrate your circuit breaker to around 80-85%. In the case of a 40 amp circuit, that would be somewhere in the 32-34 amp range. That's the number i would shoot for not exceeding just to be safe.

Worst case, if you're looking a bit high, you unplug one or both of the keezers (or flip that breaker) while you're brewing. Your beer is not going to dramatically warm up or anything more than maybe a couple degrees. You should be able to run the lights and the brew panel no problem though.

Do you have room on your main panel to expand your subpanel?

Just my 0.02. Hope it helps.

 

[CadiBrewer]

Thanks for your help. I figured I would be close. Flipping the two outlet circuits off during a brew day wouldn't be the end of the world. I'll check the lighting ballasts and the freezer specs to see what the actual draw will be.

As for my main panel, it is 200 amps and completely full. That being said, I don't think I'm drawing anywhere near 200 amps even at the peak. I have a 40 amp air conditioning circuit, but the rest are just 15 amp lighting and 20 amp outlet circuits. Gas dryer, gas stove. Just the dishwasher and other normal household appliances and two bathrooms in a small house.

I could increase the size of the breaker on the garage sub-panel and pull new wires, but I think I'll be limited to 50 amps because, unfortunately, the garage is detached and there is only 3/4 inch pvc conduit under the slab in the backyard (guy before me's bright idea). I think code would allow me to pull #6 wire and run the 50 amp breaker, but it may not be worth the effort of pulling the wires for the small increase.

Ideally, I'd like to have the brewery control panel, the lights, and a small, 120v point of use hot water heater running at the same time in the garage. Anyone have a bright idea about how I can accomplish this?

 

[itsnotrequired]

the amount of load on the 200 amp main panel isn't the limiting factor, it is that 40 amp breaker which services the sub-panel. if you draw more than 40 amps out at the sub-panel, it doesn't matter if the rest of the main panel is drawing 1, 10 or 100 amps, that 40 amp breaker will trip.

assuming you use individual thwn conductors, you can run #8 on that 50 amp breaker (also assumes 75 degree c rated terminals at the breakers, which basically everything nowadays have). they will fit in that 3/4" conduit, no problem. if it is uf cable, no dice with the #8, you would need #6 for the 50 amp breaker (and #6 uf wouldn't fit in that conduit). same for romex but romex isn't allowed underground anyway, even if in a conduit.

 

[ajdelange]

Actually, that breaker will probably not trip unless 50 or more amps are drawn, at least not right away. Most breakers have a magnetic and a thermal trip. The magnetic is there for huge overloads such as a short and will trip the breaker in milliseconds if one occurs. OTOH if you draw 25% over it may not trip for hours. You should pull the breaker out of the panel to find the maufacturer's name and model number and can then look up the trip characteristics for it on the net.

There is some stuff in the code about 'continuous' and ' non-continuous' loads with the latter being loads that are on less than 4 hours. Where a circuit contains non continuous loads they can exceed the circuit capacity by 25% that is if I am remembering this correctly. Don't take my word for it. Check the code!

 

[CadiBrewer]

Actually, that breaker will probably not trip unless 50 or more amps are drawn, at least not right away. Most breakers have a magnetic and a thermal trip. The magnetic is there for huge overloads such as a short and will trip the breaker in milliseconds if one occurs. OTOH if you draw 25% over it may not trip for hours. You should pull the breaker out of the panel to find the maufacturer's name and model number and can then look up the trip characteristics for it on the net.

The breaker is a Murray MP230GF. What I could find states that it is rated for continuous rating of 30 amps at 40deg celsius. Trips at 5mA (which I think is for the GFI protection) and 10,000 A IR (whatever that means). What else should I look for?

 

[CadiBrewer]

assuming you use individual thwn conductors, you can run #8 on that 50 amp breaker (also assumes 75 degree c rated terminals at the breakers, which basically everything nowadays have). they will fit in that 3/4" conduit, no problem. if it is uf cable, no dice with the #8, you would need #6 for the 50 amp breaker (and #6 uf wouldn't fit in that conduit). same for romex but romex isn't allowed underground anyway, even if in a conduit.

They way it is currently set up, there is 8/3 romex under the crawl space of the house, to a junction box at the end of the house. From there, it transitions to individual (I think they are THHN) conductors in the 3/4 inch pvc conduit. Are you saying that I could replace the THHN with THWN and go with a 50 amp breaker at the main panel?

 

[itsnotrequired]

physically, breakers can be overloaded as ajdelange describes but is a code violation and not good design practice. if you want to run more than 40 amps at a time, get a 50 amp breaker.

the '10,000 A IR' is the fault current rating of the circuit breaker. this means it can be exposed to a current of up to 10,000 amps running through it and still have the capability to operate properly and interrupt the current. above 10,000 amps, there is the risk the breaker will weld or blow up or similar and not safely interrupt the current. it is a testing/rating/listing thing. it is all but impossible to ever see currents that high in a home, even if you bolted hot and ground together.

regarding your setup, thhn conductors are not rated for below ground use, even if in a conduit. underground conduits are considered 'wet' environments, thhn is only rated up to 'damp' environments. thwn is rated for wet, good to go in that underground conduit. to take advantage of #8 on a 50 amp breaker, the entire circuit would need to be thwn. #8 romex is not allowed on a 50 amp breaker.

 

[ajdelange]

The breaker is a Murray MP230GF. What I could find states that it is rated for continuous rating of 30 amps at 40deg celsius. Trips at 5mA (which I think is for the GFI protection) and 10,000 A IR (whatever that means). What else should I look for?

Actually that's a 30 amp breaker. 40 Amps would represent a 33% overload and it would trip between 100 and 700 seconds after a load of that size is applied. You can get the detailed characteristics from www.allelectricneedsinc.com/content/MP1515.pdf

10,000 amperes is the magnitude of the current it can interrupt. The series impedance of the fault and the wires would have to be less than 120/10000 = .012 Ω for a larger fault current than that.

 

[ajdelange]

physically, breakers can be overloaded as ajdelange describes but is a code violation and not good design practice.

No, not at all. It's done all the time. The breaker in question (a thirty amp breaker) can, for example, sustain a 300% (90 Amp) load for 5 - 20 seconds. This would be plenty of time to start a motor rated at 30 FLA, for example.

if you want to run more than 40 amps at a time, get a 50 amp breaker.

Clearly, if you want to run 45 amps continuously you should have a 50 amp breaker. Thirty isn't going to do it.

 

[CadiBrewer]

Actually that's a 30 amp breaker. 40 Amps would represent a 33% overload and it would trip between 100 and 700 seconds after a load of that size is applied. You can get the detailed characteristics from www.allelectricneedsinc.com/content/MP1515.pdf

Thanks, AJ and others. I made a mistake. That's the 30 amp breaker I installed in the garage sub-panel for the brewery control panel. As you first indicated, I'll need to pull the 40 amp breaker from the main panel to see what breaker model it is and look up the characteristics.

 

[itsnotrequired]

No, not at all. It's done all the time. The breaker in question (a thirty amp breaker) can, for example, sustain a 300% (90 Amp) load for 5 - 20 seconds. This would be plenty of time to start a motor rated at 30 FLA, for example.

i'm not saying it isn't done but it absolutely a code violation to connect a load greater than the breaker rating for that type of load.

motors are a completely different animal and there are whole sections of the code dedicated to it. these are very specific rules that do not apply to non-motor loads.

 

[CadiBrewer]

regarding your setup, thhn conductors are not rated for below ground use, even if in a conduit. underground conduits are considered 'wet' environments, thhn is only rated up to 'damp' environments. thwn is rated for wet, good to go in that underground conduit. to take advantage of #8 on a 50 amp breaker, the entire circuit would need to be thwn. #8 romex is not allowed on a 50 amp breaker.

I checked the panel and the subpanel and here's what I found. I did run thwn under the slab, so I'm ok with that part. On the wire, it says "THWN-2 or THNN gasoline and oil rated" plus a bunch of other stuff. The rest of the run in the crawl space is romex. If I want to go with the 50 amp breaker in the main panel, I'll need to change the wire under the house to individual THWN wires.

Can you guys help me with two things? First, I checked the wattage and amperage on everything that I'll be running during a brew session. Here's what I have:

Kal clone brewery control panel - 23 amps
Freezer - 1.69 amps, 12 amps at start up
Freezer - 1.7 amps - no listing for amps at start up - different brand, but same sized 7 CF freezer, so I assume around the same 12 amps at start up
Television - 115 watts, 115v
Cable receiver - 80 watts, 115v
Lights - (4) 36w LED shop lights 144 watts total - I'll be changing out my flourescents for LED shop lights

Will a 50 amp breaker work for all of this at one time? I could calculate the wattage to amps to come up with a total, but I don't know the max I should run safely on 50 amps, and I have no idea how to handle the start up amps on the two freezers.

Secondly, I looked up the section of the NEC where it indicates that I can use #8 wires on a 50 amp circuit if I use THWN wire and if my connections are all rated at a certain temperature. My question is, how do I know what temperature my connections are rated at? If I change out the romex to individual conductors, I'll have whatever the 50 amp breaker is at the main panel on one end, one connection under the house where the new THWN I'll be running is spliced with the existing THWN under my patio slab, joined with wire nuts, and then the subpanel in the garage. How do I figure out the temp rating of these connections? And is the temp rating requirement 90 deg Celsius for the #8 wire to take advantage of a 50 amp circuit?

Thanks for all of your help. I want to learn this stuff, not just run the wire and hope things work.

 

[CadiBrewer]

Bumped for some help. Thanks!

 

[ccfoo242]

Seems like you would be safest by unplugging one of the freezers while brewing. When maintaining a boil would it be pulling 23 amps or is that just theoretical max?

 

[ajdelange]

Given those loads you are fine with the 30 Amp breaker. If I split the 120V Loads between the two phases you would have a 28.5 amp load on one and a 27 amp load on the other. Not more than 3 AWG. 8 conductors rated for 75 °C in a conduit or buried are good for up to 50 amps and your 30 amp breaker will never pop at 28 (unless it gets very hot in the panel for some reason). Do not worry about the startup currents for the freezer compressors. Twelve amps represents about a 30% overload for a 30 amp breaker. Don't want to go look yours up again but the ones in my panels would pop between 30 and 200 seconds at that level of overload. While the probability that the compressors on both machines would come on at the same time is very small you might want to be sure the freezers are on separate phases. This wouldn't protect against restoration of power after a long outage but you wouldn't probably have the kettle on then. Even the resulting 50% overload would give the freezers 7 - 50 seconds to start up before the breaker popped.

Given the AWG 8 you could, of course, put a 50 amp breaker in the panel.

 

[ajdelange]

i'm not saying it isn't done but it absolutely a code violation to connect a load greater than the breaker rating for that type of load.

Of course it's done as there is no way around it and it isn't a code violation. Suppose I install 10 spots of 100W each on a 120V circuit connected to 1 switch. They would draw 0.83 amps each in steady state and when all turned on together would present a load of 8.3 amps - well below the capabilities of a 15 amp breaker and No. 14 wire. But what happens when the homeowner throws that switch? There is inrush current of something like 11 times that (based on what I just measured for a 75 W bulb). That's 95 amps or an overload of 636%. Omigosh! But no need to despair. Everyone, except apparently itsnotrequired, knows that it isn't I that causes problems, it is I^2*t where t is the length of time the current flows. This is proportional to the energy delivered to the wiring and hence the amount of temperature rise it experiences from the overload. Accordingly, protective devices are set to trip on I^2*t, not just I. For example, the GE breakers in my panels will allow current of 6.4 times the breaker rating to flow for a minimum of 600 ms to a maximum of about 4s. The heavy current flow only lasts for a ms or so and then begins to diminish. The rms current for the first 40 mS is about 2.78 times the steady state load or in this case 23.1 amps which is 154% of the rating of the breaker. These breakers will handle a 154% load for at least 50 and possibly as long as 500 seconds which is plenty as this level of load is over in 40 mS and everything is settled out within about 100 ms.

motors are a completely different animal and there are whole sections of the code dedicated to it. these are very specific rules that do not apply to non-motor loads.

Lots of things in homes have motors in them and, for example, Article 440 which deals with air conditioning, talks about proper sizing and overload protection using the same thermo magnetic breakers we are talking about here. The problem with this argument is that the electrician doesn't know when he pulls a circuit whether the occupant will plug a dell phone charger into the receptacle or a room air conditioner which has a compressor motor. So he puts in a UL approved thermal magnetic breaker which will happily run at more that its rating for hours if the overload is modest enough. There is no code violation that I can find but perhaps someone with a deeper familiarity with it than mine can find one.

Mind you I don't advocate putting a 120% load on a breaker just because I know it won't trip for 2000 seconds. But my genius electrician did do just that to me with an air compressor for which he pulled an 15 amp circuit. The overload is about 15% but an air compressor typically doesn't run more than a few minutes so the breaker doesn't trip. I'm not happy with this (or a lot of other stuff he did) but I'm not going to pull new wire.

 

[itsnotrequired]

i know how breakers work, dude. i'm pointing out is a code violation to connect loads to a circuit in excess of the rating of the breaker (in general). so even though 40.1 amps on a 40 amp breaker may not trip for days, it is still a code violation to set the circuit up that way. it practically isn't a problem but still a violation. just like a #14 wire can carry 15.1 amps without bursting into flames but it is still a violation to load it that way. the code isn't always practical.

to the op, your existing 40 amp breaker is fine . the worst case is if both freezers happen to start at the exact same time while the brew panel is at full throttle the breaker might trip but not likely. practically, it is very unlikely for those appliances to start at the exact same time. i say give it a test, worst case you upgrade to teh 50 amp breaker and replace some romex.

 

[ajdelange]

i know how breakers work, dude. i'm pointing out is a code violation to connect loads to a circuit in excess of the rating of the breaker (in general). so even though 40.1 amps on a 40 amp breaker may not trip for days, it is still a code violation to set the circuit up that way. it practically isn't a problem but still a violation.

We actually may be agreeing. Your answer to the question "Which, if any, of the following three installations is a code violation?" will tell me whether that's the case so if you don't mind answering...

A)Ten flush mounted 75W halogen ceiling lights on a 15 amp breaker. Startup current 18.6 A rms for the first 20 mS. Steady state current 5.3A rms (true rms as the current waveform looks as if there are switchers in the bulbs).

B)Twelve 8ft T8 fluorescents in 3 luminaires on a 15 amp breaker.
Startup current: 30.2A rms over first 2 mS (peak current ~60A). Steady state current: 9.36A rms

C)A 240 V receptacle in a garage on a 20 Amp breaker.
With a 1 HP compressor plugged in startup current is 69.7 A rms over the first second. Running current is 18.1 A rms after 1.5 sec.

So which represent code violations?

BTW these are all actual installations. A) is in the corridor between my house and brewery/garage, B) describes the lights in my brewery and C) is an outlet in my garage. Obviously I remembered the details of C incorrectly in my previous post.

 

[ajdelange]

I guess no one is going to take a stab so I'll give the answers. The electrician committed no code violations in any of the three cases. In each he installed wiring connected to an approved circuit breaker with a rating less than the ampacity of the wires he ran. The approved breakers are approved because they permit the inrush currents associated with the loads to which those circuits are wired even though those loads are, for a short time period, appreciably higher, than the breaker rating, without endangering the wiring.

The electrician did not violate code when he installed the outlet because he did not (nor did I at the time) know what I might plug into it. And the inspector had no problems with any of these three circuits.

I eventually plugged an air compressor into that outlet and have never plugged anything else. Had I asked the electrician to permanently wire the compressor to the 20 amp circuit then he would have committed a code violation because the motor is rated for continuous duty and a continuous duty motor has a requirement for conductors that must handle currents greater than the nameplate load. Twenty amps won't quite cut it.

 

[itsnotrequired]

meant to reply to your earlier post, agreed no violations. regarding your latest post, no problem with a breaker rated less than the ampacity of the conductors it serves, happens all the time

not sure why you are so hung up on inrush...

 

[The10mmKid]

You'll be fine.

Often not mentioned is the current draw after reaching boiling is less than 100%

You have to back off the duty cycle of the controller to keep from a violent boil.

'da Kid

 

[ajdelange]

not sure why you are so hung up on inrush...

I'm not. This was all for the benefit of the gentleman here who seems to be.