Okay, let's break down the cable sizing for your outbuilding upgrade, focusing on the ground and neutral considerations.

Understanding the Cables You're Considering 2-2-4-6 MHF

This means two #2 AWG aluminum conductors (for the hot legs), one #4 AWG aluminum conductor (likely for the neutral), and one #6 AWG aluminum conductor (for the ground).

2-2-2-4 MHF

This means two #2 AWG aluminum conductors (for the hot legs), one #2 AWG aluminum conductor (likely for the neutral), and one #4 AWG aluminum conductor (for the ground).

Hot Conductors

Your load calculation of 79 amps, rounded up to 90 amps, is correct. NEC Table 310.16 allows #2 AWG aluminum conductors for 90 amps, assuming the cable is rated for 75°C termination (which is standard for most MHF cable). You need two of these for your 240V service.

Neutral Conductor

This is where it gets a little more nuanced, as it often does. You're correct that the neutral doesn't always need to be the same size as the hots, especially in a 240V service. The key considerations are:1.

Neutral Load Calculation

You need to estimate the maximum unbalanced load that will be placed on the neutral. This is the difference in current between the two hot legs. Think about the loads you'll have:

Washer, Dryer

These will have a 120V motor component and maybe a 120V control component; that's where the neutral current will come from.

Lights and Outlets

These will contribute to the unbalanced load. Estimate the maximum current they will draw.

Workbench

If tools connected to this workbench will be pulling 120V, they will be contributing to the unbalanced load.

Hot Water Heater

This is a 240V appliance and contributes nothing to the neutral current.2.

NEC Article 220.61 (Optional Feeder or Service Load Calculation)

This article allows for a reduced neutral load calculation in certain cases. Specifically, 220.61(B) states that the neutral load shall be permitted to be equal to the maximum unbalanced load determined by Section 220.
61. However, this provision only applies where there is no appreciable amount of nonlinear loads.3.

Minimum Neutral Size

The NEC requires a minimum neutral size. NEC Article 250.24(C)(1) states that the grounded conductor shall not be smaller than the grounding electrode conductor.
4.

Minimum Size for a Service

Table 250.66 indicates that, for a 90 amp service using aluminum, the grounding electrode conductor cannot be smaller than #6 AWG aluminum. This applies to services only. For feeders, use NEC 250.
122.

Grounding Conductor



NEC Table 250.122 determines the minimum size of the equipment grounding conductor (EGC) based on the size of the overcurrent protection device (your 90 amp breaker). For a 90 amp overcurrent device, Table 250.122 requires a #8 AWG copper or #6 AWG aluminum equipment grounding conductor. The grounding conductor provides a low-impedance path for fault current to return to the source, tripping the breaker quickly and safely. This is critical for safety.

Direct Burial Considerations



MHF Cable (Mobile Home Feeder)

MHF cable is suitable for direct burial, according to the NEC. However, you need to check the specific markings on the cable itself to confirm it's rated for direct burial. Most MHF cable is rated sunlight resistant as well.

Burial Depth

NEC 300.5 specifies minimum burial depths for direct-burial cables. For a 240V circuit, the minimum depth is typically 24 inches. Check local codes and regulations as they can sometimes be more stringent.

Recommendation

Based on your information, and assuming that your neutral loads will not be excessive: Conductors

2-#2 AWG Aluminum (Hot)

Neutral Conductor

#4 AWG Aluminum (NEC 250.24(C)(1) states the neutral conductor shall not be smaller than the grounding electrode conductor size from Table 250.66)

Grounding Conductor

#6 AWG Aluminum (NEC 250.122)

Cable to Use

2-2-4-6 MHF

Important Considerations and Actions1. Local Codes and Inspections

This is critical. Local codes can vary and may supersede the NEC in some areas.

Contact your local building inspector or electrical inspector before you start any work. They can confirm the cable sizes and burial requirements specific to your location.2. Comprehensive Load Calculation

Do a very thorough load calculation, itemizing everything you'll be connecting to the outbuilding, including the amperage and voltage of each appliance, lights, and tools.3.

Professional Consultation

If you have any doubts or concerns, consult with a licensed electrician. This is especially important given that you're upgrading the service size, which involves potentially hazardous voltages and currents. A professional can perform a proper load calculation, ensure compliance with all codes, and guarantee the safety of the installation.4.

GFCI Protection

Be sure to install GFCI protection for all 120V receptacles in the outbuilding, as required by the NEC.5.

Proper Terminations

Use connectors rated for aluminum conductors (AL/CU rated). Apply an anti-oxidant paste (like Noalox or Penetrox) to the aluminum conductors before terminating them to prevent corrosion. Torque the connections to the manufacturer's specifications.6.

Markings

Make sure the cable you use is marked for the voltage and amperage it's rated for and is listed for direct burial.

Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. This information is for general guidance only. It is essential to consult with a qualified electrician and comply with all applicable codes and regulations before performing any electrical work. Electrical work can be dangerous, and incorrect wiring can lead to fire, injury, or death. Flag for review